ELM ELM327LP Automatically searches for protocol Datasheet

ELM327L
OBD to RS232 Interpreter
Description
Features
The ELM327L is a low voltage version of our
popular ELM327 integrated circuit. It supports all of
the features of our current ELM327 IC, and only
differs physically in how pin 6 is used.
The following pages discuss the ELM327L in
detail, how to use it and how to configure it, as well
as providing some background information on the
protocols that are supported. There are also
schematic diagrams and tips to help you to interface
to microprocessors, construct a basic scan tool, and
to use the low power mode. Because of the similarity
with the ELM327, much of this data sheet has been
copied from the ELM327 data sheet.
Throughout this document, we will use the term
‘ELM327’ to refer to either the ELM327 or the
ELM327L. If there is something different between
the two, we will be specific as to which it applies to.
• Works with a 2.0V to 5.5V supply
• Universal serial (RS232) interface
• Automatically searches for protocols
• Fully configurable with AT commands
• Low power CMOS design
Connection Diagram
PDIP and SOIC
(top view)
MCLR
OBD Tx LED
Vmeasure
OBD Rx LED
J1850 Volts
RS232 Tx LED
J1850 Bus+
RS232 Rx LED
Applications
Memory
CAN Rx
Filter Cap
CAN Tx
LFmode
ISO L
VSS
ISO K
• Diagnostic trouble code readers
XT1
VDD
• Automotive scan tools
XT2
VSS
• Teaching aids
Block Diagram
VPW In
RS232 Rx
ISO In
RS232 Tx
PWM In
PwrCtrl / Busy
J1850 Bus-
IgnMon / RTS
4.00 MHz
Memory
LFmode
5
7
XT1
9
XT2
10
MCLR
1
Vmeasure
2
RS232Rx
18
RS232Tx
17
PwrCtrl / Busy
16
IgnMon / RTS
15
RS232
Interface
Command
and
Protocol
Interpreter
Power
Control
A/D
Converter
ISO 15765-4
SAE J1939
23
Filter Cap
ELM327L DSA
6
LDO
Regulator
25
26
27
28
ISO 9141-2
ISO 14230-4
24
21
22
12
SAE J1850
PWM & VPW
3
4
14
13
11
OBD interfaces
status LEDs
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ELM327L
Contents
The Basics
Description................................................................................... 1
Features....................................................................................... 1
Applications..................................................................................1
Block Diagram..............................................................................1
Connection Diagram.................................................................... 1
Pin Descriptions........................................................................... 4
ELM327L Unused Pins................................................................ 6
ELM327L Absolute Maximum Ratings......................................... 7
ELM327L Electrical Characteristics............................................. 7
Using the ELM327
Overview...................................................................................... 9
Communicating with the ELM327................................................ 9
AT Commands........................................................................... 11
AT Command Summary.............................................................11
AT Command Descriptions........................................................ 13
Reading the Battery Voltage...................................................... 30
OBD Commands........................................................................ 31
Talking to the Vehicle.................................................................32
Bus Initiation...............................................................................34
Interpreting Trouble Codes........................................................ 35
Resetting Trouble Codes........................................................... 36
Quick Guide for Reading Trouble Codes................................... 36
Selecting Protocols.................................................................... 37
OBD Message Formats..............................................................39
Setting the Headers................................................................... 40
Multiline Responses................................................................... 43
CAN Message Types................................................................. 45
Multiple PID Requests................................................................46
Response Pending Messages................................................... 46
CAN Receive Filtering - the CRA command.............................. 47
Using the CAN Mask and Filter..................................................48
Monitoring the Bus..................................................................... 49
Restoring Order..........................................................................50
Advanced Features
Using Higher RS232 Baud Rates...............................................51
Setting Timeouts - the AT ST and AT AT Commands............... 53
SAE J1939 Messages................................................................54
Using J1939............................................................................... 56
The FMS Standard.....................................................................59
The NMEA 2000 Standard......................................................... 60
Periodic (Wakeup) Messages.................................................... 60
Altering Flow Control Messages................................................ 61
Using CAN Extended Addresses............................................... 62
CAN Input Frequency Matching................................................. 63
ELM327L DSA
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ELM327L
Contents
Advanced Features
(continued)
Programming Serial Numbers....................................................64
Saving a Data Byte.................................................................... 64
The Activity Monitor....................................................................65
Power Control............................................................................ 65
Programmable Parameters........................................................ 69
Programmable Parameter Summary......................................... 70
Design Discussions
Maximum CAN Data Rates........................................................ 75
Microprocessor Interfaces..........................................................77
Upgrading Versions....................................................................78
Example Applications.................................................................79
Figure 9 - An OBD to USB Interpreter (5.0V)...................... 81
Figure 10 - Parts List for Figure 9........................................ 82
Figure 11 - A Low Speed RS232 Interface.......................... 82
Figure 12 - A High Speed RS232 Interface......................... 83
Figure 13 - An Alternative USB Interface............................ 83
Figure 14 - Connecting to a 3.3V System............................84
Figure 15 - An OBD to USB Interpreter (3.3V).................... 85
Modifications for Low Power Standby Operation....................... 86
Misc. Information
ELM327L DSA
Error Messages and Alerts.........................................................89
Version History...........................................................................91
Ordering Information.................................................................. 91
Outline Diagrams....................................................................... 92
Copyright and Disclaimer........................................................... 92
Index.......................................................................................... 93
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ELM327L
Pin Descriptions
MCLR (pin 1)
A momentary (>2µsec) logic low applied to this input
will reset the ELM327. If unused, this pin should be
connected to a logic high (VDD) level.
Vmeasure (pin 2)
This analog input is used to measure a 0 to 5V
signal that is applied to it. Care must be taken to
prevent the voltage from going outside of the supply
levels of the ELM327, or damage may occur. If it is
not used, this pin should be tied to either VDD or VSS.
J1850 Volts (pin 3)
This output can be used to control a voltage supply
for the J1850 Bus+ output. The pin normally outputs
a logic high level when a nominal 8V is required (for
J1850 VPW), and a low level for 5V (for J1850
PWM), but this can be changed with PP 12. The total
current into or from pin 3 must not exceed 2 mA.
J1850 Bus+ (pin 4)
This active high output is used to drive the
J1850 Bus+ Line to an active (low) level. The total
current into or from pin 4 must not exceed 2 mA.
Memory (pin 5)
This input controls the default state of the memory
option. If this pin is at a high level during power-up or
reset, the memory function will be enabled by
default. If it is at a low level, then the default will be
to have it disabled. Memory can always be enabled
or disabled with the AT M1 and AT M0 commands.
This input has Schmitt trigger waveshaping.
Filter Cap (pin 6)
The ELM327L requires that a 10µF filter capacitor be
connected between pin 6 and VSS. The capacitor
should be a low ESR (<5Ω) ceramic or tantalum
type.
The ELM327 uses pin 6 for setting the initial baud
rate after power on. Since this option is not available
with the ELM327L, it defaults to an initial baud rate
of 38400 bps. If you require 9600 bps, you must set
PP 0C to 00.
LFmode (pin 7)
This input is used to select the default linefeed mode
to be used after a power-up or system reset. If it is at
ELM327L DSA
a high level, then by default messages sent by the
ELM327 will be terminated with both a carriage
return and a linefeed character. If it is at a low level,
lines will be terminated by a carriage return only.
This behaviour can always be modified by issuing an
AT L1 or AT L0 command. This input has Schmitt
trigger waveshaping.
VSS (pin 8)
Circuit common must be connected to this pin.
XT1 (pin 9) and XT2 (pin 10)
A 4.000 MHz oscillator crystal is connected between
these two pins. Loading capacitors as required by
the crystal (typically 27pF each) will also need to be
connected between each of these pins and circuit
common (Vss).
Note that this device has not been configured for
operation with an external oscillator – it expects a
crystal to be connected to these pins. Use of an
external clock source is not recommended. Also,
note that this oscillator is turned off when in the Low
Power or ‘standby’ mode of operation.
VPW In (pin 11)
This is the active high input for the J1850 VPW data
signal. When at rest (bus recessive) this pin should
be at a low logic level. This input has Schmitt trigger
waveshaping, so no special amplification is required.
ISO In (pin 12)
This is the active low input for the ISO 9141 and
ISO 14230 data signal. It is derived from the K Line,
and should be at a high logic level when at rest (bus
recessive). No special amplification is required, as
this input has Schmitt trigger waveshaping.
PWM In (pin 13)
This is the active low input for the J1850 PWM data
signal. It should normally be at a high level when at
rest (ie. bus recessive). This input has Schmitt
trigger waveshaping, so no special amplification is
required.
J1850 Bus- (pin 14)
This active high output is used to drive the J1850
Bus- Line to an active (dominant) level for J1850
PWM applications.
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ELM327L
Pin Descriptions (continued)
IgnMon / RTS (pin 15)
This input pin can serve one of two functions,
depending on how the Power Control options
(PP 0E) are set.
If both bit 7 and bit 2 of PP 0E are ‘1’s, this pin will
act as an Ignition Monitor. This will result in a switch
to the Low Power mode of operation, should the
IgnMon signal go to a low level, as would happen if
the vehicle’s ignition were turned off. An internal
‘debounce’ timer is used to ensure that the ELM327
does not shut down for noise at the input.
When the voltage at pin 15 is again restored to a
high level, and a time of 1 or 5 seconds (as set by
PP 0E bit 1) passes, the ELM327 will perform a
‘Warm Start’ and return to normal operation. A low to
high transition at pin 15 will in fact restore normal
operation, regardless of the setting of PP 0E bit 2, or
whether pin 15 was the initial cause for the low
power mode. This feature allows a system to control
how and when it switches to low power standby
operation, but still have automatic wakeup by the
ignition voltage, or even by a pushbutton.
If either bit 7 or bit 2 of PP 0E are ‘0’, this pin will
function as an active low ‘Request To Send’ input.
This can be used to interrupt the OBD processing in
order to send a new command, or as previously
mentioned, to highlight the fact that the ignition has
been turned off. Normally kept at a high level, this
input is brought low for attention, and should remain
so until the Busy line (pin 16) indicates that the
ELM327 is no longer busy, or until a prompt
character is received (if pin 16 is being used for
power control).
By default, pin 15 acts as the RTS interrupt input.
This input has Schmitt trigger waveshaping.
PwrCtrl / Busy (pin 16)
This output pin can serve one of two functions,
depending on how the Power Control options
(PP 0E) are set.
If bit 7 of PP 0E is a ‘1’ (the default), this pin will
function as a Power Control output. The normal state
of the pin will be as set by PP 0E bit 6, and the pin
will remain in that state until the ELM327 switches to
the Low Power mode of operation, when the output
changes to the opposite level. This output is typically
used to control enable inputs, but may also be used
for relay circuits, etc. with suitable buffering. The
ELM327L DSA
discussion on page 86 (‘Modifications for Low Power
Standby Operation’) provides more detail on how to
use this output.
If bit 7 of PP 0E is a ‘0’, pin 16 will function as a
‘Busy’ output, showing when the ELM327 is actively
processing a command (the output will be at a high
level), or when it is idle, ready to receive commands
(the output will be low).
By default, bit 7 of PP 0E is ‘1’, so pin 16 provides
the Power Control function.
RS232Tx (pin 17)
This is the RS232 data transmit output. The signal
level is compatible with most interface ICs (the
output is high when idle), and there is sufficient
current drive to allow interfacing using only a PNP
transistor, if desired.
RS232Rx (pin 18)
This is the RS232 receive data input. The signal
level is compatible with most interface ICs (when at
idle, the level should be high), but can be used with
other interfaces as well, since the input has Schmitt
trigger waveshaping.
VSS (pin 19)
Circuit common must be connected to this pin.
VDD (pin 20)
This pin is the positive supply pin, and should always
be the most positive point in the circuit. Internal
circuitry connected to this pin is used to provide
power on reset of the ELM327 processor, so an
external reset signal is not required. Refer to the
Electrical Characteristics section for further
information.
ISO K (pin 21) and ISO L (pin 22)
These are the active high output signals which are
used to drive the ISO 9141 and ISO 14230 buses to
an active (dominant) level. Many new vehicles do not
require the L Line – if yours does not, you can simply
leave pin 22 open-circuited.
CAN Tx (pin 23) and CAN Rx (pin 24)
These are the two CAN interface signals that must
be connected to a CAN transceiver IC (see the
Example Applications section for more information).
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ELM327L
Pin Descriptions (continued)
If unused, pin 24 must be connected to a logic high
(VDD) level. The pin 24 CAN Rx input has Schmitt
trigger waveshaping.
RS232 Rx LED (pin 25), RS232 Tx LED (pin 26),
OBD Rx LED (pin 27) and OBD Tx LED (pin 28)
These four output pins are normally high, and are
driven to low levels when the ELM327 is transmitting
or receiving data. These outputs are suitable for
directly driving most LEDs through current limiting
resistors, or interfacing to other logic circuits. If
unused, these pins may be left open-circuited.
Note that pin 28 can also be used to turn off all of the
Programmable Parameters, if you can not do so by
using the normal interface - see page 70 for details.
ELM327L Unused Pins
When people only want to implement a portion of what
the ELM327L is capable of, they often ask what to do
with the unused pins. The rule is that unused outputs
may be left open-circuited with nothing connected to
them, but unused inputs must always be terminated.
The ELM327L is a CMOS integrated circuit that can
not have any inputs left floating (or you might damage
the IC). Connect unused inputs as follows:
Pin
1
2
5
7
11
12
13
15
18
24
Level
H
H*
H*
H*
H*
L*
L*
H
H
H
Note that the inputs that are shown with an asterisk (*)
may be connected to either a High (VDD) or a Low
(VSS) level, but the level shown is preferred. Pin 6 is
not shown for the ELM327L as it must be connected to
a capacitor.
ELM327L DSA
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ELM327L
ELM327L Absolute Maximum Ratings
Storage Temperature....................... -65°C to +150°C
Note:
These values are given as a design guideline only.
The ability to operate to these levels is neither
inferred nor recommended, and stresses beyond
those listed here will likely damage the device.
Ambient Temperature with
Power Applied....................................-40°C to +85°C
Voltage on VDD with respect to VSS..... -0.3V to +7.5V
Voltage on any other pin with
respect to VSS........................... -0.3V to (VDD + 0.3V)
Current through pins 3 or 4
(source or sink)...................................................2 mA
Current through all other output pins
(source or sink).................................................25 mA
ELM327L Electrical Characteristics
All values are for operation at 25°C. For more information, refer to note 1 below.
Characteristic
Minimum
Supply voltage, VDD
2.0
VDD rate of rise
0.05
Brown-out reset voltage
1.7
A/D conversion time
Typical
5.5
128
IgnMon debounce time
50
V
1.8
1.9
V
msec
AT RV to beginning of response
µsec
to wake from Low Power mode
65
msec
AT LP to PwrCtrl output time
1.0
sec
LP ALERT to PwrCtrl output time
2.0
sec
Reset time
800
msec
2
msec
AT Z
AT WS
Conditions
V/msec see note 2
7
Pin 18 wake pulse duration
Maximum Units
Measured from the end of the
command to the start of the ID
message (ELM327 v2.1)
continued on the next page…
Notes:
1. This integrated circuit is based on Microchip Technology Inc.’s PIC18F25K80 device. For more detailed
device specifications, and possibly clarification of those given, please refer to the Microchip documentation
(available at www.microchip.com).
2. This spec must be met in order to ensure that a correct power on reset occurs. It is quite easily achieved
using most common types of supplies, but may be violated if one uses a slowly varying supply voltage, as
may be obtained through direct connection to solar cells or some charge pump circuits.
ELM327L DSA
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ELM327L
ELM327L Electrical Characteristics (continued)
The following values are for operation at 25°C, with VDD = 3.3V.
Characteristic
Average current, IDD
Input logic levels
(Schmitt thresholds)
Minimum
Maximum Units
normal
5.0
mA
low power
0.03
mA
rising
1.5
falling
Output current drive
Typical
0.7
source
2.6
V
Conditions
All input pins use Schmitt trigger
waveshaping - see note 3
1.3
V
2.3
mA
VO = 3.05V
8
mA
VO = 0.25V
see note 4
sink
The following values are for operation at 25°C, with VDD = 5.0V.
Characteristic
Average current, IDD
Input logic levels
(Schmitt thresholds)
Minimum
Maximum Units
normal
5.5
mA
low power
0.13
mA
rising
2.1
falling
Output current drive
Typical
1.0
4.0
V
Conditions
All input pins use Schmitt trigger
waveshaping - see note 3
1.8
V
source
3.3
mA
VO = 4.75V
sink
11
mA
VO = 0.25V
see note 4
Notes (cont’d):
3. The Microchip 18F25K80 documentation states that all of the inputs used by the ELM327L have Schmitt
waveshaping on them. Sample testing however, has shown that pin 11 may respond as a standard ‘CMOS’
type input (ie not Schmitt) with a threshold voltage approximately equal to the average of the Schmitt
thresholds. For this reason, circuit designs should not present slowly changing waveforms at pin 11.
4. This applies to all output pins, except pins 3 and 4. Although pins 3 and 4 may seem capable of similar
currents, Microchip Technology states in their literature that these pins must not source or sink any more
than 2 mA.
ELM327L DSA
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ELM327L
Overview
The following describes how to use the ELM327 to
obtain information from your vehicle.
We begin by discussing just how to ‘talk’ to the IC
using a PC, then explain how to change options using
‘AT’ commands, and finally we show how to use the
ELM327 to obtain trouble codes (and reset them). For
the more advanced experimenters, there are also
sections on how to use some of the programmable
features of this integrated circuit as well.
Using the ELM327 is not as daunting as it first
seems. Many users will never need to issue an ‘AT’
command, adjust timeouts, or change the headers. For
most, all that is required is a PC or smart device with a
terminal program (such as HyperTerminal or ZTerm),
and a little knowledge of OBD commands, which we
will provide in the following sections…
Communicating with the ELM327
The ELM327 expects to communicate with a PC
through an RS232 serial connection. Although modern
computers do not usually provide a serial connection
such as this, there are several ways in which a ‘virtual
serial port’ can be created. The most common devices
are USB to RS232 adapters, but there are several
others such as PC cards, ethernet devices, or
Bluetooth to serial adapters.
No matter how you physically connect to the
ELM327, you will need a way to send and receive
data. The simplest method is to use one of the many
‘terminal’ programs that are available (HyperTerminal,
ZTerm, etc.), to allow typing the characters directly
from your keyboard.
To use a terminal program, you will need to adjust
several settings. First, ensure that your software is set
to use the proper ‘COM’ port, and that you have
chosen the proper data rate - this will usually be 38400
baud (if PP 0C has not been changed). If you select
the wrong ‘COM’ port, you will not be able to send or
receive any data. If you select the wrong data rate, the
information that you send and receive will be all
garbled, and unreadable by you or the ELM327. Don’t
forget to also set your connection for 8 data bits, no
parity bits, and 1 stop bit, and to set it for the proper
‘line end’ mode. All of the responses from the ELM327
are terminated with a single carriage return character
and, optionally, a linefeed character (depending on
your settings).
Properly connected and powered, the ELM327 will
energize the four LED outputs in sequence (as a lamp
test) and will then send the message:
ELM327 v2.1
>
In addition to identifying the version of this IC,
receiving this string is a good way to confirm that the
computer connections and terminal software settings
ELM327L DSA
are correct (however, at this point no communications
have taken place with the vehicle, so the state of that
connection is still unknown).
The ‘>’ character that is shown on the second line
is the ELM327’s prompt character. It indicates that the
device is in the idle state, ready to receive characters
on the RS232 port. If you did not see the identification
string, you might try resetting the IC again with the AT
Z (reset) command. Simply type the letters A T and Z
(spaces are optional), then press the return key:
>AT Z
That should cause the leds to flash again, and the
identification string to be printed. If you see strange
looking characters, then check your baud rate - you
have likely set it incorrectly.
Characters sent from the computer can either be
intended for the ELM327’s internal use, or for
reformatting and passing on to the vehicle. The
ELM327 can quickly determine where the received
characters are to be directed by monitoring the
contents of the message. Commands that are
intended for the ELM327’s internal use will begin with
the characters ‘AT’, while OBD commands for the
vehicle are only allowed to contain the ASCII codes for
hexadecimal digits (0 to 9 and A to F).
Whether it is an ‘AT’ type internal command or a
hex string for the OBD bus, all messages to the
ELM327 must be terminated with a carriage return
character (hex ‘0D’) before it will be acted upon. The
one exception is when an incomplete string is sent and
no carriage return appears. In this case, an internal
timer will automatically abort the incomplete message
after about 20 seconds, and the ELM327 will print a
single question mark (‘?’) to show that the input was
not understood (and was not acted upon).
Messages that are not understood by the ELM327
(syntax errors) will always be signalled by a single
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ELM327L
Communicating with the ELM327 (continued)
question mark. These include incomplete messages,
incorrect AT commands, or invalid hexadecimal digit
strings, but are not an indication of whether or not the
message was understood by the vehicle. One must
keep in mind that the ELM327 is a protocol interpreter
that makes no attempt to assess the OBD messages
that you send for validity – it only ensures that
hexadecimal digits were received, combined into
bytes, then sent out the OBD port, and it does not
know if a message sent to the vehicle was in error.
While processing OBD commands, the ELM327
will continually monitor for either an active RTS input,
or an RS232 character received. Either one will
interrupt the IC, quickly returning control to the user,
while possibly aborting any initiation, etc. that was in
progress. After generating a signal to interrupt the
ELM327, software should always wait for either the
prompt character (‘>’ or hex 3E), or a low level on the
Busy output before beginning to send the next
command.
Finally, it should be noted that the ELM327 is not
case-sensitive, so the commands ‘ATZ’, ‘atz’, and
‘AtZ’ are all exactly the same to the ELM327. All
commands may be entered as you prefer, as no one
method is faster or better. The ELM327 also ignores
space characters and all control characters (tab, etc.),
so they can be inserted anywhere in the input if that
improves readability.
One other feature of the ELM327 is the ability to
repeat any command (AT or OBD) when only a single
carriage return character is received. If you have sent
a command (for example, 01 0C to obtain the rpm),
you do not have to resend the entire command in
order to resend the request to the vehicle - simply
send a carriage return character, and the ELM327 will
repeat the command for you. The memory buffer only
remembers one command though, and there is no
provision in the current ELM327 to provide storage for
any more.
Please Note:
An issue with the EUSART used in a previous version of the ELM327 resulted in the
possibilty of NULL characters (byte value 00) being inserted into the RS232 data. This
was very rare, but we recommended that software filter for NULL characters and remove
them if found. Although there are no known issues with the ELM327L’s EUSART, we still
recommend that all NULL characters be fitered from data that is received from the
ELM327L.
ELM327L DSA
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ELM327L
AT Commands
Several parameters within the ELM327 can be
adjusted in order to modify its behaviour. These do not
normally have to be changed before attempting to talk
to the vehicle, but occasionally the user may wish to
customize these settings – for example by turning the
character echo off, adjusting a timeout value, or
changing the header bytes. In order to do this, internal
‘AT’ commands must be used.
Those familiar with PC modems will immediately
recognize AT commands as a standard way in which
modems are internally configured. The ELM327 uses
essentially the same method, always watching the
data sent by the PC, looking for messages that begin
with the character ‘A’ followed by the character ‘T’. If
found, the next characters will be interpreted as an
internal configuration or ‘AT’ command, and will be
executed upon receipt of a terminating carriage return
character. If the command is just a setting change, the
ELM327 will reply with the characters ‘OK’, to say that
it was successfully completed.
Some of the following commands allow passing
numbers as arguments in order to set the internal
values. These will always be hexadecimal numbers
which must generally be provided in pairs. The
hexadecimal conversion chart in the OBD Commands
section (page 31) may be helpful if you wish to
interpret the values. Also, you should be aware that for
the on/off types of commands, the second character is
the number 1 or the number 0, the universal terms for
on and off.
The remainder of this page, and the two pages
following provide a summary of all of the commands
that the current version of the ELM327 recognizes. A
more complete description of each command begins
on page 13. Note that the settings which are shown
with an asterisk (*) are the default values.
AT Command Summary
General Commands
Programmable Parameter Commands
<CR>
repeat the last command
PP xx OFF disable Prog Parameter xx
BRD hh
try Baud Rate Divisor hh
PP FF OFF all Prog Parameters disabled
BRT hh
set Baud Rate Timeout
PP xx ON
enable Prog Parameter xx
D
set all to Defaults
PP FF ON
all Prog Parameters enabled
E0, E1
Echo off, or on*
PP xx SV yy for PP xx, Set the Value to yy
FE
Forget Events
PPS
I
print the version ID
L0, L1
Linefeeds off, or on
LP
go to Low Power mode
M0, M1
Memory off, or on
RD
Read the stored Data
SD hh
Save Data byte hh
WS
Warm Start (quick software reset)
Z
reset all
@1
display the device description
@2
display the device identifier
print a PP Summary
Voltage Reading Commands
CV dddd
Calibrate the Voltage to dd.dd volts
CV 0000
restore CV value to factory setting
RV
Read the input Voltage
Other
IGN
read the IgnMon input level
@3 cccccccccccc store the @2 identifier
ELM327L DSA
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ELM327L
AT Command Summary (continued)
OBD Commands
J1850 Specific Commands (protocols 1 and 2)
AL
Allow Long (>7 byte) messages
IFR0, 1, 2
IFRs off, auto*, or on
AMC
display Activity Monitor Count
IFR H, S
IFR value from Header* or Source
AMT hh
set the Activity Mon Timeout to hh
AR
Automatically Receive
AT0, 1, 2
Adaptive Timing off, auto1*, auto2
BD
perform a Buffer Dump
BI
Bypass the Initialization sequence
DP
Describe the current Protocol
DPN
Describe the Protocol by Number
H0, H1
Headers off*, or on
MA
Monitor All
MR hh
Monitor for Receiver = hh
MT hh
Monitor for Transmitter = hh
NL
Normal Length messages*
PC
Protocol Close
R0, R1
Responses off, or on*
RA hh
set the Receive Address to hh
S0, S1
printing of Spaces off, or on*
SH xyz
Set Header to xyz
FI
perform a Fast Initiation
IB 10
set the ISO Baud rate to 10400*
IB 48
set the ISO Baud rate to 4800
IB 96
set the ISO Baud rate to 9600
IIA hh
set ISO (slow) Init Address to hh
KW
display the Key Words
KW0, KW1 Key Word checking off, or on*
SI
perform a Slow (5 baud) Initiation
SW hh
Set Wakeup interval to hh x 20 msec
SW 00
Stop sending Wakeup messages
WM [1 - 6 bytes] set the Wakeup Message
SH xxyyzz Set Header to xxyyzz
SH wwxxyyzz Set Header to wwxxyyzz
SP h
Set Protocol to h and save it
SP Ah
Set Protocol to Auto, h and save it
SP 00
Erase stored protocol
SR hh
Set the Receive address to hh
SS
use Standard Search order (J1978)
ST hh
Set Timeout to hh x 4 msec
TA hh
set Tester Address to hh
TP h
Try Protocol h
TP Ah
Try Protocol h with Auto search
ELM327L DSA
ISO Specific Commands (protocols 3 to 5)
CAN Specific Commands (protocols 6 to C)
CEA
turn off CAN Extended Addressing
CEA hh
use CAN Extended Address hh
CAF0, CAF1 Automatic Formatting off, or on*
CF hhh
set the ID Filter to hhh
CF hhhhhhhh set the ID Filter to hhhhhhhh
CFC0, CFC1 Flow Controls off, or on*
CM hhh
set the ID Mask to hhh
CM hhhhhhhh set the ID Mask to hhhhhhhh
CP hh
set CAN Priority to hh (29 bit)
CRA
reset the Receive Address filters
CRA hhh
set CAN Receive Address to hhh
CRA hhhhhhhh set the Rx Address to hhhhhhhh
CS
show the CAN Status counts
CSM0, CSM1 Silent Monitoring off, or on*
CTM1
set Timer Multiplier to 1*
CTM5
set Timer Multiplier to 5
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ELM327L
AT Command Summary (continued)
CAN Specific Commands (continued)
D0, D1
display of the DLC off*, or on
FC SM h
Flow Control, Set the Mode to h
FC SH hhh
FC, Set the Header to hhh
FC SH hhhhhhhh Set the Header to hhhhhhhh
FC SD [1 - 5 bytes] FC, Set Data to [...]
PB xx yy
Protocol B options and baud rate
RTR
send an RTR message
V0, V1
use of Variable DLC off*, or on
J1939 CAN Specific Commands (protocols A to C)
DM1
monitor for DM1 messages
JE
use J1939 Elm data format*
JHF0, JHF1 Header Formatting off, or on*
JS
use J1939 SAE data format
JTM1
set Timer Multiplier to 1*
JTM5
set Timer Multiplier to 5
MP hhhh
Monitor for PGN 0hhhh
MP hhhh n “
“ and get n messages
MP hhhhhh Monitor for PGN hhhhhh
MP hhhhhh n
“
“ and get n messages
AT Command Descriptions
The following describes each AT Command that the
current version of the ELM327 supports:
<CR>
[ repeat the last command ]
Sending a single carriage return character causes
the ELM327 to repeat the last command that it
performed. This is typically used when you wish to
obtain updates to a value at the fastest possible rate for example, you may send 01 0C to obtain the engine
rpm, then send only a carriage return character each
time you wish to receive an update.
AL
[ Allow Long messages ]
The standard OBDII protocols restrict the number
of data bytes in a message to seven, which the
ELM327 normally does as well (for both send and
receive). If AL is selected, the ELM327 will allow long
sends (eight data bytes) and long receives (unlimited
in number). The default is AL off (and NL selected).
AMC
[ display Activity Monitor Count ]
The Activity Monitor uses a counter to determine
just how active the ELM327's OBD inputs are. Every
time that activity is detected, this counter is reset,
while if there is no activity, the count goes up (every
0.655 seconds). This count then represents the time
since activity was last detected, and may be useful
when writing your own logic based on OBD activity.
The counter will not increment past FF (internal logic
stops it there), and stays at 00 while monitoring.
ELM327L DSA
AMT hh
[ set the Act Mon Timeout to hh ]
When the Activity Monitor Count (ie time) exceeds
a certain threshold, the ELM327 decides that there is
no OBD activity. It might then give an ACT ALERT
message or switch to Low Power operation, depending
on how the bits of PP 0F are set. The threshold setting
is determined by either PP 0F bit 4, or by the AT AMT
value, should you provide it. The actual time to alarm
will be (hh+1) x 0.65536 seconds. Note that a value of
00 is accepted for AMT, but is used to block all Activity
Monitor outputs.
AR
[ Automatically set the Receive address ]
Responses from the vehicle will be acknowledged
and displayed by the ELM327, if the internally stored
receive address matches the address that the
message is being sent to. With the auto receive mode
in effect, the value used for the receive address will be
chosen based on the current header bytes, and will
automatically be updated whenever the header bytes
are changed.
The value that is used for the receive address is
determined based on such things as the contents of
the first header byte, and whether the message uses
physical addressing, functional addressing, or if the
user has set a value with the SR or RA commands.
Auto Receive is turned on by default, and is not
used by the J1939 protocol.
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ELM327L
AT Command Descriptions (continued)
AT0, AT1 and AT2
[ Adaptive Timing control ]
When receiving responses from a vehicle, the
ELM327 has traditionally waited the time set by the
AT ST hh setting for a response. To ensure that the IC
would work with a wide variety of vehicles, the default
value was set to a conservative (slow) value. Although
it was adjustable, many people did not have the
equipment or experience to determine a better value.
The Adaptive Timing feature automatically sets the
timeout value for you, to a value that is based on the
actual response times that your vehicle is responding
in. As conditions such as bus loading, etc. change, the
algorithm learns from them, and makes appropriate
adjustments. Note that it always uses your AT ST hh
setting as the maximum setting, and will never choose
one which is longer.
There are three adaptive timing settings that are
available for use. By default, Adaptive Timing option 1
(AT1) is enabled, and is the recommended setting.
AT0 is used to disable Adaptive Timing (so the timeout
is always as set by AT ST), while AT2 is a more
aggressive version of AT1 (the effect is more
noticeable for very slow connections – you may not
see much difference with faster OBD systems). The
J1939 protocol does not support Adaptive Timing – it
uses fixed timeouts as set in the standard.
BD
[ perform an OBD Buffer Dump ]
All messages sent and received by the ELM327
are stored temporarily in a set of twelve memory
storage locations called the OBD Buffer. Occasionally,
it may be of use to view the contents of this buffer,
perhaps to see why an initiation failed, to see the
header bytes in the last message, or just to learn more
of the structure of OBD messages. You can ask at any
time for the contents of this buffer to be ‘dumped’
(ie printed) – when you do, the ELM327 sends a length
byte (representing the length of the message in the
buffer) followed by the contents of all twelve OBD
buffer locations. For example, here’s one ‘dump’:
>AT BD
05 C1 33 F1 3E 23 C4 00 00 10 F8 00 00
The 05 is the length byte - it tells us that only the
first 5 bytes (ie C1 33 F1 3E and 23) are valid. The
remaining bytes are likely left over from a previous
operation.
The length byte always represents the actual
number of bytes received, whether they fit into the
ELM327L DSA
OBD buffer or not. This may be useful when viewing
long data streams (with AT AL), as it represents the
actual number of bytes received, mod 256. Note that
only the first twelve bytes received are stored in the
buffer.
BI
[ Bypass the Initialization sequence ]
This command should be used with caution. It
allows an OBD protocol to be made active without
requiring any sort of initiation or handshaking to occur.
The initiation process is normally used to validate the
protocol, and without it, results may be difficult to
predict. It should not be used for routine OBD use, and
has only been provided to allow the construction of
ECU simulators and training demonstrators.
BRD hh
[ try Baud Rate Divisor hh ]
This command is used to change the RS232 baud
rate divisor to the hex value provided by hh, while
under computer control. It is not intended for casual
experimenting - if you wish to change the baud rate
from a terminal program, you should use PP 0C.
Since some interface circuits are not able to
operate at high data rates, the BRD command uses a
sequence of sends and receives to test the interface,
with any failure resulting in a fallback to the previous
baud rate. This allows several baud rates to be tested
and a reliable one chosen for the communications.
The entire process is described in detail in the ‘Using
Higher RS232 Baud Rates’ section, on pages 51 and
52.
If successful, the actual baud rate (in kbps) will be
4000 divided by the divisor (hh). The value 00 is not
accepted by the BRD command.
BRT hh
[ set Baud Rate Timeout to hh ]
This command allows the timeout used for the
Baud Rate handshake (ie. AT BRD) to be varied. The
time delay is given by hh x 5.0 msec, where hh is a
hexadecimal value. The default value for this setting is
0F, providing 75 msec. Note that a value of 00 does
not result in 0 msec - it provides the maximum time of
256 x 5.0 msec, or 1.28 seconds.
CAF0 and CAF1
[ CAN Auto Formatting off or on ]
These commands determine whether the ELM327
assists you with the formatting of the CAN data that is
sent and received. With CAN Automatic Formatting
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ELM327L
AT Command Descriptions (continued)
enabled (CAF1), the formatting (PCI) bytes will be
automatically generated for you when sending, and will
be removed when receiving. This means that you can
continue to issue OBD requests (01 00, etc.) as usual,
without regard to the extra bytes that CAN diagnostics
systems require. Also, with formatting on, any extra
(unused) data bytes that are received in the frame will
be removed, and any messages with invalid PCI bytes
will be ignored. (When monitoring, however, messages
with invalid PCI bytes are all shown, with a ‘<DATA
ERROR’ message beside them).
Multi-frame responses may be returned by the
vehicle with ISO 15765 and SAE J1939. To make
these more readable, the Auto Formatting mode will
extract the total data length and print it on one line,
then show each line of data with the segment number
followed by a colon (‘:’), and then the data bytes.
You may also see the characters 'FC:' on a line (if
you are experimenting). This identifies a Flow Control
message that has been sent as part of the multi-line
message signalling. Flow Control messages are
automatically generated by the ELM327 in response to
a ‘First Frame’ reply, as long as the CFC setting is on
(it does not matter if auto formatting is on or not).
Another type of message – the RTR (or ‘Remote
Transfer Request’) – will be automatically hidden for
you when in the CAF1 mode, since they contain no
data. When auto formatting is off (CAF0), you will see
the characters 'RTR' printed when a remote transfer
request frame has been received.
Turning the CAN Automatic Formatting off (CAF0),
will cause the ELM327 to print all of the data bytes as
received. No bytes will be hidden from you, and none
will be inserted for you. Similarly, when sending data
with formatting off, you must provide all of the required
data bytes exactly as they are to be sent – the
ELM327 will not add a PCI byte for you (but it will add
some trailing 'padding' bytes to ensure that the
required eight data bytes are sent). This allows the
ELM327 to be used with protocols that have special
formatting requirements.
Note that turning the display of headers on (with
AT H1) will override some of the CAF1 formatting of
the received data, so that the received bytes will
appear much like in the CAF0 mode (ie. as received).
It is only the printing of the received data that will be
affected when both CAF1 and H1 modes are enabled,
though; when sending data, the PCI byte will still be
created for you and padding bytes will still be added.
Auto Formatting on (CAF1) is the default setting.
ELM327L DSA
CEA
[ turn off the CAN Extended Address ]
The CEA command is used to turn off the special
features that are set with the CEA hh command.
CEA hh
[ set the CAN Extended Address to hh ]
Some (non-OBD) CAN protocols extend the
addressing fields by using the first of the eight data
bytes as a target (receiver) address. This command
allows the ELM327 to interact with those protocols.
Sending the CEA hh command causes the
ELM327 to insert the hh value as the first data byte of
all CAN messages that you send. It also adds one
more filtering step to received messages, only passing
ones that have the Tester Address in the first byte
position (in addition to requiring that ID bits match the
patterns set by AT CF and CM, or CRA). The AT CEA
hh command can be sent at any time, and changes
are effective immediately, allowing for changes of the
address ‘on-the-fly’. There is a more lengthy
discussion of extended addressing in the ‘Using CAN
Extended Addresses’ section on page 62.
The CEA mode of operation is off by default, and
once on, can be turned off at any time by sending AT
CEA, with no address. Note that the CEA setting has
no effect when J1939 formatting is on.
CF hhh
[ set the CAN ID Filter to hhh ]
The CAN Filter works in conjunction with the CAN
Mask to determine what information is to be accepted
by the receiver. As each message is received, the
incoming CAN ID bits are compared to the CAN Filter
bits (when the mask bit is a ‘1’). If all of the relevant
bits match, the message will be accepted, and
processed by the ELM327, otherwise it will be
discarded. This three nibble version of the CAN Filter
command makes it a little easier to set filters with 11
bit ID CAN systems. Only the rightmost 11 bits of the
provided nibbles are used, and the most significant bit
is ignored. The data is actually stored as four bytes
internally however, with this command adding leading
zeros for the other bytes. See the CM command(s) for
more details.
CF hh hh hh hh [ set the CAN ID Filter to hhhhhhhh ]
This command allows all four bytes (actually 29
bits) of the CAN Filter to be set at once. The 3 most
significant bits will always be ignored, and may be
given any value. This command may be used to enter
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ELM327L
AT Command Descriptions (continued)
11 bit ID filters as well, since they are stored in the
same locations internally (entering AT CF 00 00 0h hh
is exactly the same as entering the shorter AT CF hhh
command).
CFC0 and CFC1
[ CAN Flow Control off or on ]
The ISO 15765-4 CAN protocol expects a ‘Flow
Control’ message to always be sent in response to a
‘First Frame’ message, and the ELM327 automatically
sends these without any intervention by the user. If
experimenting with a non-OBD system, it may be
desirable to turn this automatic response off, and the
AT CFC0 command has been provided for that
purpose.
As of firmware version 2.0, these commands also
enable or disable the sending of J1939 TP.CM_CTS
messages in response to TP.CM_RTS requests.
During monitoring (AT MA, MR, or MT), there are
never any Flow Controls sent no matter what the CFC
option is set to. The default setting is CFC1 - Flow
Controls on.
CM hhh
[ set the CAN ID Mask to hhh ]
There can be a great many messages being
transmitted in a CAN system at any one time. In order
to limit what the ELM327 views, there needs to be a
system of filtering out the relevant ones from all the
others. This is accomplished by the filter, which works
in conjunction with the mask. A mask is a group of bits
that show the ELM327 which bits in the filter are
relevant, and which ones can be ignored. A ‘must
match’ condition is signalled by setting a mask bit to
'1', while a 'don't care' is signalled by setting a bit to '0'.
This three digit variation of the CM command is used
to provide mask values for 11 bit ID systems (the most
significant bit is always ignored).
Note that a common storage location is used
internally for the 29 bit and 11 bit masks, so an 11 bit
mask could conceivably be assigned with the next
command (CM hh hh hh hh), should you wish to do the
extra typing. The values are right justified, so you
would need to provide five leading zeros followed by
the three mask bytes.
for the length. Note that the three most significant bits
that you provide in the first digit will be ignored.
CP hh
[ set CAN Priority bits to hh ]
This command is used to assign the five most
significant bits of the 29 bit CAN ID that is used for
sending messages (the other 24 bits are set with the
AT SH command). Many systems use these bits to
assign a priority value to messages, and to determine
the protocol. Any bits provided in excess of the five
required are ignored, and not stored by the ELM327 (it
only uses the five least significant bits of this byte).
The default value for these priority bits is hex 18,
which can be restored at any time with the AT D
command.
CRA
[ reset the CAN Rx Addr ]
The AT CRA command is used to restore the CAN
receive filters to their default values. Note that it does
not have any arguments (ie no data).
CRA hhh
[ set the CAN Rx Addr to hhh ]
Setting the CAN masks and filters can be difficult
at times, so if you only want to receive information
from one address (ie. one CAN ID), then this
command may be very welcome. For example, if you
only want to see information from 7E8, simply send AT
CRA 7E8, and the ELM327 will make the necessary
adjustments to both the mask and the filter for you.
If you wish to allow the reception of a range of
values, you can use the letter X to signify a ‘don’t care’
condition. That is, AT CRA 7EX would allow all IDs
that start with 7E to pass (7E0, 7E1, etc.). For a more
specific range of IDs, you may need to assign a mask
and filter.
To reverse the changes made by the CRA
command, simply send AT CRA or AT AR.
CRA hhhhhhhh [set the CAN Rx Addr to hhhhhhhh]
This command is identical to the previous one,
except that it is used with 29 bit CAN IDs. Sending
either AT CRA or AT AR will also reverse any changes
made by this command.
CM hh hh hh hh [ set the CAN ID Mask to hhhhhhhh ]
This command is used to assign mask values for
29 bit ID systems. See the discussion under the
CM hhh command as it is essentially identical, except
ELM327L DSA
CS
[ show the CAN Status counts ]
The CAN protocol requires that statistics be kept
regarding the number of transmit and receive errors
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ELM327L
AT Command Descriptions (continued)
detected. If there should be a significant number of
errors (due to a hardware or software problem), the
device will go off-line in order to not affect other data
on the bus. The AT CS command lets you see both
the transmitter (Tx) and the receiver (Rx) error counts,
in hexadecimal. If the transmitter should be off (count
>FF), you will see ‘OFF’ rather than a specific count.
CSM0 and CSM1
[ CAN Silent Monitoring off or on ]
The ELM327 was designed to be completely silent
while monitoring a CAN bus. Because of this, it is able
to report exactly what it sees, without colouring the
information in any way. Occasionally (when bench
testing, or when connecting to a dedicated CAN port),
it may be preferred that the ELM327 does not operate
silently (ie generates ACK bits, etc.), and this is what
the CSM command is for. CSM1 turns it on, CSM0
turns it off, and the default value is determined by
PP 21. Be careful when experimenting with this. If you
should choose the wrong baud rate then monitor the
CAN bus with the silent monitoring off, you will disturb
the flow of data. Always keep the silent monitoring on
until you are certain that you have chosen the correct
baud rate.
CTM1
[ set the Timer Multiplier to 1 ]
This command causes all timeouts set by AT ST
to be multiplied by a factor of 1. Note that this currently
only affects the CAN protocols (6 to C). CTM1 is the
default setting.
CTM5
[ set the Timer Multiplier to 5 ]
This command causes all timeouts set by AT ST
to be multiplied by a factor of 5. Note that this currently
only affects the CAN protocols (6 to C).
This command was originally added (as JTM5) to
assist with the retrieving of some J1939 messages.
We have since had several requests to allow it to
affect all CAN modes, and so have modified the JTM5
code and added the new CTM1/CTM5 commands. If
using CTM5, we caution that the Adaptive Timing code
does not monitor changes in the setting, so we advise
turning it off (with AT AT0).
By default, this multiplier is off.
CV dddd
[ Calibrate the Voltage to dd.dd volts ]
The voltage reading that the ELM327 shows for an
AT RV request can be calibrated with this command.
ELM327L DSA
The argument (‘dddd’) must always be provided as 4
digits, with no decimal point (it assumes that the
decimal place is between the second and the third
digits).
To use this feature, simply use an accurate meter
to read the actual input voltage, then use the CV
command to change the internal calibration (scaling)
factor. For example, if the ELM327 shows the voltage
as 12.2V while you measure 11.99 volts, then send
AT CV 1199 and the ELM327 will recalibrate itself for
that voltage (it will actually read 12.0V due to digit
roundoff). See page 30 for some more information on
how to read voltages and perform the calibration.
CV 0000
[ restore the factory Calibration Value ]
If you are experimenting with the CV dddd
command but do not have an accurate voltmeter as a
reference, you may soon get into trouble. If this
happens, you can always send AT CV 0000 to restore
the ELM327 to the original calibration value.
The ELM327’s factory calibration setting assumes
that a 1:5.7 ratio resistor divider is being used (this is
what the 47K and 10K resistors provide in Figure 9).
The ELM327L also assumes this same ratio, if VDD is
above 4V. If V DD is 4V or less, the ELM327L assumes
a 1:11 ratio as would be acheived with a 47K and 4.7K
resistor divider, as we show in Figure 15.
D
[ set all to Defaults ]
This command is used to set the options to their
default (or factory) settings, as when power is first
applied. The last stored protocol will be retrieved from
memory, and will become the current setting (possibly
closing other protocols that are active). Any settings
that the user had made for custom headers, filters, or
masks will be restored to their default values, and all
timer settings will also be restored to their defaults.
D0 and D1
[ display of DLC off or on ]
The ISO 15765-4 CAN protocol requires all
messages to have 8 data bytes, so displaying the
number of data bytes (the DLC) is not very useful.
When experimenting with other CAN protocols,
however, it may be useful to be able to see what the
data lengths are. The D0 and D1 commands control
the display of the DLC digit (the headers must also be
on in order to see it). When displayed, the single DLC
digit will appear between the ID (header) bytes and the
data bytes. The default setting is determined by PP 29.
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ELM327L
AT Command Descriptions (continued)
DM1
[ monitor for DM1s ]
The SAE J1939 Protocol broadcasts trouble codes
periodically, by way of Diagnostic Mode 1 (DM1)
messages. This command sets the ELM327 to
continually monitor for this type of message for you,
following multi-segment transport protocols as
required. Note that a combination of masks and filters
could be set to provide a similar output, but they would
not allow multiline messages to be detected. The DM1
command adds the extra logic that is needed for
multiline messages.
This command is only available when a CAN
Protocol (A, B, or C) has been selected for J1939
formatting. It returns an error if attempted under any
other conditions.
DP
[ Describe the current Protocol ]
The ELM327 automatically detects a vehicle’s
OBD protocol, but does not normally report what it is.
The DP command is a convenient means of asking
what protocol the IC is currently set to (even if it has
not yet ‘connected’ to the vehicle).
If a protocol is chosen and the automatic option is
also selected, AT DP will show the word 'AUTO' before
the protocol description. Note that the description
shows the actual protocol names, not the numbers
used by the protocol setting commands.
DPN
[ Describe the Protocol by Number ]
This command is similar to the DP command, but
it returns a number which represents the current
protocol. If the automatic search function is also
enabled, the number will be preceded with the letter
‘A’. The number is the same one that is used with the
set protocol and test protocol commands.
E0 and E1
[ Echo off or on ]
These commands control whether or not the
characters received on the RS232 port are echoed
(retransmitted) back to the host computer. Character
echo can be used to confirm that the characters sent
to the ELM327 were received correctly. The default is
E1 (or echo on).
FC SD [1-5 bytes]
[ Flow Control Set Data to… ]
The data bytes that are sent in a CAN Flow
ELM327L DSA
Control message may be defined with this command.
One to five data bytes may be specified, with the
remainder of the data bytes in the message being
automatically set to the default CAN filler byte, if
required by the protocol. Data provided with this
command is only used when Flow Control modes 1 or
2 have been enabled.
FC SH hhh
[ Flow Control Set Header to… ]
The header (or more properly ‘CAN ID’) bytes
used for CAN Flow Control messages can be set using
this command. Only the right-most 11 bits of those
provided will be used - the most significant bit is
always removed. This command only affects Flow
Control mode 1.
FC SH hhhhhhhh
[ Flow Control Set Header to… ]
This command is used to set the header (or ‘CAN
ID’) bits for Flow Control responses with 29 bit CAN ID
systems. Since the 8 nibbles define 32 bits, only the
right-most 29 bits of those provided will be used - the
most significant three bits are always removed. This
command only affects Flow Control mode 1.
FC SM h
[ Flow Control Set Mode to h ]
This command sets how the ELM327 responds to
First Frame messages when automatic Flow Control
responses are enabled. The single digit provided can
either be ‘0’ (the default) for fully automatic responses,
‘1’ for completely user defined responses, or ‘2’ for
user defined data bytes in the response. Note that FC
modes 1 and 2 can only be enabled if you have
defined the needed data and possibly ID bytes. If you
have not, you will get an error. More complete details
and examples can be found in the Altering Flow
Control Messages section (page 61).
FE
[ Forget Events ]
There are certain events which may change how
the ELM327 responds from that time onwards. One of
these is the occurrence of a fatal CAN error (ERR94),
which blocks subsequent searching through CAN
protocols if PP 2A bit 5 is ‘1’. Normally, an event such
as this will affect all searches until the next power off
and on, but it can be ‘forgotten’ using software, with
the AT FE command.
Another example is an ‘LV RESET’ event which
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ELM327L
AT Command Descriptions (continued)
will prevent searches through CAN protocols if PP 2A
bit 4 is ‘1’. It may also be forgotten with the AT FE
command.
FI
[ perform a Fast Initiation ]
One version of the Keyword protocol uses what is
known as a 'fast initiation' sequence to begin
communications. Usually, this sequence is performed
when the first message needs to be sent, and then the
message is sent immediately after. Some ECUs may
need more time between the two however, and having
a separate initiation command allows you to control
this time. Simply send AT FI, wait a little, then send the
message. You may need to experiment to get the right
amount of delay.
Another use for this command might be if you
would like to perform a fast initiation with an ISO 9141
type protocol (ie 3 - CARB format). Simply follow these
steps to do that:
AT
AT
AT
AT
SP 5
FI
SP 3
BI
I
[ Identify yourself ]
Issuing this command causes the chip to identify
itself, by printing the startup product ID string (currently
‘ELM327 v2.1’). Software can use this to determine
exactly which integrated circuit it is talking to, without
having to reset the IC.
IB 10
[ set the ISO Baud rate to 10400 ]
This command restores the ISO 9141-2 and
ISO 14230-4 baud rates to the default value of 10400.
IB 48
This command is used to change the baud rate
used for the ISO 9141-2 and ISO 14230-4 protocols
(numbers 3, 4, and 5) to 4800 baud, while relaxing
some of the requirements for the initiation byte
transfers. It may be useful for experimenting with some
vehicles. Normal (10,400 baud) operation may be
restored at any time with the IB 10 command.
IB 96
and you should be able to then communicate with the
ECU. Note that a protocol close (ie AT PC) is not
required in the above code, as the ELM327
automatically performs one when you switch protocols.
Protocol 5 must be selected to use the AT FI
command, or an error will result.
[ Headers off or on ]
These commands control whether or not the
additional (header) bytes of information are shown in
the responses from the vehicle. These are not
normally shown by the ELM327, but may be of interest
(especially if you receive multiple responses and wish
to determine what modules they were from).
Turning the headers on (with AT H1) actually
shows more than just the header bytes – you will see
the complete message as transmitted, including the
check-digits and PCI bytes, and possibly the CAN data
length code (DLC) if it has been enabled with PP 29 or
AT D1. The current version of this IC does not display
the CAN CRC code, nor the special J1850 IFR bytes
(which some protocols use to acknowledge receipt of a
message).
ELM327L DSA
[ set the ISO Baud rate to 9600 ]
This command is used to change the baud rate
used for the ISO 9141-2 and ISO 14230-4 protocols
(numbers 3, 4, and 5) to 9600 baud, while relaxing
some of the requirements for the initiation byte
transfers. It may be useful for experimenting with some
vehicles. Normal (10,400 baud) operation may be
restored at any time with the IB 10 command.
IFR0, IFR1, and IFR2
H0 and H1
[ set the ISO Baud rate to 4800 ]
[ IFR control ]
The SAE J1850 protocol allows for an In-Frame
Response (IFR) byte to be sent after each message,
usually to acknowledge the correct receipt of that
message. The ELM327 automatically generates and
sends this byte for you by default, but you can override
this behaviour with this command.
The AT IFR0 command will disable the sending of
all IFRs, no matter what the header bytes require.
AT IFR2 is the opposite - it will cause an IFR byte to
always be sent, no matter what the header bytes say.
The AT IFR1 command is the default mode, with the
sending of IFRs determined by the ‘K’ bit of the first
header byte (for both PWM and VPW).
IFR H and IFR S
[ IFR from Header or Source ]
The value sent in the J1850 In-Frame Response
(IFR) byte is normally the same as the value sent as
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ELM327L
AT Command Descriptions (continued)
the Source (or Tester) Address byte that was in the
header of the request. There may be occasions when
it is desirable to use some other value, however, and
this set of commands allows for this.
If you send AT IFR S, the ELM327 will use the
value currently defined as the Source (or Tester)
Address, even if another value was sent in the Header
bytes. This is not what is normally required, and
caution should be used when using AT IFR S.
AT IFR H restores the sending of the IFR bytes to
those provided in the Header, and is the default
setting.
IGN
[ read the IgnMon input level ]
This command reads the signal level at pin 15. It
assumes that the logic level is related to the ignition
voltage, so if the input is at a high level, the response
will be ‘ON’, and a low level will report ‘OFF’.
This feature is most useful if you wish to perform
the power control functions using your own software. If
you disable the Low Power automatic response to a
low input on this pin (by setting bit 2 of PP 0E to 0),
then pin 15 will function as the RTS input. A low level
on the input will not turn the power off, but it will
interrupt any OBD activity that is in progress. All you
need to do is detect the ‘STOPPED’ message that is
sent when the ELM327 is interrupted, and then check
the level at pin 15 using AT IGN. If it is found to be
OFF, you can perform an orderly shutdown yourself.
IIA hh
[ set the ISO Init Address to hh ]
The ISO 9141-2 and ISO 14230-4 standards state
that when beginning a session with an ECU, the
initiation sequence is to be directed to a specific
address ($33). If you wish to experiment by directing
the slow five baud sequence to another address, it is
done with this command. For example, if you prefer
that the initiation be performed with the ECU at
address $7A, then simply send:
>AT IIA 7A
and the ELM327 will use that address when called to
do so (protocols 3 or 4). The full eight bit value is used
exactly as provided – no changes are made to it (ie no
adding of parity bits, etc.)
Note that setting this value does not affect any
address values used in the header bytes. The ISO init
address is restored to $33 whenever the defaults, or
ELM327L DSA
the ELM327, are reset.
JE
[ enables the J1939 ELM data format ]
The J1939 standard requires that PGN requests
be sent with the byte order reversed from the standard
‘left-to-right’ order, which many of us would expect. For
example, to send a request for the engine temperature
(PGN 00FEEE), the data bytes are actually sent in the
reverse order (ie EE FE 00), and the ELM327 would
normally expect you to provide the data in that order
for passing on to the vehicle.
When experimenting, this constant need for byte
reversals can be quite confusing, so we have defined
an ELM format that reverses the bytes for you. When
the J1939 ELM (JE) format is enabled, and you have a
J1939 protocol selected, and you provide three data
bytes to the ELM327, it will reverse the order for you
before sending them to the ECU. To request the
engine temperature PGN, you would send 00 FE EE
(and not EE FE 00). The ‘JE’ type of automatic
formatting is enabled by default.
JHF0 and JHF1 [ J1939 Header Formatting off or on ]
When printing responses, the ELM327 normally
formats the J1939 ID (ie Header) bits in such a way as
to isolate the priority bits and group all the PGN
information, while keeping the source address byte
separate. If you prefer to see the ID information as four
separate bytes (which a lot of the J1939 software
seems to do), then simply turn off the formatting with
JHF0. The CAF0 command has the same effect (and
overrides the JHF setting), but also affects other
formatting. The default setting is JHF1.
JS
[ enables the J1939 SAE data format ]
The AT JS command disables the automatic byte
reordering that the JE command performs for you. If
you wish to send data bytes to the J1939 vehicle
without any manipulation of the byte order (ie in the
order specified by the SAE documents), then select JS
formatting.
As an example, when sending a request for
engine temperature (PGN 00FEEE) with the data
format set to JS, you must present the bytes to the
ELM327 as EE FE 00 (this is also known as littleendian byte ordering).
The JS type of data formatting is off by default.
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ELM327L
AT Command Descriptions (continued)
JTM1
[ set the J1939 Timer Multiplier to 1 ]
This used to set the AT ST time multiplier to 1, for
the SAE J1939 protocol. As of firmware v2.1, this
command now simply calls the CTM1 command.
JTM5
[ set the J1939 Timer Multiplier to 5 ]
This used to set the AT ST time multiplier to 5, for
the SAE J1939 protocol. As of firmware v2.1, this
command now simply calls the CTM5 command.
AT L1, linefeeds will be generated after every carriage
return character, and for AT L0, they will be off. Users
will generally wish to have this option on if using a
terminal program, but off if using a custom computer
interface (as the extra characters transmitted will only
serve to slow the communications down). The default
setting is determined by the voltage at pin 7 during
power on (or reset). If the level is high, then linefeeds
are on by default; otherwise they will be off.
LP
KW
[ display the Key Words ]
When the ISO 9141-2 and ISO 14230-4 protocols
are initialized, two special bytes (key words) are
passed to the ELM327 (the values are used internally
to determine whether a particular protocol variation
can be supported by the ELM327). If you wish to see
what the value of these bytes were, simply send the
AT KW command.
KW0 and KW1
[ Key Word checks off or on ]
The ELM327 looks for specific bytes (called key
words) to be sent to it during the ISO 9141-2 and
ISO14230-4 initiation sequences. If the bytes are not
found, the initiation is said to have failed (you might
see ‘UNABLE TO CONNECT’ or perhaps ‘BUS INIT:
...ERROR’). This might occur if you are trying to
connect to a non-OBD compliant ECU, or perhaps to
an older one.
If you wish to experiment with non-standard
systems, you may have to tell the ELM327 to perform
the initiation sequence, but ignore the contents of the
bytes that are sent and received. To do this, send:
>AT KW0
After turning keyword checking off, the ELM327
will still require the two key word bytes in the
response, but will not look at the actual values of the
bytes. It will also send an acknowledgement to the
ECU, and will wait for the final response from it (but
will not stop and report an error if none is received).
This may allow you to make a connection in an
otherwise ‘impossible’ situation. Normal behaviour can
be returned with AT KW1, which is the default setting.
L0 and L1
[ Linefeeds off or on ]
This option controls the sending of linefeed
characters after each carriage return character. For
ELM327L DSA
[ go to the Low Power mode ]
This command causes the ELM327 to shut off all
but ‘essential services’ in order to reduce the power
consumption to a minimum. The ELM327 will respond
with an ‘OK’ (but no carriage return) and then, one
second later, will change the state of the PwrCtrl
output (pin 16) and will enter the low power (standby)
mode. The IC can be brought back to normal operation
through a character received at the RS232 input or a
rising edge at the IgnMon (pin 15) input, in addition to
the usual methods of resetting the IC (power off then
on, a low on pin 1, or a brownout). See the Power
Control section (page 65) for more information.
M0 and M1
[ Memory off or on ]
The ELM327 has internal ‘non-volatile’ memory
that is capable of remembering the last protocol used,
even after the power is turned off. This can be
convenient if the IC is often used for one particular
protocol, as that will be the first one attempted when
next powered on. To enable this memory function, it is
necessary to either use an AT command to select the
M1 option, or to have chosen ‘memory on’ as the
default power on mode (by connecting pin 5 of the
ELM327 to a high logic level).
When the memory function is enabled, each time
that the ELM327 finds a valid OBD protocol, that
protocol will be memorized (stored) and will become
the new default. If the memory function is not enabled,
protocols found during a session will not be
memorized, and the ELM327 will always start at power
up using the same (last saved) protocol.
If the ELM327 is to be used in an environment
where the protocol is constantly changing, it would
likely be best to turn the memory function off, and
issue an AT SP 0 command once. The SP 0 command
tells the ELM327 to start in an 'Automatic' protocol
search mode, which is the most useful for an unknown
environment. ICs come from the factory set to this
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ELM327L
AT Command Descriptions (continued)
mode. If, however, you have only one vehicle that you
regularly connect to, storing that vehicle’s protocol as
the default would make the most sense.
The default setting for the memory function is
determined by the voltage level at pin 5 during power
up (or system reset). If it is connected to a high level
(VDD), then the memory function will be on by default.
If pin 5 is connected to a low level, the memory saving
will be off by default.
MA
[ Monitor All messages ]
This command places the ELM327 into a bus
monitoring mode, in which it continually monitors for
(and displays) all messages that it sees on the OBD
bus. It is a quiet monitor, not sending In Frame
Responses for J1850 systems, Acknowledges for CAN
systems (unless you turn silent mode off with CSM0),
or Wakeup (‘keep-alive’) messages for the ISO 9141
and ISO 14230 protocols. Monitoring will continue until
you stop it with activity on the RS232 input, or the RTS
pin.
To stop monitoring, simply send any single
character to the ELM327, then wait for it to respond
with a prompt character (‘>’), or a low level output on
the Busy pin. (Setting the RTS input to a low level will
interrupt the device as well.) Waiting for the prompt is
necessary as the response time varies depending on
what the IC was doing when it was interrupted. If for
instance it was in the middle of printing a line, it will
first complete that line and then print ‘STOPPED’,
before returning to the command state and sending a
prompt character. If it were simply waiting for input, it
would return much quicker. Note that the character
which stops the monitoring will always be discarded,
and will not affect subsequent commands.
If this command is used with CAN protocols, and if
the CAN filter and/or mask were previously set (with
CF, CM or CRA), then the MA command will be
affected by those settings. For example, if the receive
address had been set previously with CRA 4B0, then
the AT MA command would only be able to ‘see’
messages with an ID of 4B0. This may not be what is
desired - you may want to reset the masks and filters
(with AT AR or AT CRA) first.
All of the monitoring commands (MA, MR and MT)
operate by closing the current protocol (an AT PC is
executed internally), before configuring the IC for
monitoring the data. When the next OBD command is
to be transmitted, the protocol will again be initialized,
ELM327L DSA
and you may see messages stating this.
‘SEARCHING...’ may also be seen, depending on
what changes were made while monitoring.
MP hhhh
[ Monitor for PGN hhhh ]
The AT MA, MR and MT commands are quite
useful for when you wish to monitor for a specific byte
in the header of a typical OBD message. For the SAE
J1939 Protocol, however, it is often desirable to
monitor for the multi-byte Parameter Group Numbers
(or PGNs), which can appear in either the header, or
the data bytes. The MP command is a special J1939
only command that is used to look for responses to a
particular PGN request.
Note that this MP command provides no means to
set the first two digits of the requested PGN, and they
are always assumed to be 00. For example, the DM2
PGN has an assigned value of 00FECB (see SAE
J1939-73). To monitor for DM2 messages, you would
issue AT MP FECB, eliminating the 00, since the
MP hhhh command always assumes that the PGN is
preceded by two zeros.
This command is only available when a CAN
Protocol (A, B, or C) has been selected for SAE J1939
formatting. It returns an error if attempted under any
other conditions. Note also that this version of the
ELM327 only displays responses that match the
criteria, not the requests that are asking for the PGN
information.
MP hhhh n
[ Monitor for PGN, get n messages ]
This is very similar to the above command, but
adds the ability to set the number of messages that
should be fetched before the ELM327 automatically
stops monitoring and prints a prompt character. The
value ‘n’ may be any single hex digit.
MP hhhhhh
[ Monitor for PGN hhhhhh ]
This command is very similar to the MP hhhh
command, but it extends the number of bytes provided
by one, so that there is complete control over the PGN
definition (it does not make the assumption that the
Data Page bit is 0, as the MP hhhh command does).
This allows for future expansion, should additional
PGNs be defined with the Data Page bit set. Note that
only the Data Page bit is relevant in the extra byte the other bits are ignored.
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ELM327L
AT Command Descriptions (continued)
MP hhhhhh n
[ Monitor for PGN, get n messages ]
This is very similar to the previous command, but it
adds the ability to set the number of messages that
should be fetched before the ELM327 automatically
stops monitoring and prints a prompt character. The
value ‘n’ may be any single hex digit.
MR hh
[ Monitor for Receiver hh ]
This command is very similar to the AT MA
command except that it will only display messages that
were sent to the hex address given by hh. These are
messages which are found to have the value hh in the
second byte of a traditional three byte OBD header, in
bits 8 to 15 of a 29 bit CAN ID, or in bits 8 to 10 of an
11 bit CAN ID. Any single RS232 character aborts the
monitoring, as with the MA command.
Note that if this command is used with CAN
protocols, and if the CAN filter and/or mask were
previously set (with CF, CM or CRA), then the MR
command will over-write the previous values for these
bits only - the others will remain unchanged. As an
example, if the receive address has been set with
CRA 4B0, and then you send MR 02, the 02 will
replace the 4, and the CAN masks/filters will only allow
IDs that are equal to 2B0. This is often not what is
desired - you may want to reset the masks and filters
(with AT AR) first.
As with the AT MA command, this command
begins by performing an internal Protocol Close.
Subsequent OBD requests may show ‘SEARCHING’
or ‘BUS INIT’, etc. messages when the protocol is
reactivated.
MT hh
[ Monitor for Transmitter hh ]
This command is also very similar to the AT MA
command, except that it will only display messages
that were sent by the transmitter with the hex address
given by hh. These are messages which are found to
have that value in the third byte of a traditional three
byte OBD header, or in bits 0 to 7 for CAN IDs. As with
the MA and MR monitoring modes, any RS232 activity
(single character) aborts the monitoring.
Note that if this command is used with CAN
protocols, and if the CAN filter and/or mask were
previously set (with CF, CM or CRA), then the MT
command will over-write the previous values for these
bits only - the others will remain unchanged. As an
example, if the receive address has been set with
ELM327L DSA
CRA 4B0, and then you send MT 20, the 20 will
replace the B0, and the CAN masks/filters will only
allow IDs that are equal to 420. This is often not what
is desired - you may want to reset the masks and
filters (with AT AR) first.
As with the AT MA command, this command
begins by performing an internal Protocol Close.
Subsequent OBD requests may show ‘SEARCHING’
or ‘BUS INIT’, etc. messages when the protocol is
reactivated.
NL
[ Normal Length messages ]
Setting the NL mode on forces all sends and
receives to be limited to the standard seven data bytes
in length, similar to the other ELM32x OBD ICs. To
allow longer messages, use the AL command.
The ELM327 does not require a change to AL to
allow longer message lengths for the KWP protocols to
be received (as determined by the header length
values). You can simply leave the IC set to the default
setting of NL, and all of the received bytes will be
shown.
PB xx yy
[ set Protocol B parameters ]
This command allows you to change the protocol
B (USER1) options and baud rate without having to
change the associated Programmable Parameters.
This allows for easier testing, and program control.
To use this feature, simply set xx to the value for
PP 2C, and yy to the value for PP 2D, and issue the
command. The next time that the protocol is initialized
it will use these values. For example, assume that you
wish to try monitoring a system that uses 11 bit CAN at
33.3 kbps. If you do not want any special formatting,
this means a value of 11000000 or C0 hex for PP 2C,
and 15 decimal or 0F hex for PP 2D. Send both of
these values to the ELM327 in one command:
>AT PB C0 0F
then monitor:
>AT MA
if you see CAN ERRORs, and realize that you wanted
an 83.3 kbps baud rate, simply close the protocol, and
then send the new values:
>AT PC
OK
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ELM327L
AT Command Descriptions (continued)
PP hh ON
>AT PB C0 06
OK
>AT MA
Values passed in this way do not affect those that
are stored in the 2C and 2D Programmable
Parameters, and are lost if the ELM327 is reset. If you
want to make your settings persist over power cycles,
then you may wish to store them in the Programmable
Parameter for CAN protocols USER1 or USER2.
PC
[ Protocol Close ]
There may be occasions where it is desirable to
stop (deactivate) a protocol. Perhaps you are not using
the automatic protocol finding, and wish to manually
activate and deactivate protocols. Perhaps you wish to
stop the sending of idle (wakeup) messages, or have
another reason. The PC command is used in these
cases to force a protocol to close.
PP hh OFF
[ turn Prog. Parameter hh OFF ]
This command disables Programmable Parameter
number hh. Any value assigned using the PP hh SV
command will no longer be used, and the factory
default setting will once again be in effect. The actual
time when the new value for this parameter becomes
effective is determined by its type. Refer to the
Programmable Parameters section (page 70) for more
information on the types.
Note that ‘PP FF OFF’ is a special command that
disables all of the Programmable Parameters, as if you
had entered PP OFF for every possible one.
It is possible to alter some of the Programmable
Parameters so that it may be difficult, or even
impossible, to communicate with the ELM327. If this
occurs, there is a hardware means of resetting all of
the Programmable Parameters at once. Connect a
jumper from circuit common to pin 28, holding it there
while powering up the ELM327 circuit. Hold it in
position until you see the RS232 Receive LED begin to
flash (which indicates that all of the PPs have been
turned off). At this point, remove the jumper to allow
the IC to perform a normal startup. Note that a reset of
the PPs occurs quite quickly – if you are holding the
jumper on for more than a few seconds and do not see
the RS232 receive light flashing, remove the jumper
and try again, as there may be a problem with your
connection.
ELM327L DSA
[ turn Programmable Parameter hh ON ]
This command enables Programmable Parameter
number hh. Once enabled, any value assigned using
the PP hh SV command will be used where the factory
default value was before. (All of the programmable
parameter values are set to their default values at the
factory, so enabling a programmable parameter before
assigning a value to it will not cause problems.) The
actual time when the value for this parameter becomes
effective is determined by its type. Refer to the
Programmable Parameters section (page 70) for more
information on the types.
Note that ‘PP FF ON’ is a special command that
enables all of the Programmable Parameters at the
same time.
PP hh SV yy [ Prog. Param. hh: Set the Value to yy ]
A value is assigned to a Programmable Parameter
using this command. The system will not be able to
use this new value until the Programmable Parameter
has been enabled, with PP hh ON.
PPS
[ Programmable Parameter Summary ]
The complete range of current Programmable
Parameters are displayed with this command (even
those not yet implemented). Each is shown as a PP
number followed by a colon and the value that is
assigned to it. This is followed by a single digit – either
‘N’ or ‘F’ to show that it is ON (enabled), or OFF
(disabled), respectively. See the Programmable
Parameters section for a more complete discussion.
R0 and R1
[ Responses off or on ]
These commands control the ELM327’s automatic
receive (and display) of the messages returned by the
vehicle. If responses have been turned off, the IC will
not wait for a reply from the vehicle after sending a
request, and will return immediately to wait for the next
RS232 command (the ELM327 does not print anything
to say that the send was successful, but you will see a
message if it was not).
R0 may be useful to send commands blindly when
using the IC for a non-OBD network application, or
when simulating an ECU in a learning environment. It
is not recommended that this option used for normal
OBD communications, however, as the vehicle may
have difficulty if it is expecting an acknowledgement
and never receives one.
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ELM327L
AT Command Descriptions (continued)
An R0 setting will always override any ‘number of
responses digit’ that is provided with an OBD request.
The default setting is R1, or responses on.
The ELM327 treats an RTR just like any other
message sent, and will wait for a response from the
vehicle (unless AT R0 has been chosen).
RA hh
RV
[ set the Receive Address to hh ]
Depending on the application, users may wish to
manually set the address to which the ELM327 will
respond. Issuing this command will turn off the AR
mode, and force the IC to only accept responses
addressed to hh. Use caution with this setting, as
depending on what you set it to, you may end up
accepting (ie. acknowledging with an IFR) a message
that was actually meant for another module. To turn off
the RA filtering, simply send AT AR.
This command is not very effective for use with the
CAN protocols, as it only monitors for one portion of
the ID bits, and that is not likely enough for most CAN
applications - the CRA command may be a better
choice. Also, this command has no effect on the
addresses used by the J1939 protocols, as the J1939
routines derive them from the header values, as
required by the SAE standard.
The RA command is exactly the same as the SR
command, and can be used interchangeably. Note that
CAN Extended Addressing does not use this value - it
uses the one set by the AT TA command.
RD
[ Read the Data in the user memory ]
The byte value stored with the SD command is
retrieved with this command. There is only one
memory location, so no address is required.
RTR
[ send an RTR message ]
This command causes a special ‘Remote Frame’
CAN message to be sent. This type of message has
no data bytes, and has its Remote Transmission
Request (RTR) bit set. The headers and filters will
remain as previously set (ie the ELM327 does not
make any assumptions as to what format a response
may have), so adjustments may need to be made to
the mask and filter. This command must be used with
an active CAN protocol (one that has been sending
and receiving messages), as it can not initiate a
protocol search. Note that the CAF1 setting normally
eliminates the display of all RTRs, so if you are
monitoring messages and want to see the RTRs, you
will have to turn off formatting, or else turn the headers
on.
ELM327L DSA
[ Read the input Voltage ]
This initiates the reading of the voltage present at
pin 2, and the conversion of it to a decimal voltage. By
default, it is assumed that the input is connected to the
voltage to be measured through a 47KΩ and 10KΩ
resistor divider (with the 10KΩ connected from pin 2 to
Vss), and that the ELM327 supply is a nominal 5V.
This will allow for the measurement of input voltages
up to about 28V, with an uncalibrated accuracy of
typically about 2%.
S0 and S1
[ printing of Spaces off or on ]
These commands control whether or not space
characters are inserted in the ECU response.
The ELM327 normally reports ECU responses as
a series of hex characters that are separated by space
characters (to improve readability), but messages can
be transferred much more quickly if every third byte
(the space) is removed. While this makes the message
less readable for humans, it can provide significant
improvements for computer processing of the data. By
default, spaces are on (S1), and space characters are
inserted in every response.
SD hh
[ Save Data byte hh ]
The ELM327 is able to save one byte of
information for you in a special nonvolatile memory
location, which is able to retain its contents even if the
power is turned off. Simply provide the byte to be
stored, then retrieve it later with the read data (AT RD)
command. This location is ideal for storing user
preferences, unit ids, occurrence counts, or other
information.
SH xyz
[ Set the Header to 00 0x yz ]
Entering CAN 11 bit ID words (headers) normally
requires that extra leading zeros be added (eg. AT SH
00 07 DF), but this command simplifies doing so. The
AT SH xyz command accepts a three digit argument,
takes only the right-most 11 bits from that, adds
leading zeros, and stores the result in the header
storage locations for you. As an example, AT SH 7DF
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ELM327L
AT Command Descriptions (continued)
is a valid command, and is quite useful for working
with 11 bit CAN systems. It actually results in the
header bytes being stored internally as 00 07 DF.
SH xx yy zz
[ Set the Header to xx yy zz ]
This command allows the user to manually control
the values that are sent as the three header bytes in a
message. These bytes are normally assigned values
for you (and are not required to be adjusted), but there
may be occasions when it is desirable to change them
(particularly if experimenting with physical addressing).
If experimenting, it is not necessary but may be better
to set the headers after a protocol is active. That way,
wakeup messages, etc. that get set on protocol
activation will use the default values.
The header bytes are defined with hexadecimal
digits - xx will be used for the first or priority/type byte,
yy will be used for the second or receiver/target byte,
and zz will be used for the third or transmitter/source
byte. These remain in effect until set again, or until
restored to their default values with the D, WS, or Z
commands.
If new values for header bytes are set before the
vehicle protocol has been determined, and if the
search is not set for fully automatic (ie other than
protocol 0), these new values will be used for the
header bytes of the first request to the vehicle. If that
first request should fail to obtain a response, and if the
automatic search is enabled, the ELM327 will then
continue to search for a protocol using default values
for the header bytes. Once a valid protocol is found,
the header bytes will revert to the values assigned with
the AT SH command.
This command is used to assign all header bytes,
whether they are for a J1850, ISO 9141, ISO 14230, or
a CAN system. The CAN systems will use these three
bytes to fill bits 0 to 23 of the ID word (for a 29 bit ID),
or will use only the rightmost 11 bits for an 11 bit CAN
ID (and any extra bits assigned will be ignored). The
additional 5 bits needed for a 29 bit system are set
with the AT CP command.
If assigning header values for the KWP protocols
(4 and 5), care must be taken when setting the first
header byte (xx) value. The ELM327 will always insert
the number of data bytes for you, but how it is done
depends on the values that you assign to this byte. If
the second digit of this first header byte is anything
other than 0 (zero), the ELM327 assumes that you
wish to have the length value inserted in that first byte
ELM327L DSA
when sending. In other words, providing a length value
in the first header byte tells the ELM327 that you wish
to use a traditional 3 byte header, where the length is
stored in the first byte of the header.
If you provide a value of 0 for the second digit of
the first header byte, the ELM327 will assume that you
wish that value to remain as 0, and that you want to
have a fourth header (length) byte inserted into the
message. This is contrary to the ISO 14230-4 OBD
standard, but it is in use by many KWP2000 systems
for (non-OBD) data transfer, so may be useful when
experimenting.
SH ww xx yy zz
[ Set the Header to ww xx yy zz ]
This four byte version of the AT SH command
allows setting a complete 29 bit CAN ID in one
instruction. Alternatively, AT SP (for the five most
significant bits) and AT SH (for the other three bytes)
may be used.
SI
[ perform a Slow Initiation ]
Protocols 3 and 4 use what is sometimes called a
5 baud, or slow initiation sequence in order to begin
communications. Usually, the sequence is performed
when the first message needs to be sent, and then the
message is sent immediately after. Some ECUs may
need more time between the two however, and having
a separate initiation command allows you to control
this time. Simply send AT SI, wait a little, then send
the message. You may need to experiment a little to
get the right amount of delay. Protocol 3 or 4 must be
selected to use the AT SI command, or an error will
result.
SP h
[ Set Protocol to h ]
This command is used to set the ELM327 for
operation using the protocol specified by 'h', and to
also save it as the new default. Note that the protocol
will be saved no matter what the AT M0/M1 setting is.
The ELM327 supports 12 different protocols (two
can be user-defined). They are:
0 - Automatic
1 - SAE J1850 PWM (41.6 kbaud)
2 - SAE J1850 VPW (10.4 kbaud)
3 - ISO 9141-2 (5 baud init, 10.4 kbaud)
4 - ISO 14230-4 KWP (5 baud init, 10.4 kbaud)
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ELM327L
AT Command Descriptions (continued)
5 - ISO 14230-4 KWP (fast init, 10.4 kbaud)
6 - ISO 15765-4 CAN (11 bit ID, 500 kbaud)
7 - ISO 15765-4 CAN (29 bit ID, 500 kbaud)
8 - ISO 15765-4 CAN (11 bit ID, 250 kbaud)
9 - ISO 15765-4 CAN (29 bit ID, 250 kbaud)
A - SAE J1939 CAN (29 bit ID, 250* kbaud)
B - USER1 CAN (11* bit ID, 125* kbaud)
C - USER2 CAN (11* bit ID, 50* kbaud)
* default settings (user adjustable)
The first protocol shown (0) is a convenient way of
telling the ELM327 that the vehicle’s protocol is not
known, and that it should perform a search. It causes
the ELM327 to try all protocols if necessary, looking for
one that can be initiated correctly. When a valid
protocol is found, and the memory function is enabled,
that protocol will then be remembered, and will
become the new default setting. When saved like this,
the automatic mode searching will still be enabled, and
the next time the ELM327 fails to connect to the saved
protocol, it will again search all protocols for another
valid one. Note that some vehicles respond to more
than one protocol - if searching, you may see more
than one type of response.
ELM327 users often use the AT SP 0 command to
reset the search protocol before starting (or restarting)
a connection. This works well, but since it is used so
often, and since writes to EEPROM result in an
unnecessary delay (of about 30 msec), the AT SP0
command sets the protocol to 0, but does not perform
a write to EEPROM. Similarly, the SP A0 and SP 0A
commands do not perform writes to EEPROM, either.
Saving this value to EEPROM would not provide any
advantage (and would be very short-lived, as the
ELM327 will soon be finding the vehicle’s protocol and
over-writing the ‘0’ value in EEPROM). If you really
want to store the value ‘0’ in the internal EEPROM,
you must use the AT SP 00 command.
If another protocol (other than 0) is selected with
this command (eg. AT SP 3), that protocol will become
the default, and will be the only protocol used by the
ELM327. Failure to initiate a connection in this
situation will result in a response such as ‘BUS INIT:
...ERROR’, and no other protocols will be attempted.
This is a useful setting if you know that your vehicle(s)
only use the one protocol, but is also one that can
cause a lot of problems if you do not understand it.
ELM327L DSA
SP 00
[ erase the Stored Protocol ]
To speed up protocol initiation and detection, the
SP 0 command sets the protocol to automatic, but
does not perform a (very time-consuming) write to
EEPROM. Some users felt it was necessary to be able
to actually write to the ELM327's EEPROM, however,
so we provided this command. It should not normally
be used when connecting to a vehicle.
SP Ah
[ Set Protocol to Auto, h ]
This variation of the SP command allows you to
choose a starting (default) protocol, while still retaining
the ability to automatically search for a valid protocol
on a failure to connect. For example, if your vehicle is
ISO 9141-2, but you want to occasionally use the
ELM327 circuit on other vehicles, you might use the
AT SP A3 command, so that the first protocol tried will
then be yours (3), but it will also automatically search
for other protocols. Don't forget to disable the memory
function if doing this, or each new protocol detected
will become your new default.
SP Ah will save the protocol information even if
the memory option is off (but SP A0 and SP 0A do notif you must write 0 to the EEPROM, use command AT
SP 00). Note that the ‘A’ can come before or after the
h, so AT SP A3 can also be entered as AT SP 3A.
SR hh
[Set the Receive address to hh ]
Depending on the application, users may wish to
manually set the address to which the ELM327 will
respond. Issuing this command will turn off the AR
mode, and force the IC to only accept responses
addressed to hh. Use caution with this setting, as
depending on what you set it to, you may accept a
message that was actually meant for another module,
possibly sending an IFR when you should not. To turn
off the SR filtering, simply send AT AR.
This command has limited use with CAN, as it only
monitors one byte of the ID bits, and that is not likely
selective enough for most CAN applications (the CRA
command may be a better choice). Also, the command
has no effect on the addresses used by the J1939
protocols, as the J1939 routines set their own receive
addresses based on the ID bit (header) values.
This SR command is exactly the same as the RA
command, and can be used interchangeably with it.
Note that CAN Extended Addressing does not use this
value - it uses the one set by the AT TA command.
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ELM327L
AT Command Descriptions (continued)
SS
[ use the Standard Sequence for searches ]
SAE standard J1978 specifies a protocol search
order that scan tools should use. It follows the number
order that we have assigned to the ELM327 protocols.
In order to provide a faster search, the ELM327 does
not normally follow this order, but it will if you
command it to with AT SS.
ST hh
[ Set Timeout to hh ]
After sending a request, the ELM327 waits a
preset time for a response before it can declare that
there was ‘NO DATA’ received from the vehicle. The
same timer setting may also be used after a response
has been received, while waiting to see if any more is
coming (but this depends on the AT AT setting). The
AT ST command allows this timer to be adjusted, in
increments of 4 msec (or 20 msec if in a CAN protocol,
with CTM5 selected).
When Adaptive Timing is enabled, the AT ST time
sets the maximum time that is to be allowed, even if
the adaptive algorithm determines that the setting
should be longer. In most circumstances, it is best to
simply leave the AT ST time at the default setting, and
let the adaptive timing algorithm determine what to use
for the timeout.
The ST timer is set to 32 by default (giving a time
of approximately 200 msec), but this default setting
can be adjusted by changing PP 03. Note that a value
of 00 does not result in a time of 0 msec – it will
restore the timer to the default value. Also, during
protocol searches, an internally set minimum time is
used - you may select longer times with AT ST, but not
shorter ones.
SW hh
[ Set Wakeup to hh ]
Once a data connection has been established,
some protocols require that there be data flow every
few seconds, just so that the ECU knows to maintain
the communications path open. If the messages do not
appear, the ECU will assume that you are finished,
and will close the channel. The connection will need to
be initialized again to reestablish communications.
The ELM327 will automatically generate periodic
messages, as required, in order to maintain a
connection. Any replies to these messages are
ignored by the ELM327, and are not visible to the user.
(Currently, only protocols 3, 4, and 5 support these
messages - nothing is available for CAN. If you require
ELM327L DSA
CAN periodic messages, you must use the ELM329.)
The time interval between these periodic ‘wakeup’
messages can be adjusted in 20 msec increments
using the AT SW hh command, where hh is any
hexadecimal value from 00 to FF. The maximum
possible time delay of just over 5 seconds results
when a value of FF (decimal 255) is used. The default
setting (92) provides a nominal delay of 3 seconds
between messages.
Note that the value 00 (zero) is special, as it will
stop the periodic (wakeup) messages. This provides a
control for experimenters to stop the messages while
keeping the rest of the protocol functioning normally,
and is not intended to be used regularly. Issuing
AT SW 00 will not change a prior setting for the time
between wakeup messages, if the protocol is reinitialized. Once periodic messages have been turned
off with AT SW, they can only be reestablished by
closing and reinitializing the protocol.
TA hh
[ set the Tester Address to hh ]
This command is used to change the current
tester (ie. scan tool) address that is used in the
headers, periodic messages, filters, etc. The ELM327
normally uses the value that is stored in PP 06 for this,
but the TA command allows you to temporarily
override that value.
Sending AT TA will affect all protocols, including
J1939. This provides a convenient means to change
the J1939 address from the default value of F9,
without affecting other settings.
Although this command may appear to work ‘on
the fly’, it is not recommended that you try to change
this address after a protocol is active, as the results
may be unpredictable.
TP h
[ Try Protocol h ]
This command is identical to the SP command,
except that the protocol that you select is not
immediately saved in internal EEPROM memory, so
does not change the default setting. Note that if the
memory function is enabled (AT M1), and this new
protocol that you are trying is found to be valid, that
protocol will then be stored in memory as the new
default.
TP Ah
[ Try Protocol h with Auto ]
This command is very similar to the AT TP
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ELM327L
AT Command Descriptions (continued)
command above, except that if the protocol that is tried
should fail to initialize, the ELM327 will then
automatically sequence through the other protocols,
attempting to connect to one of them.
V0 and V1
[ Variable data lengths off or on ]
Many CAN protocols (ie ISO 15765-4) expect to
send eight data bytes at all times. The V0 and V1
commands may be used to override this behaviour (for
any CAN protocol) should you wish.
Choosing V1 will cause the current CAN protocol
to send variable data length messages, just as bit 6 of
PP 2C and PP 2E do for protocols B and C. It does not
matter what the protocol should be doing - V1 will
override that. This allows experimenting with variable
data length messages on demand.
If you select V0 (the default setting), the forced
sending of variable length CAN messages is turned
off. The format of the sent messages reverts to the
protocol’s settings.
WM [1 to 6 bytes]
[ set Wakeup Message to… ]
This command allows the user to override the
default settings for the wakeup messages (sometimes
known as the ‘periodic idle’ messages). Simply provide
the message that you wish to have sent (typically three
header bytes and one to three data bytes), and the
ELM327 will add the checksum and send them as
required, at the rate determined by the AT SW setting.
Default settings will send the bytes 68 6A F1 01 00
for ISO 9141, and C1 33 F1 3E for KWP.
WS
and the chip will be put into the idle state, waiting for
characters on the RS232 bus. Note that any baud rate
that was set with the AT BRD command will be lost,
and the ELM327 will return to the default baud rate
setting.
[ Warm Start ]
@1
[ display the device description ]
This command displays the device description
string. The default text is ‘OBDII to RS232 Interpreter’.
@2
[ display the device identifier ]
A device identifier string that was recorded with
the @3 command is displayed with the @2 command.
All 12 characters and a terminating carriage return will
be sent in response, if they have been defined. If no
identifier has been set, the @2 command returns an
error response (‘?’). The identifier may be useful for
storing product codes, production dates, serial
numbers, or other such codes.
See the ‘Programming Serial Numbers’ section for
more information.
@3 cccccccccccc
[ store the device identifier ]
This command is used to set the device identifier
code. Exactly 12 characters must be sent, and once
written to memory, they can not be changed (ie you
may only use the @3 command one time). The
characters sent must be printable (ascii character
values 0x21 to 0x5F inclusive).
If you are developing software to write device
identifiers, you may be interested in the ELM328 IC, as
it allows multiple writes using the @3 command (but it
can not send OBD messages).
This command causes the ELM327 to perform a
complete reset. It is very similar to the AT Z command,
but does not include the power on LED test. Users
may find this a convenient way to quickly ‘start over’
without having the extra delay of the AT Z command.
If using variable RS232 baud rates (ie AT BRD
commands), it is preferred that you reset the IC using
this command rather than AT Z, as AT WS will not
affect the chosen RS232 baud rate.
Z
[ reset all ]
This command causes the chip to perform a
complete reset as if power were cycled off and then on
again. All settings are returned to their default values,
ELM327L DSA
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ELM327L
Reading the Battery Voltage
Before learning the OBD Commands, we will show
an example of how to use an AT Command. We will
assume that you have built (or purchased) a circuit
which is similar to that of Figure 9 in the Example
Applications section (page 80). This circuit provides a
connection to read the vehicle’s battery voltage, which
many will find very useful.
If you look in the AT Command list, you will see
there is one command that is listed as RV [Read the
input Voltage]. This is the command which you will
need to use. First, be sure that the prompt character is
shown (that is the ‘>’ character), then simply enter ‘AT’
followed by RV, and press return (or enter):
>AT RV
Note that we used upper case characters for this
request, but it was not required, as the ELM327 will
accept upper case (AT RV) as well as lower case
(at rv) or any combination of these (At rV). It does not
matter if you insert space characters (‘ ’) within the
message either, as they are ignored by the ELM327.
A typical response to this command will show a
voltage reading, followed by another prompt character:
the CV value, as the ELM327 knows that it should be
between the second and the third digits.
At this point, the internal calibration values have
been changed (ie. written to EEPROM), and the
ELM327 now knows that the voltage at the input is
actually 12.47V. To verify that the changes have taken
place, simply read the voltage again:
>AT RV
12.5V
The ELM327 always rounds off the measurement
to one decimal place, so the 12.47V actually appears
as 12.5V (but the second decimal place is maintained
internally for accuracy and is used in the calculations).
The ELM327 may be calibrated with any reference
voltage that you have available, but note that the CV
command always expects to receive four characters
representing the voltage at the input. If you had used a
9V battery for your reference, and it is actually 9.32V,
then you must add a leading zero to the actual voltage
when calibrating the IC:
>AT CV 0932
OK
12.6V
>
The accuracy of this reading depends on several
factors. As shipped from the factory, the ELM327
voltage reading circuitry will typically be accurate to
about 2%. For many, this is all that is needed. Some
people may want to calibrate the circuitry for more
accurate readings, however, so we have provided a
special ‘Calibrate Voltage’ command for this.
To change the internal calibration constants, you
will need to know the actual battery voltage to more
accuracy than the ELM327 shows. Many quality digital
multimeters can do this, but you should verify the
accuracy before making a change.
Let us assume that you have connected your
accurate multimeter, and you find that it reads 12.47V.
The ELM327 is a little high at 12.6V, and you would
like it to read the same as your meter. Simply calibrate
the ELM327 to the measured voltage using the CV
command:
If you should get into trouble with this command
(for example, if you set calibration values to something
arbitrary and do not have a voltmeter on hand to
provide accurate values), you can restore the settings
to the original (factory) values with the CV 0000
command. Simply send:
>AT CV 0000
OK
The other AT Commands are used in the same
manner. Simply type the letters A and T, then follow
with the command you want to send and any
arguments that are required. Then press return (or
enter, depending on your keyboard). Remember - you
can always insert space characters as often as you
wish if it improves the readability for you, as they are
ignored by the ELM327.
>AT CV 1247
OK
Note that you should not provide a decimal point in
ELM327L DSA
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ELM327L
OBD Commands
If the bytes that you send to the ELM327 do not
begin with the letters ‘A’ and ‘T’, they are assumed to
be OBD commands for the vehicle. Each pair of ASCII
bytes will be tested to ensure that they are valid
hexadecimal digits, and will then be combined into
data bytes for transmitting to the vehicle.
OBD commands are actually sent to the vehicle
embedded in a data packet. Most standards require
that three header bytes and an error checksum byte
be included with every OBD message, and the
ELM327 adds these extra bytes to your command
bytes for you. The initial (default) values for these
extra bytes are usually appropriate for most requests,
but if you wish to change them, there is a mechanism
to do so (see the ‘Setting the Headers’ section).
Most OBD commands are only one or two bytes in
length, but some can be longer. The ELM327 will limit
the number of bytes that can be sent to the maximum
number allowed by the standards (usually seven bytes
or 14 hexadecimal digits). Attempts to send more
bytes will result in an error – the entire command is
then ignored and a single question mark printed.
Hexadecimal digits are used for all of the data
exchange with the ELM327 because it is the data
format used most often in the OBD standards. Most
mode request listings use hexadecimal notation, and it
is the format most frequently used when results are
shown. With a little practice, it should not be very
difficult to deal in hex numbers, but some people may
want to use a table such as Figure 1, or keep a
calculator nearby. Dealing with the hex digits can not
be avoided - eventually all users need to manipulate
the results in some way (combining bytes and dividing
by 4 to obtain rpm, dividing by 2 to obtain degrees of
advance, converting temperatures, etc.).
As an example of sending a command to the
vehicle, assume that A6 (or decimal 166) is the
command that is required to be sent. In this case, the
user would type the letter A, then the number 6, then
would press the return key. These three characters
would be sent to the ELM327 by way of the RS232
port. The ELM327 would store the characters as they
are received, and when the third character (the
carriage return) was received, would begin to assess
the other two. It would see that they are both valid hex
digits, and would convert them to a one byte value (the
decimal value is 166). The header bytes and a
checksum byte would then be added, and a total of
five bytes would typically be sent to the vehicle. Note
that the carriage return character is only a signal to the
ELM327L DSA
ELM327, and is not sent to the vehicle.
After sending the command, the ELM327 listens
on the OBD bus for replies, looking for ones that are
directed to it. If a message address matches, the
received bytes will be sent on the RS232 port to the
user, while messages received that do not have
matching addresses will be ignored (but are often still
available for viewing with the AT BD command).
The ELM327 will continue to wait for messages
addressed to it until there are none found in the time
that was set by the AT ST command. As long as
messages continue to be received, the ELM327 will
continue to reset this timer, and look for more. Note
that the IC will always respond to a request with some
reply, even if it is to say ‘NO DATA’ (meaning that
there were no messages found, or that some were
found but they did not match the receive criteria).
Hexadecimal
Number
Decimal
Equivalent
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Figure 1. Hex to Decimal Conversion
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ELM327L
Talking to the Vehicle
The standards require that each OBD command or
request that is sent to the vehicle must adhere to a set
format. The first byte sent (known as the ‘mode’)
describes the type of data being requested, while the
second byte (and possibly a third or more) specifies
the actual information that is required. The bytes which
follow after the mode byte are known as the
‘parameter identification’ or PID number bytes. The
modes and PIDs are described in detail in documents
such as the SAE J1979, or ISO 15031-5 standards,
and may also be defined by the vehicle manufacturers.
The SAE J1979 standard currently defines ten
possible diagnostic test modes, which are:
01
02
03
04
05
06
07
08
09
0A
- show current data
- show freeze frame data
- show diagnostic trouble codes
- clear trouble codes and stored values
- test results, oxygen sensors
- test results, non-continuously monitored
- show ‘pending’ trouble codes
- special control mode
- request vehicle information
- request permanent trouble codes
Vehicles are not required to support all of the
modes, and within modes, they are not required to
support all possible PIDs (some of the first OBDII
vehicles only supported a very small number of them).
Within each mode, PID 00 is reserved to show which
PIDs are supported by that mode. Mode 01, PID 00
must be supported by all vehicles, and can be
accessed as follows…
Ensure that your ELM327 interface is properly
connected to the vehicle, and powered. Most vehicles
will not respond without the ignition key in the ON
position, so turn the ignition to on, but do not start the
engine. If you have been experimenting, the state of
your interface may be unknown, so reset it by sending:
>AT Z
You will see the interface leds flash, and then the
IC should respond with ‘ELM327 v2.1’, followed by a
prompt character. Now, you may choose a protocol
that the ELM327 should connect with, but it is usually
easier to simply select protocol ‘0’ which tells the IC to
search for one:
>AT SP 0
That’s all that you need to do to prepare the
ELM327L DSA
ELM327 for communicating with a vehicle. At the
prompt, issue the mode 01 PID 00 command:
>01 00
The ELM327 should say that it is ‘SEARCHING...’
for a protocol, then it should print a series of numbers,
similar to these:
41 00 BE 1F B8 10
The 41 in the above signifies a response from a
mode 01 request (01 + 40 = 41), while the second
number (00) repeats the PID number requested. A
mode 02, request is answered with a 42, a mode 03
with a 43, etc. The next four bytes (BE, 1F, B8, and
10) represent the requested data, in this case a bit
pattern showing the PIDs that are supported by this
mode (1=supported, 0=not). Although this information
is not very useful for the casual user, it does prove that
the connection is working.
Another example requests the current engine
coolant temperature (ECT). Coolant temperature is
PID 05 of mode 01, and can be requested as follows:
>01 05
The response will be of the form:
41 05 7B
The 41 05 shows that this is a response to a
mode 1 request for PID 05, while the 7B is the desired
data. Converting the hexadecimal 7B to decimal, one
gets 7 x 16 + 11 = 123. This represents the current
temperature in degrees Celsius, but with the zero
offset to allow for subzero temperatures. To convert to
the actual coolant temperature, you need to subtract
40 from the value obtained. In this case, then, the
coolant temperature is 123 - 40 or 83°C.
A final example shows a request for the engine
rpm. This is PID 0C of mode 01, so at the prompt type:
>01 0C
If the engine is running, the response might be:
41 0C 1A F8
The returned value (1A F8) is actually a two byte
hex number that must be converted to a decimal value
to be useful. Converting it, we get a value of 6904,
which seems like a very high value for engine rpm.
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ELM327L
Talking to the Vehicle (continued)
That is because rpm is sent in increments of 1/4 rpm!
To convert to the actual engine speed, we need to
divide the 6904 by 4. A value of 1726 rpm is much
more reasonable.
Note that these examples asked the vehicle for
information without regard for the type of OBD protocol
that the vehicle uses. This is because the ELM327
takes care of all of the data formatting and translation
for you. Unless you are going to do more advanced
functions, there is really no need to know what the
protocol is.
The above examples showed only a single line of
response for each request, but the replies often
consist of several separate messages, either from
multiple ECUs responding, or from one ECU providing
messages that need to be combined to form one
response (see ‘Multiline Responses’ on page 43). In
order to be adaptable to this variable number of
responses, the ELM327 normally waits to see if any
more are coming. If no response arrives within a
certain time, it assumes that the ECU is finished. This
same timer is also used when waiting for the first
response, and if that never arrives, causes ‘NO DATA’
to be printed.
There is a way to speed up the retrieval of
information, if you know how many responses will be
sent. By telling the ELM327 how many lines of data to
receive, it knows when it is finished, so does not have
to go through the final timeout, waiting for data that is
not coming. Simply add a single hex digit after the
OBD request bytes - the value of the digit providing the
maximum number of responses to obtain, and the
ELM327 does the rest. For example, if you know that
there is only one response coming for the engine
temperature request that was previously discussed,
you can send:
>01 05 1
and the ELM327 will return immediately after obtaining
only one response. This may save a considerable
amount of time, as the default time for the AT ST timer
is 200 msec. (The ELM327 still sets the timer after
sending the request, but that is only in case the single
response does not arrive.)
Some protocols (like J1850 PWM) require an
acknowledgement from the ELM327 for every
message sent. If you provide a number for the
responses that is too small, the ELM327 will return to
the prompt too early, and you may cause bus
ELM327L DSA
congestion while the ECU tries several times to resend
the messages that were not acknowledged. For this
reason, you must know how many responses to
expect before using this feature.
As an example, consider a request for the vehicle
identification number (VIN). This number is 17 digits
long, and typically takes 5 lines of data to be
represented. It is obtained with mode 09, PID 02, and
should be requested with:
>09 02
or with:
>09 02 5
if you know that there are five lines of data coming. If
you should mistakenly send 09 02 1, you might cause
problems.
This ability to specify the number of responses
was really added with the programmer in mind. An
interface routine can determine how many responses
to expect for a specific request, and then store that
information for use with subsequent requests. That
number can then be added to the requests and the
response time can be optimized. For an individual
trying to obtain a few trouble codes, the savings are
not really worth the trouble, and it’s easiest to just
make a request, without regard to how many
responses are expected.
We offer one additional warning when trying to
optimize the speed at which you obtain information
from vehicles. Prior to the APR2002 release of the
J1979 standard, sending J1850 requests more
frequently than every 100 msec was forbidden. With
the APR2002 update, scan tools were allowed to send
the next request without delay if it was determined that
all the responses to the previous request had been
received. Vehicles made prior to this time may not be
able to tolerate requests at too fast a rate, so use
caution with them.
Hopefully this has shown how typical requests are
made using the ELM327. If you are looking for more
information on modes and PIDs, it is available from
the SAE (www.sae.org), from ISO (www.iso.org), or
from various other sources on the web.
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ELM327L
Bus Initiation
Both the ISO 9141-2 and ISO 14230-4 (KWP2000)
standards require that the vehicle’s OBD bus be
‘initialized’ before any communications can take place.
The ISO 9141 standard allows for only a slow (2 to 3
second) initiation process, while ISO 14230 allows for
both a slow method, and a faster alternative.
The ELM327 will perform this bus initiation for you,
but generally not until a request needs to be sent (but
you can force one with the FI and SI commands). If the
bus initiation occurs during an automatic search, you
will not see any status reporting, but if you have the
Auto option off (and are set to protocols 3, 4, or 5),
then you will see a message similar to this:
BUS INIT: ...
The three dots appear only as the slow initiation
process is carried out – a fast initiation does not show
the dots. This will be followed by either the expression
‘OK’ to say it was successful, or else an error message
to indicate that there was a problem. (The most
common error encountered is in forgetting to turn the
ELM327L DSA
vehicle’s key to the ‘ON’ position before attempting to
talk to the vehicle.)
Once the bus has been initiated, communications
must take place regularly (typically at least once every
five seconds), or the bus will revert to a low-power
‘sleep’ mode. If you are not sending data requests
often enough, the ELM327 will generate requests for
you to ensure that the bus stays ‘awake’. You will
never see the responses to these, but you may see
the transmit LED flash periodically as these are being
sent.
By default, the ELM327 ensures that these
‘wakeup’ or ‘idle’ messages are sent every 3 seconds,
but this is adjustable with the AT SW command. The
contents of the wakeup message are also user
programmable with the AT WM command, if you
should wish to change them. Users generally do not
have to change either of the above, as the default
settings work well with most systems.
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ELM327L
Interpreting Trouble Codes
Likely the most common use that the ELM327 will
be put to is in obtaining the current Diagnostic Trouble
Codes (or DTCs). Minimally, this requires that a mode
03 request be made, but first one should determine
how many trouble codes are presently stored. This is
done with a mode 01 PID 01 request as follows:
>01 01
To which a typical response might be:
41 01 81 07 65 04
The 41 01 signifies a response to the request, and
the next data byte (81) is the number of current trouble
codes. Clearly there would not be 81 (hex) or 129
(decimal) trouble codes present if the vehicle is at all
operational. In fact, this byte does double duty, with
the most significant bit being used to indicate that the
malfunction indicator lamp (MIL, or ‘Check Engine
Light’) has been turned on by one of this module’s
codes (if there are more than one), while the other 7
bits of this byte provide the actual number of stored
trouble codes. In order to calculate the number of
stored codes when the MIL is on, simply subtract 128
(or 80 hex) from the number.
The above response then indicates that there is
one stored code, and it was the one that set the Check
Engine Lamp or MIL on. The remaining bytes in the
response provide information on the types of tests
supported by that particular module (see the J1979
document for further information).
In this instance, there was only one line to the
response, but if there were codes stored in other
modules, they each could have provided a line of
response. To determine which module is reporting the
trouble code, one would have to turn the headers on
(AT H1) and then look at the third byte of the three
byte header for the address of the module that sent
the information.
Having determined the number of codes stored,
the next step is to request the actual trouble codes
with a mode 03 request (there is no PID needed):
>03
A response to this could be:
43 01 33 00 00 00 00
6 bytes in the response have to be read in pairs to
show the trouble codes (the above would be
interpreted as 0133, 0000, and 0000). Note that the
response has been padded with 00’s as required by
the SAE standard for this mode – the 0000’s do not
represent actual trouble codes.
As was the case when requesting the number of
stored codes, the most significant bits of each trouble
code also contain additional information. It is easiest to
use the following table to interpret the extra bits in the
first digit as follows:
If the first hex digit received is this,
Replace it with these two characters
0
P0
Powertrain Codes - SAE defined
1
P1
“
“ - manufacturer defined
2
P2
“
“ - SAE defined
3
P3
“
“ - jointly defined
4
C0
Chassis Codes - SAE defined
5
C1
“
“ - manufacturer defined
6
C2
“
“ - manufacturer defined
7
C3
“
“ - reserved for future
8
B0
Body Codes - SAE defined
9
B1
“
“ - manufacturer defined
A
B2
“
“ - manufacturer defined
B
B3
“
“ - reserved for future
C
U0
Network Codes - SAE defined
D
U1
“
“ - manufacturer defined
E
U2
“
“ - manufacturer defined
F
U3
“
“ - reserved for future
Taking the example trouble code (0133), the first
digit (0) would then be replaced with P0, and the 0133
reported would become P0133 (which is the code for
an ‘oxygen sensor circuit slow response’). Note that
the ISO 15765-4 (CAN) protocol is very similar, but it
adds an extra data byte (in the second position),
showing how many data items (DTCs) are to follow.
To provide a few more examples, if the received
code was D016, you would replace the D with U1, and
the resulting trouble code would be U1016. Similarly,
1131 received would actually be for the code P1131.
The ‘43’ in the above response simply indicates
that this is a response to a mode 03 request. The other
ELM327L DSA
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ELM327L
Resetting Trouble Codes
The ELM327 is quite capable of resetting
diagnostic trouble codes, as this only requires issuing
a mode 04 command. The consequences should
always be considered before sending it, however, as
more than the MIL (or ‘Check Engine Light’) will be
reset. In fact, issuing a mode 04 will:
- reset the number of trouble codes
- erase any diagnostic trouble codes
- erase any stored freeze frame data
- erase the DTC that initiated the freeze frame
- erase all oxygen sensor test data
- erase mode 06 and 07 information
- not erase permanent (mode 0A) trouble codes
(these are reset by the ECU only)
Clearing of all of this data is not unique to the
ELM327 – it occurs whenever any scan tool is used to
reset the codes. The biggest problem with losing this
data is that your vehicle may run poorly for a short
time, while it performs a recalibration.
To avoid inadvertently erasing stored information,
the SAE specifies that scan tools must verify that a
mode 04 is intended (‘Are you sure?’) before actually
sending it to the vehicle, as all trouble code
information is immediately lost when the mode is sent.
Remember that the ELM327 does not monitor the
content of the messages, so it will not know to ask for
confirmation of the mode request – this would have to
be the duty of a software interface, if one is written.
As stated, to actually erase diagnostic trouble
codes, one need only issue a mode 04 command. A
response of 44 from the vehicle indicates that the
mode request has been carried out, the information
erased, and the MIL turned off. Some vehicles may
require a special condition to occur (eg. the ignition on
but the engine must not be running) before they will
respond to a mode 04 command.
That is all there is to clearing trouble codes. Once
again, do not accidentally send the 04 code!
Quick Guide for Reading Trouble Codes
If you do not use your ELM327 for some time, this
entire data sheet may seem like quite a bit to review
when your ‘Check Engine’ light eventually comes on,
and you just want to know why. We offer this section
as a quick guide to the basics that you will need.
To get started, connect the ELM327 circuit to your
PC or PDA and communicate with it using a terminal
program such as HyperTerminal, ZTerm, ptelnet, or a
similar program. It should normally be set to either
9600 or 38400 baud, with 8 data bits, and no parity or
handshaking.
The chart at the right provides a quick procedure
on what to do next:
Ignition Key to ON,
but vehicle not running
>AT SP 0
OK
>0101
to see how many codes
(2nd digit of the 3rd byte)
>03
to see the codes
(ignore the first byte and
read the others in pairs)
FIX THE VEHICLE !
>04
to reset the codes
ELM327L DSA
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ELM327L
Selecting Protocols
The ELM327 supports several different OBD
protocols (see Figure 2, at right). As a user, you may
never have to choose which one it should use (since
the factory settings cause an automatic search to be
performed for you), but while experimenting, you may
want to specify a protocol to be used.
For example, if you know that your vehicle uses
the SAE J1850 VPW protocol, you may want the
ELM327 to use only that protocol, and no others. If
that is what you want, simply determine the protocol
number (from Figure 2), then use the ‘Set Protocol’ AT
Command as follows:
>AT SP 2
OK
From this point on, the default protocol (used after
every power-up or AT D command) will be protocol 2
(or whichever one that you have chosen). Verify this
by asking the ELM327 to describe the protocol:
>AT DP
SAE J1850 VPW
Now what happens if your friend has a vehicle that
uses ISO 9141-2? How do you now use the ELM327
interface for that vehicle, if it is set for J1850?
One possibility is to change your protocol selection
to allow for the automatic searching for another
protocol, on failure of the current one. This is done by
putting an ‘A’ before the protocol number, as follows:
>AT SP A2
OK
Description
0
Automatic
1
SAE J1850 PWM (41.6 kbaud)
2
SAE J1850 VPW (10.4 kbaud)
3
ISO 9141-2 (5 baud init)
4
ISO 14230-4 KWP (5 baud init)
5
ISO 14230-4 KWP (fast init)
6
ISO 15765-4 CAN (11 bit ID, 500 kbaud)
7
ISO 15765-4 CAN (29 bit ID, 500 kbaud)
8
ISO 15765-4 CAN (11 bit ID, 250 kbaud)
9
ISO 15765-4 CAN (29 bit ID, 250 kbaud)
A
SAE J1939 CAN (29 bit ID, 250* kbaud)
B
User1 CAN (11* bit ID, 125* kbaud)
C
User2 CAN (11* bit ID, 50* kbaud)
*user adjustable
Figure 2. ELM327 Protocol Numbers
memory will only occur after a valid protocol is found,
and only if the memory function is enabled (M0/M1).
For the previous example, all that needs to be sent is:
>AT TP A2
OK
>AT DP
AUTO, SAE J1850 VPW
Now, the ELM327 will try protocol 2, but will then
automatically begin searching for another protocol
should the attempt to connect with protocol 2 fail (as
would happen when you try to connect to your friend’s
vehicle).
The Set Protocol commands cause an immediate
write to the internal EEPROM, before even attempting
to connect to the vehicle. This write is time-consuming,
affects the setting for the next powerup, and may not
actually be appropriate, if the protocol selected is not
correct for the vehicle. To allow a test before a write
occurs, the ELM327 offers one other command - the
Try Protocol (TP) command.
Try Protocol is very similar to Set Protocol. It is
used in exactly the same way as the AT SP command,
the only difference being that a write to internal
ELM327L DSA
Protocol
Many times, it is very difficult to even guess at a
protocol to try first. In these cases, it is best to simply
let the ELM327 decide what to use. This is done by
telling it to use protocol 0 (with either the SP or the TP
commands).
To have the ELM327 automatically search for a
protocol to use, simply send:
>AT SP 0
OK
and when the next OBD command is to be sent, the
ELM327 will automatically look for one that responds.
You will see a ‘SEARCHING...’ message, followed by
a response, after which you can ask the ELM327 what
protocol it found (by sending AT DP).
The first versions of the ELM327 used the SAE
recommended search order (protocol 1, 2, 3, etc.), but
recent versions of the IC modify the search order
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ELM327L
Selecting Protocols (continued)
based on any active inputs that are present. If you
need to follow the SAE J1978 order, you should send
the ELM327 an AT SS command first, or step through
each protocol with the TP command.
The automatic search works well with OBDII
systems, but may not be what you need if you are
experimenting. During a search, the ELM327 ignores
any headers that you have previously defined (since
there is always a chance that your headers may not
result in a response), and it uses the default OBD
header values for each protocol. It will also use
standard requests (ie 01 00) during the searches. If
this is not what you want, the results may be a little
frustrating.
To use your own header (and data) values when
attempting to connect to an ECU, do not tell the
ELM327 to use protocol 0. Instead, tell it to either use
ELM327L DSA
only your target protocol (ie. AT SP n), or else tell it to
use yours with automatic searches allowed on failure
(ie AT SP An). Then send your request, with headers
assigned as required. The ELM327 will then attempt to
connect using your headers and your data, and only if
that fails (and you have chosen the protocol with AT
SP An) will it search using the standard OBD default
values.
In general, 99% of all users find that enabling the
memory (setting pin 5 to 5V) and using the ‘Auto’
option when searching (you may need to send AT
SP 0) works very well. After the initial search, the
protocol used by your vehicle becomes the new
default, but it is still able to search for another, without
your having to say AT SP 0 again.
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ELM327L
OBD Message Formats
To this point we have only discussed the contents
(data portion) of an OBD message, and made only
passing mention of other parts such as headers and
checksums, which all messages use to some extent.
On Board Diagnostics systems are designed to be
very flexible, providing a means for several devices to
communicate with one another. In order for messages
to be sent between devices, it is necessary to add
information describing the type of information being
sent, the device that it is being sent to, and perhaps
which device is doing the sending. Additionally, the
importance of the message becomes a concern as
well – crankshaft position information is certainly of
considerably more importance to a running engine
than a request for the number of trouble codes stored,
or the vehicle serial number. So to convey importance,
messages are also assigned a priority.
The information describing the priority, the
intended recipient, and the transmitter are usually
needed by the recipient even before they know the
type of request that the message contains. To ensure
that this information is obtained first, OBD systems
transmit it at the start (or head) of the message. Since
these bytes are at the head, they are usually referred
priority
TA
SA
receiver
transmitter
3 header bytes
to as header bytes. Figure 3 below shows a typical
OBD message structure that is used by the
SAE J1850, ISO 9141-2, and ISO 14230-4 standards.
It uses 3 header bytes as shown, to provide details
concerning the priority, the receiver, and the
transmitter. Note that many texts refer to the receiver
as the ‘Target Address’ (TA), and the transmitter as
the ‘Source Address’ (SA).
Another concern when sending any message is
that errors might occur in the transmission, and the
received data may be falsely interpreted. To detect
errors, the various protocols all provide some form of
check on the received data. This may be as simple as
a sum calculation (ie a ‘running total’ of byte values)
that is sent at the end of a message. If the receiver
also calculates a sum as bytes are received, then the
two values can be compared and if they do not agree,
the receiver will know that an error has occurred.
Since simple sums might not detect multiple errors, a
more reliable (and more complicated) sum called a
Cyclic Redundancy Check (or ‘CRC’) is often used. All
of the protocols specify how errors are to be detected,
and the various ways of handling them if they occur.
The OBD data bytes are thus normally
up to 7 data bytes
checksum
Figure 3. An OBD Message
‘header’ bytes
ID bits (11 or 29)
data bytes (8 in total)
PCI
7 data bytes
checksum
Figure 4. A CAN OBD Message
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ELM327L
OBD Message Formats (continued)
encapsulated within a message, with ‘header’ bytes at
the beginning, and a ‘checksum’ at the end. The
J1850, ISO 9141-2, and ISO 14230-4 protocols all use
essentially the same structure, with three header
bytes, a maximum of seven data bytes and one
checksum byte.
The ISO 15765-4 (CAN) protocol uses a very
similar structure (see Figure 4), the main difference
really only relating to the structure of the header. CAN
header bytes are not referred to as header bytes –
they are called ‘ID bits’ instead. The initial CAN
standard defined the ID bits as being 11 in number,
while the more recent CAN standard now allows for
either 11 or 29 bit IDs.
The ELM327 does not normally show any of these
extra bytes unless you turn that feature on with the
Headers On command (AT H1). Issuing that allows
you to see the header bytes and the checksum byte
(for the J1850, ISO 9141 and ISO 14230 protocols).
For the CAN protocols, you will see the ID bits, and
other items which are normally hidden such as the PCI
byte for ISO 15765, or the data length codes (if they
are enabled with PP 29, or AT D1). Note that the
ELM327 does not display the checksum information
for CAN systems, or the IFR bytes for J1850 systems.
It is not necessary to ever have to set these
header byes, or to perform a checksum calculation, as
the ELM327 will always do this for you. The header
bytes are adjustable however, should you wish to
experiment with advanced messages such as those
for physical addressing. The next section provides a
discussion on how to do this…
Setting the Headers
The emissions related diagnostic trouble codes
that most people are familiar with are described in the
SAE J1979 standard (ISO15031-5). They represent
only a portion of the data that a vehicle may have
available – much more can be obtained if you are able
to direct the requests elsewhere.
Accessing most OBDII diagnostics information
requires that requests be made to what is known as a
a ‘functional address.’ Any processor that supports the
function will respond to the request (and theoretically,
many different processors can respond to a single
functional request). In addition, every processor (or
ECU) will also respond to what is known as their
physical address. It is the physical address that
uniquely identifies each module in a vehicle, and
permits you to direct more specific queries to only one
particular module.
To retrieve information beyond that of the OBDII
requirements then, it will be necessary to direct your
requests to either a different functional address, or to
an ECU’s physical address. This is done by changing
the data bytes in the message header.
As an example of functional addressing, let us
assume that you want to request that the processor
responsible for Engine Coolant provide the current
Fluid Temperature. You do not know its address, so
ELM327L DSA
you consult the SAE J2178 standard and determine
that Engine Coolant is functional address 48. SAE
standard J2178 also tells you that for your J1850 VPW
vehicle, a priority byte of A8 is appropriate. Finally,
knowing that a scan tool is normally address F1, you
have enough information to specify the three header
bytes (A8 48 and F1). To tell the ELM327 to use these
new header bytes, all you need is the Set Header
command:
>AT SH A8 48 F1
OK
The three header bytes assigned in this manner
will stay in effect until changed by the next AT SH
command, a reset, or an AT D.
Having set the header bytes, you now need only
send the secondary ID for fluid temperature (10) at the
prompt. If the display of headers is turned off, the
conversation could look like this:
>10
10 2E
The first byte in the response echoes the request,
as usual, while the data that we requested is the 2E
byte. You may find that some requests, being of a low
priority, may not be answered immediately, possibly
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ELM327L
Setting the Headers (continued)
causing a ‘NO DATA’ result. In these cases, you may
want to adjust the timeout value, perhaps first trying
the maximum (ie use AT ST FF). Many vehicles will
simply not support these extra addressing modes.
The other, and more common method of obtaining
information is by physical addressing, in which you
direct your request to a specific device, not to a
functional group. To do this, you again need to
construct a set of header bytes that direct your query
to the physical address of the processor, or ECU. If
you do not know the address, recall that the sender of
information is usually shown in the third byte of the
header. By monitoring your system for a time with the
headers turned on (AT H1), you can quickly learn the
main addresses of the senders. The SAE document
J2178 assigns address ranges to these devices if you
are unsure of which might be most appropriate.
When you know the address that you wish to
‘speak to,’ simply use it for the second byte in the
header (assume an address of 10 for this example).
Combine that with your knowledge of SAE J2178 to
choose a priority/type byte (assume a value of E4 for
this example, as if the vehicle is J1850 PWM). Finally,
you need to identify yourself to the target, so that
responses can be returned to you. As is customary for
diagnostic tools, we’ll use an address of F1. As before,
these three bytes are then assigned to the header with
the set header command:
>AT SH E4 10 F1
OK
From this point on, all messages that the ELM327
sends will use these three bytes for the header. All that
needs to be done now is to request data from the
vehicle. For physical addressing, this is often done
using mode 22:
22 and others are described in more detail in the SAE
standards document J2190, ‘Enhanced E/E Diagnostic
Test Modes’.
The ISO14230-4 standard defines its header bytes
a little differently. Advanced experimenters will be
aware that for ISO 14230-4, the first header byte must
always include the length of the data field, which
varies from message to message. From that, one
might assume that the you would need to redefine the
header for every message that is to be sent – not so!
The ELM327 always determines the number of bytes
that you are sending, and inserts that length for you, in
the proper place for the header that you are using. If
you are using the standard ISO 14230-4 header, the
length will be put into the first header byte, and you
need only provide the two (most significant) bits of this
byte when defining the header. What you place in the
rest of the byte will be ignored by the ELM327 unless
you set it to 0. If it is 0, it is assumed that you are
experimenting with KWP four byte headers, and the
ELM327 then creates the fourth header byte for you.
Again, you do not need to provide any length to be put
into this byte – it is done for you.
Addressing within the CAN (ISO 15765-4)
protocols is quite similar in many ways. First, consider
the 29 bit standard. The ELM327 splits the 29 bits into
a CAN Priority byte and the three header bytes that we
are now familiar with. This is how they are combined
for use by the ELM327:
>AT CP vv
>AT SH xx yy zz
vv
xx
yy
zz
5 bits only
>22 11 6B
62 11 6B 00 00
vv
The response to this command is of the same
format to those seen for ‘standard’ OBD requests. The
request has been repeated (with 40 added to the
mode value in order to show that this is a response),
and this is followed by the actual data (00 00 in this
case). The PIDs used with mode 22 are usually
proprietary to each manufacturer and are generally not
published widely, so you may have difficulty in
determining the ones to use with your vehicle. Elm
Electronics does not maintain lists of this information,
and cannot provide any further details for you. Mode
ELM327L DSA
xx
yy
zz
29 bit ID
The CAN standard states that for diagnostics, the
priority byte (‘vv’ in the diagram) will always be 18 (it is
the default value used by the ELM327). Since it is
rarely changed, it can be assigned separately from the
other header bytes, using the CP command.
The next byte (‘xx’) describes the type of message
that this is, and is set to hex DB for functional
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ELM327L
Setting the Headers (continued)
addressing, and to DA if using physical addressing.
The next two bytes are as defined previously for the
other standards – ‘yy’ is the receiver (or Target
Address), and ‘zz’ is the transmitter (or Source
Address). For the functional diagnostic requests, the
receiver is always 33, and the transmitter is F1, which
is very similar to ISO 14230-4.
Those that are familiar with the SAE J1939
standard will likely find this header structure to be very
similar (J1939 is a CAN standard for use by ‘heavyduty vehicles’ such as trucks and buses). It uses
slightly different terminology, but there is a direct
parallel between the bytes used by J1939 for the
headers and the grouping of the bytes in the ELM327.
Pages 54 and 55 provide more details of the J1939
message structure.
Another method to define all 29 CAN ID bits at
once is with the four byte version of the SH command.
Simply provide all 8 nibbles in one command:
>AT SH vv xx yy zz
vv
xx
yy
zz
29 bit ID
functional addressing (ID/header = 7DF) and physical
addressing (7En) for making requests. Generally, you
do not know physical addresses at first, but you do
know (from the standard) that the OBD functional
address is 7DF.
In order to learn the physical addresses, turn the
headers on, and watch what address the replies are
from, then use that information to make physical
requests if desired. For example, if the headers are on,
and you send 01 00, you might see:
>01 00
7E8 06 41 00 BE 3F B8 13 00
The 7E8 shows that ECU#1 was the one
responding. In order to talk directly to that ECU, all you
need do is to set the header to the appropriate value (it
is 7E0 to talk to the 7E8 device – see ISO 15765-4 for
more information). From that point on, you can ‘talk’
directly to the ECU using its physical address, as
shown here:
>AT SH 7E0
OK
>01 00
7E8 06 41 00 BE 3F B8 13 00
As with the AT CP command, only 5 bits are used
from the first byte (the 3 most significant bits are
ignored).
The final header format to discuss is that used by
11 bit CAN systems. They also use a priority and
address structure, but shorten it into roughly three
nibbles rather than three bytes. The ELM327 uses the
same storage locations for these values, so you may
use the methods just discussed to assign 11 bit values
(but only the least significant bits are used for any
messages - the others are ignored).
11 bit CAN ‘headers’ are normally assigned using
a special ‘short’ version of the AT SH command that
uses only three hex digits. Since three digits are
actually 12 bits, the most significant bit is ignored, and
can have any value. As an example of this instruction,
to assign an ID of 7DF, simply send:
>01 05
7E8 03 41 05 46 00 00 00 00
This has just been a quick overview on how to
change headers (and ID bits). Hopefully it has been
enough to at least help to get you started. We do not
recommend experimenting too much without a copy of
the standards though, as some aspects are extremely
difficult to figure out without them.
>AT SH 7DF
OK
The OK shows that the ELM327 has accepted
your value.
The 11 bit ISO15765-4 CAN standard defines both
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ELM327L
Multiline Responses
There are occasions when a vehicle must respond
with more information than one ‘message’ is able to
show. In these cases, it responds with several lines
which the receiver must assemble into one complete
message.
One example of this is a request for the 17 digit
vehicle identification number, or VIN. This is available
from newer vehicles using a mode 09, PID 02 request
(but was not initially a requirement, so many older
vehicles do not support it). Here is one example of a
response that might be obtained from a J1850 vehicle:
>0902
49 02
49 02
49 02
49 02
49 02
01
02
03
04
05
00
44
30
35
33
00
34
30
42
34
00
47
52
31
35
31
50
35
32
36
The first two bytes (49 and 02) on each line of the
response are used to show that the information is in
reply to an 09 02 request. The next byte shows which
response it is, while the remaining four bytes are the
data bytes that are being sent. Assembling the data in
the order specified by the third byte, and ignoring the
first few 00’s (they are filler bytes - see J1979) gives:
31 44 34 47 50 30 30 52 35 35 42 31
32 33 34 35 36
The data values actually represent the ASCII
codes for the digits of the VIN. Using an ASCII table to
convert these into characters gives the following VIN
for the vehicle:
1 D 4 G P 0 0 R 5 5 B 1 2 3 4 5 6
CAN systems will display this information in a
somewhat different fashion. Here is a typical response
from a CAN vehicle:
>0902
014
0: 49 02 01 31 44 34
1: 47 50 30 30 52 35 35
2: 42 31 32 33 34 35 36
The CAN Formatting has been left on (the default),
making the reading of the data easier. With formatting
on, the sequence numbers are shown with a colon (‘:’)
after each. CAN systems add this single hex digit (it
goes from 0 to F then repeats), to aid in reassembling
ELM327L DSA
the data, instead of the byte value that the J1850
vehicle did.
The first line of this response says that there are
014 bytes of information in total. That is 14 in hex, or
20 in decimal terms, which agrees with the 6 + 7 + 7
bytes shown on the three lines. The VIN numbers are
generally 17 digits long, however, so how do we
assemble the number from 20 digits?
This is done by discarding the first three bytes of
the message. The first two are the familiar 49 02, as
this is a response to an 09 02 request, so are not part
of the VIN. The third byte (the ‘01’), tells the number of
data items that are to follow (the vehicle can only have
one VIN, and this agrees with that). Ignoring the third
byte leaves 17 data bytes which are the serial number
(purposely chosen to be identical to the those of the
previous example). All that is needed is a conversion
to ASCII, in order to read them, exactly as before.
From these two examples, you can see that the
format of the data received may depend on the
protocol used to transmit it. For this reason, a copy of
the SAE J1979 standard would be essential if you are
planning to do a lot of work with this, for example if you
were writing software to display the received data.
The following shows an example of a different type
of multiline response that can occur when two or more
ECUs respond to one request. Here is a typical
response to an 01 00 request:
>01 00
41 00 BE 3E B8 11
41 00 80 10 80 00
This is difficult to decipher without knowing a little
more information. First, turn the headers on to actually
see ‘who’ is doing the talking:
>AT H1
OK
>01 00
48 6B 10 41 00 BE 3E B8 11 FA
48 6B 18 41 00 80 10 80 00 C0
Now, if you analyze the header, you can see that
the third byte shows ECU 10 (the engine controller)
and ECU 18 (the transmission) are both responding
with a reply to the 01 00 request. With modern
vehicles, this type of response occurs often, and you
should be prepared for it.
A final example shows how similar messages
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ELM327L
Multiline Responses (continued)
might occasionally be ‘mixed up’ in a CAN system. We
ask the vehicle for the Calibration ID (09 04) and are
presented with the following response:
>09 04
013
0: 49 04
1: 32 38
013
0: 49 04
2: 00 00
1: 32 38
2: 00 00
01 35 36 30
39 34 39 41 43
01
00
39
00
35
00
35
00
36
00
34
00
30
00 31
41 43
00 00
which is quite confusing. The first group (the 013, 0:, 1:
group) seems to make some sense (but the number of
data bytes do not agree with the response), and the
following group is very confusing, as it has two
segment twos. It seems that two ECUs are responding
and the information is getting mixed up. Which ECU do
the responses belong to? The only way to know is to
turn on the headers, and repeat your request:
From these, the messages can be assembled in
their proper order. To do this, look at the byte following
the CAN ID - it is what is known as the PCI byte, and
is used to tell what type of data follows. In this case,
the PCI byte begins with either a 1 (for a ‘First Frame’
message), or a 2 (for the ‘Consecutive Frames’). The
second half of the PCI byte shows the order in which
the information is to be assembled (ie. the segment
number). In this case, the segment numbers are
already in order, but if they had not been, it would
have been necessary to rearrange the messages to
place them in order.
Each OBD standard has some minor peculiarities,
but hopefully this has helped you with some of the
more common ones. If you are still having trouble, we
urge you to purchase the relevant standard, and study
it.
>AT H1
OK
>09
7E8
7E8
7E9
7E8
7E9
7E9
04
10
21
10
22
21
22
13
32
13
00
32
00
49
38
49
00
38
00
04
39
04
00
39
00
01
34
01
00
35
00
35
39
35
00
34
00
36
41
36
00
41
00
30
43
30
31
43
00
This time, the order appears to be the same, but
be aware that it may not be – that is why the standard
requires that sequence codes be transmitted with
multiline responses.
Looking at the first digits of these responses, you
can see that some begin with 7E8, and some begin
with 7E9, which are the CAN IDs representing ECU#1
and ECU#2, respectively. Grouping the responses by
ECU gives:
7E8 10 13 49 04 01 35 36 30
7E8 21 32 38 39 34 39 41 43
7E8 22 00 00 00 00 00 00 31
and
7E9 10 13 49 04 01 35 36 30
7E9 21 32 38 39 35 34 41 43
7E9 22 00 00 00 00 00 00 00
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ELM327L
CAN Message Types
The ISO 15765-4 (CAN) standard defines several
message types that are to be used with diagnostic
systems. Currently, there are four that may be used:
SF FF CF FC -
the Single Frame
the First Frame (of a multiframe message)
the Consecutive Frame ( ‘ ‘ )
the Flow Control frame
The Single Frame message contains storage for
up to seven data bytes in addition to what is known as
a PCI (Protocol Control Information) byte. The PCI
byte is always the first of the data bytes, and tells how
many data bytes are to follow. If the CAN Auto
Formatting option is on (CAF1) then the ELM327 will
create this byte for you when sending, and remove it
for you when receiving. (If the headers are enabled,
you will see it in the responses.)
If you turn the Auto Formatting off (with CAF0), it
is expected that you will provide all of the data bytes to
be sent. For diagnostics systems, this means the PCI
byte and the data bytes. The ELM327 will not modify
your data in any way, except to add extra padding
bytes for you, to ensure that you always send as many
data data bytes as are required (eight for ISO15765).
You do not need to set the Allow Long (AT AL) option
in order to send eight bytes, as the IC overrides it for
you.
A First Frame message is used to say that a multiframe message is about to be sent, and tells the
receiver just how many data bytes to expect. The
length descriptor is limited to 12 bits, so a maximum of
4095 byes can be received at once using this method.
Consecutive Frame messages are sent after the
First Frame message to provide the remainder of the
data. Each Consecutive Frame message includes a
single hex digit ‘sequence number’ that is used to
determine the order when reassembling the data. It is
expected that if a message were corrupted and resent,
it could be out of order by a few packets, but not by
more than 16, so the single digit is normally more than
adequate. As seen previously, the serial number for a
vehicle is often a multiframe response:
>0902
014
0: 49 02 01 31 44 34
1: 47 50 30 30 52 35 35
2: 42 31 32 33 34 35 36
First Frame message. The length (014) was actually
extracted from that message by the ELM327 and
printed on the first line as shown. Following the First
Frame line are two Consecutive Frames (that begin
with 1: and 2:). To learn more details of the exact
formatting, you may want to send a request such as
the one above, then repeat the same request with the
headers enabled (AT H1). This will show the PCI bytes
that are actually used to send these components of the
total message.
The Flow Control frame is one that you do not
normally have to deal with. When a First Frame
message is sent as part of a reply, the ELM327 must
tell the sender some technical things (such as how
long to delay between Consecutive Frames, etc.) and
does so by replying immediately with a Flow Control
message. These are predefined by the ISO 15765-4
standard, so can be automatically inserted for you. If
you wish to generate custom Flow Control messages,
then refer to the ‘Altering Flow Control Messages’
section, on page 61.
If a Flow Control frame is detected while
monitoring, the line will be displayed with ‘FC: ’ before
the data, to help you with decoding of the information.
There is a final type of message that is
occasionally reported, but is not supported by the
diagnostics standard. The (Bosch) CAN standard
allows for the transmission of a data request without
sending any data in the requesting message. To
ensure that the message is seen as such, the sender
also sets a special flag in the message (the RTR bit),
which is seen at each receiver. The ELM327 always
looks for this flag, or for zero data bytes, and may
report to you that an RTR was detected while
monitoring. This is shown by the characters RTR
where data would normally appear, but only if the CAN
Auto Formatting is off, or headers are enabled. Often,
when monitoring a CAN system with an incorrect baud
rate chosen, RTRs may be seen.
Note that the CAN system is quite robust with
several error detecting methods in place, so that
during normal data transmission you will rarely see
any errors. When monitoring buses however, you may
well see errors (especially if the ELM327 is set to an
incorrect baud rate). As a diagnostic aid, when errors
do occur, the ELM327 will print all bytes (no matter
what CAF, etc., is set to), followed by the message
‘<RX ERROR’.
In this example, the line that begins with 0: is the
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ELM327L
Multiple PID Requests
The SAE J1979 (ISO 15031-5) standard allows
requesting multiple PIDs with one message, but only if
you connect to the vehicle with CAN (ISO 15765-4).
Up to six parameters may be requested at once, and
the reply is one message that contains all of the
responses.
For example, let us say that you need to know
engine load (04), engine coolant temperature (05),
manifold pressure (0B), and engine rpm (0C) on a
regular basis. You could send four separate requests
for them (01 04, then 01 05, then 01 0B, etc.) or you
could put them all into one message like this:
message is a response to an 01 request.
Following the 41 is the actual information, with the
PID numbers followed by their data bytes. You will
need to know how many data bytes to expect in order
to make sense of it in most cases.
The order in which you ask for the PIDs should not
matter. For example, the previous request might have
been sent as:
>01 0B 04 0C 05
00A
0: 41 0B 21 04 3F 0C
1: 17 B8 05 44 00 00 00
>01 04 05 0B 0C
to which, a typical reply might be:
00A
0: 41 04 3F 05 44 0B
1: 21 0C 17 B8 00 00 00
The reply is a multiline one, as was discussed in a
previous section. Looking at the reply in detail, the first
line tells us that it is 00A (decimal 10) bytes long, so
we only pay attention to the first ten bytes of the
following lines (and ignore the final three 00’s on the
last line). The first byte is 41, which tells us that the
in which case, the responses might be as shown
above (but the order in which the PIDs appear in the
response does not have to match the order in which
they were requested).
Using this technique, you can make more efficient
use of the data bus. The cost is the extra work that you
must do in creating the requests, and in parsing each
response. If you are writing software to do this, the
time initially taken may well be worth it, but if you are
typing requests at a terminal screen, it is very unlikely
that this will be of benefit to you.
Response Pending Messages
The ELM327 has always been a protocol
converter device, receiving data using one protocol,
and sending it with another. Unless the data was
meant to control the ELM327, it was always passed on
without regard to the content of the data. Beginning
with v2.1, that is changing.
The ELM327 now checks each reply to see if it is
a special ‘Response Pending’ message, which may be
used by the ECU to say “Wait, I’m busy.” According to
the SAE J1979 standard, the scan tool should then
wait up to 5 seconds for the requested information to
arrive (and if another ‘Response Pending’ arrives, the
5 second timer should be reset so that the timing starts
over).
The Response Pending reply will always be of the
form:
7F xx 78
requested.
If bit 2 of PP 2A is set (it is by default), the
ELM327 will support this part of J1979, changing the
timeout to 5 seconds for you if it sees a Response
Pending message. This will only occur for the CAN
and ISO14230 (KWP) protocols as per the standard.
The CAN protocol is not normally limited to only
ISO15765, but may be if you set PP 2A b0 to ‘1’.
Note that the current implementation of this
feature does not keep track of multiple ECUs, some of
which may reply immediately, and some that may reply
with response pending messages. For this reason,
there may conceivably be circumstances when you
may need to filter for only one ECU address when
receiving a Response Pending reply.
where the xx represents the Mode (or SID) that was
being requested. There is no feedback as to the PID
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ELM327L
CAN Receive Filtering - the CRA Command
When receiving CAN data, the ELM327 actually
retrieves every message from the CAN bus, and then
decides whether or not to display it, based on criteria
that you or the firmware set. The ‘CAN Receive
Address’ or CRA command may be used to define
this criteria for you, in one simple step.
As an example, assume that the only messages
that you wish to see are those that have the CAN ID
7E9. To see only them, tell the ELM327 that the
receive address should be 7E9:
>AT CRA 7E9
and the ELM327 will set the necessary values so that
the only messages that are accepted are the ones with
ID 7E9.
If you do not want an exact address, but would
prefer to see a range of values, for example all the
OBD addresses (those that begin with 7E), then simply
use an ‘X’ for the digit that you do not want the
ELM327 to be specific about. That is, to see all
messages with CAN IDs that start with 7E (7E0, 7E1,
7E2,... 7EE, and 7EF), send:
>AT CRA 7EX
and the ELM327 will set the necessary values for you.
This command works exactly the same way for the
29 bit IDs. For example, if you wish to see all
messages that are being sent from the engine ECU
(address 10) to the scan tool (address F1), then you
can send:
and the ELM327 takes care of the details for you.
When working with J1939 data, the ELM327
normally formats the data for you, in order to separate
the priority from the PGN information. This is usually
not a concern when using the CRA command, except
when you are trying to filter for a specific priority. For
example, you might typically see:
>AT MA
3 0FE6C 00 FF FF FF FF FF FF 40 B5
6 0FEEE 00 15 50 FF FF FF FF FF FF
6 0FEF5 00 FE FF FF FF 19 00 23...
The single priority digit out front (the 3 or 6 above)
as well as the leading 0 with the PGN information are
actually part of the first two digits (5 bits) of the ID, and
need to be interpreted as such, in order to use the
CRA command. It may be easier if you turn off the
J1939 header formatting in order to see this:
>AT JHF0
OK
>AT MA
0C FE 6C 00 FF FF FF FF FF FF 40 B5
18 FE EE 00 15 50 FF FF FF FF FF FF
18 FE F5 00 FE FF FF FF 19 00 23...
This more clearly shows the four bytes that need
to be defined for the CRA command to be set. To
search for all messages that begin with 6 0FEF5, you
would actually need to send the command:
>AT CRA 18 FE F5 XX
>AT CRA XX XX F1 10
and all the settings will be taken care of for you.
If you wish to be more specific and see only the
OBD replies sent by the engine to the scan tool, you
would say:
>AT CRA 18 DA F1 10
and again, the ELM327 makes the necessary changes
for you.
Perhaps you do not care which device is sending
the information, but you want to see all messages that
start with 18 DA and are being sent to the scan tool.
For this, use the character ‘X’ to tell the ELM327 that
you do not care what value a digit has:
In summary then, the CRA command allows you
to tell the ELM327 what ID codes to look for, and the
letter ‘X’ may be used in it to represent any single digit
that you do not want the ELM327 to be specific about.
This is usually selective enough for most applications,
but occasionally, there is a need to be specific down to
the bit level, rather than to the nibble. For those
applications, you will need to program a separate
mask and filter, as we show in the next section.
>AT CRA 18 DA F1 XX
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Using the CAN Mask and Filter
Filtering of CAN messages (that is, deciding which
to keep and which to reject), is usually handled most
easily with the CRA command. The CRA command
only allows for definition to the nibble level, however if you need more selectivity (to the bit level), you must
program the mask and filter.
Internally, the ELM327 configures an ‘acceptance
filter’ with 1’s and 0’s based on the type of message
that it wishes to receive (OBD, J1939, etc.). This
pattern is then compared to the ID bits of all incoming
messages. If the two patterns match, then the entire
message is accepted, and if they do not, the message
is rejected.
Having to match all 11 or 29 bits of the ID may be
very restrictive in some cases (and would require a
very large number of filters for some applications). To
allow a little more flexibility in what to accept, and what
to reject, a mask is also defined, in addition to the
filter. This mask acts just like the type worn on your
face - some features are exposed and some are
hidden. If the mask has a ‘1’ in a bit position, that bit in
the filter must match with the bit in the ID, or the
message will be rejected. If the mask bit is a ‘0’, then
the ELM327 does not care if that filter bit matches with
the message ID bit or not.
As an example, consider the standard response to
an 11 bit OBD request. ISO15765-4 states that all
responses will use IDs in the range from 7E8 to 7EF.
That is:
1. There must always be a ‘7 ‘ (binary 111) as the
first nibble (so the filter should have the value 111
or 7). All 3 bits are relevant (so the mask should
be binary 111 or 7). Note that this first nibble is
only 3 bits wide for the 11 bit CAN ID.
2. There must always be an ‘E’ (binary 1110) in
the second position, so the filter needs to be of
value 1110 or E. Since all 4 bits are relevant, the
mask needs to be of value 1111 or F.
3. If you analyze the patterns for the binary
numbers from 8 to F, you will see that the only
thing in common is that the most significant bit is
always set. That is, the mask will have a value of
1000 since only that one bit is relevant, and you
do not care what the other bits are. The filter
needs to be assigned a value that has a 1 in the
first position, but we do not care what is in the
other three positions. In the following, we use 0’s
in the positions (but it doesn’t really matter).
ELM327L DSA
Putting this together, the filter will have a value:
111 1110 1000 = 7E8
and the mask will have a value:
111 1111 1000 = 7F8
In order to make these active, you will need to
issue both a CAN Filter and a CAN Mask command:
>AT CF 7E8
OK
>AT CM 7F8
OK
From that point on, only the IDs from 7E8 to 7EF
will be accepted by the chip.
The 29 bit IDs work in exactly the same way. For
example, assume that you wish to receive only
messages of the form:
18 DA F1 XX
where XX is the address of the ECU that is sending
the message, but you do not care what the value is
(this is the standard OBD response format). Putting 0’s
in the mask for don’t care bits, then the mask needs to
be set as follows:
>AT CM 1F FF FF 00
OK
(as every bit except those in the last byte are relevant)
while the filter may be set to:
>AT CF 18 DA F1 00
OK
Note that if a filter has been set, it will be used for
all CAN messages, so setting filters and masks may
cause standard OBD requests to be ignored, and you
may begin seeing ‘NO DATA’ replies. If this happens,
and you are unsure of why, you may want to reset
everything to the default values (with AT CRA, AT D,
or possibly AT WS) and start over.
Quite likely, you will never have to use the CM and
CF commands. If you do, then creating your own
masks and filters can be difficult. You may find it
helpful to draw the bit patterns first, and think about
which ones matter, and which ones do not. It may also
help to connect to a vehicle, apply test settings, and
send AT MA to see how the settings affect the
displayed data.
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ELM327L
Monitoring the Bus
Some vehicles use the OBD bus for information
transfer during normal vehicle operation, passing a
great deal of information over it. A lot can be learned if
you have the good fortune to connect to one of these
vehicles, and are able to decipher the contents of the
messages.
To see how your vehicle uses the OBD bus, you
can enter the ELM327’s ‘Monitor All’ mode, by sending
the command AT MA from your terminal program. This
will cause the IC to display any information that it sees
on the OBD bus, regardless of transmitter or receiver
addresses (it will show all). Note that the ELM327
remains silent while monitoring, so periodic ‘wakeup’
messages are not sent (if you have an ISO 9141 or
ISO 14230 bus that had been initialized previously, it
may ‘go to sleep’), IFRs are not sent, and the CAN
module does not acknowledge messages.
The monitoring mode can be stopped by putting a
logic low level on the RTS pin, or by sending a single
RS232 character to the ELM327. Any convenient
character can be used to interrupt the IC – there are
no restrictions on whether it is printable, etc. Note that
any character that you send will be discarded, and will
have no effect on any subsequent commands.
The time it takes to respond to such an interrupt
will depend on what the ELM327 is doing when the
character is received. The IC will always finish a task
that is in progress (printing a line, for example) before
printing ‘STOPPED’ and returning to wait for your
input, so it is best to wait for the prompt character (‘>’)
to be sent, or the Busy line to go low, before beginning
to send a new command.
One unexpected result may occur if you have the
automatic protocol search feature enabled, and you
tell the ELM327 to begin monitoring. If the bus is quiet,
the ELM327 will begin searching for an active protocol,
which may not be what you were expecting. Be aware
also that the ISO 9141 and ISO 14230 protocols look
identical when monitoring, so the ELM327 may stop
searching at ISO 9141, even if the actual protocol is
ISO 14230. With the Automatic searching enabled, this
should correct itself, however, when an OBD request
is later made.
If the ‘Monitor All’ command provides too much
information (it certainly does for most CAN systems),
then you can restrict the range of data that is to be
displayed. Perhaps you only want to see messages
that are being transmitted by the ECU with address 10.
To do that, you only need to type:
and all messages that contain 10 in the third byte of
the header will be displayed.
Using this command with 11 bit CAN systems can
be a little confusing at first. Recall the way in which all
header bytes are stored within the ELM327. An 11 bit
CAN ID is actually stored as the least significant 11
bits in the 3 byte ‘header storage’ location. It will be
stored with 3 bits in the receiver’s address location,
and the remaining 8 bits in the transmitter’s address
location. For this example, we have requested that all
messages created by transmitter ‘10’ be printed, so all
11 bit CAN IDs that end in 10 will be displayed (ie all
that look like ‘x10’).
The other monitoring command that is very useful
is the AT MR command, which looks for specific
addresses in the middle byte of the header. Using this
command, you can look for all messages being sent to
a particular address. For example, to use it to look for
messages being sent to the ECU with address 10,
simply send:
>AT MR 10
and all messages that contain 10 in the second byte of
the header will be displayed.
Using this command with the 11 bit CAN systems
will again need further explanation. It may be helpful to
first picture the hex number ‘10’ in the above example
as the binary number ‘0001 0000’. Recall from above
that 11 bit CAN IDs are actually stored as the least
significant 11 bits in the 3 byte ‘header storage’
locations, and only 3 bits are actually stored in the
middle byte (receiver’s address) position. When
comparing the received CAN ID to the address you
provide with the MR command then, only the rightmost 3 bits of your MR address are considered and
the other 5 bits are ignored. In this example, the
AT MR 10 effectively becomes AT MR 0 for 11 bit
CAN systems, and so all messages that begin with ‘0’
as the first digit will be displayed.
It is best not to use the AT MT or MR commands
when monitoring CAN systems. The ELM327 provides
another command (AT CRA) that allows much better
control over the received data - see the next section
for complete details.
>AT MT 10
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ELM327L
Restoring Order
There may be times when it seems the ELM327 is
out of control, and you will need to know how to
restore order. Before we continue to discuss modifying
too many parameters, this seems to be a good point to
discuss how to ‘get back to the start’. Perhaps you
have told the ELM327 to monitor all data, and there
are screens and screens of data flying by. Perhaps the
IC is now responding with ‘NO DATA’ when it did work
previously. This is when a few tips may help.
The ELM327 can always be interrupted from a
task by a single keystroke from the keyboard. As part
of its normal operation, checks are made for received
characters and if found, the IC will stop what it is doing
at the next opportunity. Often this means that it will
continue to send the information on the current line,
then stop, print a prompt character, and wait for your
input. The stopping may not always seem immediate if
the RS232 send buffer is almost full, though – you will
not actually see the prompt character until the buffer
has emptied, and your terminal program has finished
printing what it has received.
There are times when the problems seem more
serious and you don’t remember just what you did to
make them so bad. Perhaps you have ‘adjusted’ some
of the timers, then experimented with the CAN filter, or
perhaps tried to see what happens if the header bytes
are changed. All of these can be reset by sending the
‘set to Defaults’ AT Command:
the four status LEDs in sequence. A much quicker
option is available with the ELM327, however, if the
led test is not required – the ‘Warm Start’ command:
>AT WS
The AT WS command performs a software reset,
restoring exactly the same items as the AT Z does, but
it omits the LED test, making it considerably faster.
Also, it does not affect any baud rates that have been
set with the AT BRD command (which AT Z does), so
is essential if you are modifying the RS232 baud rates
with software.
Any of the above methods should be effective in
restoring order while experimenting. There is always
the chance that you may have changed a
Programmable Parameter, however, and are still
having problems with your system. In this case, you
may want to simply turn off all of the Programmable
Parameters (which forces them to their default values).
To do so, send the command:
>AT PP FF OFF
which should disable all of the changes that you have
made. Since some of the Programmable Parameters
are only read during a system reset, you may have to
follow this command with a reset command:
>AT Z
>AT D
OK
This will often be sufficient to restore order, but it
can occasionally bring unexpected results. One such
surprise will occur if you are connected to a vehicle
using one protocol, but the saved (default) protocol is
a different one. In this case, the ELM327 will close the
current session and then change the protocol to the
default one, exactly as instructed.
If the AT D does not bring the expected results, it
may be necessary to do something more drastic - like
resetting the entire IC. There are a few ways that this
can be performed with the ELM327. One way is to
simply remove the power and then reapply it. Another
way that acts exactly the same way as a power off and
then on is to send the full reset command:
after which, you can start over with what is essentially
a device with ‘factory settings’. There may be times
when even this command is not recognized, however.
If that is the case, you will need to use the hardware
method of turning the PPs off. See the section on
‘Programmable Parameters’ (pages 69 and 70) for
more details.
>AT Z
It takes approximately one second for the IC to
perform this reset, initialize everything and then test
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ELM327L
Using Higher RS232 Baud Rates
A serial interface has always been provided with
our ELM OBD products, largely due to its versatility.
Older computers, microprocessors and PDAs can all
interface easily with it, as can USB, Bluetooth,
ethernet and WiFi modules. It is simply one of the
most versatile interfaces available.
Originally, our users almost exclusively used the
traditional RS232 interface to connect our integrated
circuits to their computers. Interface circuits were
easily made, or purchased, and could be used with a
very large range of devices. The large voltage swings
and cable capacitance worked against using the
interface for very high data rates, however, so we set
the default data rate of the ELM327 to a conservative
38.4 kbps.
If your application needs a traditional RS232
interface, then by all means use one. We offer a few
suggestions in the Example Applications section that
you might try. The discrete version that we show works
extremely well at speeds of up to 57600 bps, and
depending on several factors, it may also work well at
speeds as high as 115200 bps.
If you would like to operate your interface at
speeds of 115200 bps or higher, there are several
single IC solutions that are available. These include
devices such as the ADM232A from Analog Devices
(www.analog.com), or the popular MAX232 series of
integrated circuits from Maxim Integrated Products
(www.maximintegrated.com). These are all excellent
devices that can be used for speeds of up to 115.2
kbps. We do caution that many of these devices are
only rated for operation up to 120 kbps, however, so
may not be suitable for very high data rates - be sure
to check the manufacturers data sheet before
committing to a design.
An RS232 interface is typically limited to operating
at speeds of about 230.4 kbps maximum. If you wish
to go higher than that, then you must consider
alternatives - one of which is to use USB.
Almost all computers that are made today have
replaced the once familiar RS232 port with a USB one.
Software is readily available to make these look like
traditional serial (‘COM’) ports, and modules are
available for connecting to circuits like the ELM327.
Several manufacturers offer these modules (often
called ‘bridge’ circuits) – try the CP2102 from Silicon
Labs (www.silabs.com) or the FT232R from Future
Technology Devices (www.ftdichip.com). If planning to
use higher baud rates, these interfaces are essential.
We are often asked if it is possible to use a direct
ELM327L DSA
connection to a microprocessor. That is certainly an
option, and one that allows a full speed connection at
essentially zero cost. If you are developing such an
interface, refer to page 77 for more details.
The default configuration for the ELM327 typically
provides a data rate of 38400 baud. There are two
ways that this rate can be changed – either
permanently with a Programmable Parameter, or
temporarily with an AT command.
Programmable Parameter ‘0C’ is the ELM327
device setting that stores the baud rate divisor. The
value is stored in ‘non-volatile’ memory (EEPROM)
that is not affected by power cycles or resets (but
changing this value may affect the operation of some
software packages, so be careful how you use it).
If you store a new value in PP 0C, then enable it,
your stored rate will become the new data rate after
the next reset. As an example, perhaps you would like
to have the ELM327 use a baud rate of 57.6 kbps,
rather than the factory setting of 38.4 kbps. To do this,
you need to determine the required value for PP 0C,
store this value in PP 0C, and then enable the PP.
The value stored in PP 0C is actually an internal
divisor that is used to determine the baud rate (the
baud rate in kbps is 4000 divided by the value of
PP 0C). To obtain a setting of 57.6 then, a baud rate
divisor of 69 is required (4000/69 is approximately
57.6). Since 69 in decimal is 45 in hexadecimal, you
need to tell the ELM327 to set the value of PP 0C to
45, with the set value command:
>AT PP 0C SV 45
then enable the new PP 0C value for use:
>AT PP 0C ON
from that point on, the default data rate will be 57.6K,
and not 38.4K. Note that the value that you write does
not become effective until the next full reset (a power
off/on, AT Z, or MCLR pulse).
If you are designing your own circuitry, you will
know what your circuit is capable of, and can assign a
baud rate with PP 0C. Software developers will not
usually know what hardware is to be connected,
however, so will not know what the limitations are. For
these users, we have provided the BRD command.
This command allows a new baud rate divisor to
be tested, and then accepted or rejected depending on
the results of the test. The chart shown here tries to
explain just how this command should be used.
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ELM327L
Using Higher RS232 Baud Rates (continued)
The sequence begins with the PC making a
request for a new baud rate divisor, with the BRD
command. For example, to try the 57.6K rate that was
previously discussed, the controlling PC would send:
AT BRD 45
If the ELM327 firmware is a very old version, it will
not support this command and will return with the
familiar ‘?’ response. If it does support the command, it
will respond with ‘OK’, so the software knows whether
to proceed or not. No prompt character follows the
‘OK’ reply; it is followed only by a carriage return
character (and optionally, a linefeed character).
Having sent the ‘OK’, the ELM327 then switches
to the new (proposed) baud rate, and then simply
waits the time set by the BRT command (it is 75 msec
by default). This period is to allow the PC sufficient
time to change its baud rate. When the time is up, the
ELM327 then sends the ID string (currently ‘ELM327
v2.1’) to the PC at the new baud rate (followed by a
carriage return character and optionally, a linefeed
character) and waits for a response.
Knowing that it should receive the ELM327 ID
string, the PC software compares what was actually
received to what was expected. If they match, the PC
responds with a carriage return character, but if there
is a problem, the PC sends nothing. The ELM327 is
meanwhile waiting for a valid carriage return character
to arrive. If it does (within 75 msec), the proposed
baud rate is retained, and the ELM327 says ‘OK’ at
this new rate. If it does not see a carriage return in the
75 msec ‘window’, the baud rate will revert back to the
original rate. Note that the PC might correctly output
the carriage return at this new rate, but the interface
circuitry could corrupt the character, and the ELM327
might not see a valid response, so your software must
check for an ‘OK’ response before assuming that the
new rate has been accepted.
Using this method, a program can quickly try
several baud rates, and determine the most suitable
one for the connected hardware. The new baud rate
will stay in effect until reset by an AT Z, a Power
off/on, or a MCLR input. It is not affected by the AT D
(set Defaults), or AT WS (Warm Start) commands.
PC
ELM327
Request for a new
baud rate divisor:
AT BRD hh
ELM327 responds
with ‘OK’
(if it is supported)
Program switches to
the new baud rate,
and waits for input
ELM327 switches to
new baud rate and
waits for 75 msec*
ELM327 sends
the AT I string
If the Rx is good,
Program sends a
carriage return
ELM327 waits
up to 75 msec*
for a carriage return
CR
received
?
no
Baud rate reverts
to the previous
baud rate
yes
ELM327 says ‘OK’
(and remains at the
new baud setting)
Print a prompt,
and wait for the
next command
* the 75 msec time is adjustable
with the AT BRT hh command
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ELM327L
Setting Timeouts - AT ST and AT AT Commands
Users often ask about how to obtain faster OBD
scanning rates. There is nothing that we can do about
how fast (or slow) a vehicle is to respond, but we can
optimize how the ELM327 handles the responses.
A typical vehicle request and response is shown in
the diagram below:
request is sent
ELM327
response
enabled by default, but can be disabled with the AT0
command should you not agree with what it is doing
(there is also an AT2 setting that is a little more
aggressive, should you wish to experiment). For 99%
of all vehicles, we recommend that you simply leave
the settings at their default values, and let the ELM327
make the adjustments for you.
Consider the following times taken from a J1850
VPW vehicle, in response to an 01 00 request:
ELM327
4 msec
Vehicle
58 msec
ELM waits up
to 200 msec
ELM waits 200 msec
for more responses
The ELM327 sends a request then waits up to
200 msec for a reply. If none were to come, an internal
timer would stop the waiting, and the ELM327 would
print ‘NO DATA’.
After each reply has been received, the ELM327
must wait to see if any more replies are coming (and it
uses the same internal timer to stop the waiting if no
more replies arrive). With our initial OBD products (the
ELM320, ELM322 and ELM323) we found that older
vehicles often needed a timeout setting of about 100
msec, and occasionally needed more, so we settled on
a standard default setting of 200 msec.
If a typical vehicle query response time were about
50 msec, and the timeout were set to 200 msec, the
fastest scan rate possible would only be about 4
queries per second. Changing the ST time to about
100 msec would almost double that rate, giving about
7 queries per second. Clearly, if you were to know how
long it takes for your vehicle to reply, you might be
able to improve on the scan rate, by adjusting the ST
time.
It is not easy to tell how fast a vehicle replies to
requests. For one thing, requests all have priorities
assigned, so responses may be fast at some times,
and slower at others. The physical measurement of
the time is not easy either - it requires expensive test
equipment just to make one measurement. To help
with this, we added a feature to the ELM327 called
‘Adaptive Timing’.
Adaptive Timing actually makes the response time
measurements for you, and adjusts the AT ST time to
a value that should work for most situations. It is
ELM327L DSA
Vehicle
engine
(ECU 10)
transmission
(ECU 18)
The engine controller responds very quickly, but
the transmission takes considerably longer. The
adaptive timing algorithm measures the longer
transmission response times and will use them to set
the timeout, likely to a value in the range of 90 msec.
With a timeout of 90 msec, the maximum scan rate
would be about 6 readings per second.
Surely there has to be a way to eliminate that final
timeout, if you know how many responses to expect?
There is a way – instead of sending 01 00 for the
above request, the ELM327 will also accept 01 00 2.
This tells the IC to send 01 00, then return immediately
after receiving 2 responses. It can not speed up a slow
ECU, but it will eliminate the final delay, as the
ELM327 knows the number of responses to expect.
This one change might give you 10 to 12 responses
per second, instead of the 6 obtained previously.
We do caution you to use this feature carefully. If
you set the last digit to a number that is less than the
actual number of responses, then acknowledgements
that may be required will not be sent, and some
protocols may begin resending the replies, looking for
a response. This will lead to unnecessary network
congestion, which must be avoided. Before using this
feature, always determine the number of responses
that will be coming from the vehicle, and then set the
responses digit to that value.
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ELM327L
SAE J1939 Messages
The SAE J1939 CAN standard is being used by
many types of heavy machinery – trucks, buses, and
agricultural equipment, to name a few. It uses the
familiar CAN (ISO 11898) physical interface, and
defines its own format for data transfer (which is very
similar to the ISO 15765 standard that is used for
automobiles).
The following will discuss a little of how data is
transferred using the J1939 standard. Considerably
more information is provided in the Society of
Automotive Engineers (SAE) standards documents, so
if you are going to be doing a lot of work with J1939, it
may be wise to purchase copies of them. At minimum,
the J1939-73 diagnostics, the J1939-21 data transfer,
and the J1939-71 vehicle application documents
should be purchased. Another great reference for this
work is the HS-J1939 two book set, also available from
the SAE.
The current version of the J1939 standard allows
only one data rate (250 kbps), but work is underway to
amend the standard so that an alternate rate of
500 kbps will also be allowed. For the purpose of this
discussion, the data rate is not important - it is the
format of the information that we will discuss.
All CAN messages are sent in ‘frames’, which are
data structures that have ID bits and data bytes, as
well as checksums and other items. Many of the J1939
frames are sent with eight data bytes, although there is
no requirement to do so (unlike ISO 15765, which
must always send eight data bytes in each frame). If a
J1939 message is eight bytes or less, it will be sent in
one frame, while longer messages are sent using
multiple frames, just like ISO 15765. When sending
multiple frames, a single data byte is used to assign a
‘sequence number’, which helps in determining if a
frame is missing, as well as in the reassembly of the
received message. Sequence numbers always start
with 01, so there is a maximum of 255 frames in a
message, or 1785 bytes.
One major feature of the J1939 standard is its very
orderly, well defined data structures. Related data is
defined and specified in what are called ‘parameter
groups’. Each parameter group is assigned a
‘parameter group number’, or PGN, that uniquely
defines that packet of information. Often, the
parameter groups consist of eight bytes of data (which
is convenient for CAN messages), but they are not
restricted to this. Many of the PGNs, and the data
within them (the SPNs) are defined in the J1939-71
document, and manufacturers also have the ability to
ELM327L DSA
define their own proprietary PGNs.
The ID portion of a J1939 CAN frame is always 29
bits in length. It provides information as to the type of
message that is being sent, the priority of the
message, the device address that is sending it, and
the intended recipient. Information within the ID bits is
divided roughly into byte size pieces as follows:
3 bits 2 bits
Priority
8 bits
8 bits
8 bits
PDU
Format
Destination
Address
Source
Address
PDU1 Format
The data structure formed by the 29 bit ID, and the
associated data bytes is called a Protocol Data Unit, or
PDU. When the ID bits have a destination address
specified, as is shown above, it is said to be a PDU1
Format message.
The two bits shown between the Priority and the
PDU Format are known as the Extended Data Page
(EDP), and the Data Page (DP) bits. For J1939, EDP
must always be set to ‘0’, while the DP bit is used to
extend the range of values that the PDU Format may
have. While the DP bit is typically ‘0’ now, that may not
be true in the future.
Not all J1939 information is sent to a specific
address. In fact, one of the unique features of this
standard is that there is a large amount of information
that is being continually broadcast over the network,
with receivers using it as they see fit. In this way,
multiple devices requiring the same information do not
have to make multiple requests to obtain it, information
is provided at regular time intervals, and bus loading is
reduced.
If information is being broadcast over the network
to no particular address, then the Destination Address
field is not required. The eight bits can be put to better
use, possibly by extending the PDU Format field. This
is what is done for a PDU2 Format frame, as shown
here:
3 bits 2 bits
Priority
8 bits
8 bits
8 bits
PDU
Format
Group
Extension
Source
Address
PDU2 Format
So how does one know if they are looking at a
PDU1 Format frame that contains an address, or a
PDU2 Format frame that does not? The secret lies in
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ELM327L
J1939J1939
SAE
Support
Messages
(continued)
(continued)
the values assigned to the PDU Format field. If the
PDU Format value begins with ‘F’ (when expressed as
a hexadecimal number), it is PDU2. Any other value
for the first digit means that it is a PDU1 Format frame,
which contains an address.
To summarize, PDU1 format frames are sent to a
specific address, and PDU2 frames are sent to all
addresses. To further complicate matters, however,
PDU1 frames may be sent to all addresses. This is
done by sending the message to a special ‘global
address’ which has the value FF. That is, if you see a
PDU1 message (where the first digit of the PDU
Format byte is not an F), and the Destination Address
is FF, then that message is being sent to all devices.
The J1939 recommended practices document
provides a list of addresses that should be used by
devices. It is particularly important to adhere to this list
with the ELM327, as the IC uses a fixed address
method and is not able to negotiate a different one, per
J1939-81. OBD Service Tools should use either F9 or
FA as their address (the ELM327 uses F9). If you wish
to change this, you can use the AT TA (tester address)
command, or simply define it with the header.
You may use either the AT SH or the AT CP and
AT SH commands to assign values for the J1939 ID
bits, just the same as with the other CAN protocols.
How these are used was discussed previously, but we
Use either:
or:
will repeat it here.
Since the priority (and DP and EDP) values only
rarely change, they may be assigned with the CP
command. By default, the ELM327 uses a priority of 6
(binary 110), and sets the EDP and DP bits both to 0.
The default value for the CP setting is then 110+0+0
(which would be entered as 11000 or 18 in hex). The
values for the other bytes of the J1939 ID are entered
with the AT SH command, as shown in Figure 5.
This has tried to cover the basics of the J1939
message structure, but if you want more information,
you should look at the standards mentioned
previously. One other one that gives good examples of
actual data is J1939-84 which describes the
compliance tests and shows the expected responses.
Even at 250 kbps, J1939 data is transferred at a
rate that is more than ten times faster than the
previous heavy duty vehicle standard (SAE J1708),
and several of the light duty standards. As designers
build more into each system, the amount of
information required continues to grow, however, so
the 500 kbps version of J1939 will be a welcome
addition.
>AT SH vv xx yy zz
>AT CP vv
and
>AT SH xx yy zz
The values will be used as follows:
vv
xx yy zz
5 bits
only
Priority
PDU
Format
Destination
Address
Source
Address
Figure 5. Setting the J1939 CAN ID
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ELM327L
Using J1939
This section provides a few examples which show
how to monitor an SAE J1939 data bus, and how to
make requests of devices that are connected to it.
To begin, you will need to configure the ELM327
for J1939 operation, at the correct baud rate. Protocol
A is predefined for J1939 at 250 kbps, which is what
most applications require. To use protocol A, send:
>AT SP A
Protocols B and C may also be used with J1939,
if you wish to experiment with other baud rates. To use
them for J1939, the option value (in PP 2C or 2E) must
be set to 42, and the baud rate divisor (in PP 2D or 2F)
must be set to the appropriate value. Perhaps the
simplest way to provide an alternate rate is to use the
AT PB command, that allows you to set both the
options byte (which is always 42), and the baud rate
divisor (which is 500k ÷ the desired baud rate) at the
same time. For example, to set protocol B for J1939
operation at 500 kbps, simply send:
>AT PB 42 01
then send:
>AT SP B
to begin. Note that this setting will not be maintained if
the IC is reset, so if you want a more permanent
setting, you should store the values in PP 2C and 2D.
J1939 often provides slowly broadcast information,
and for this reason, the ELM327 automatically sets a
response timeout (AT ST time) as required by the
message (typically 1.25 seconds, but it varies). If this
is too long for you, or if you are using an older version
of our product (ie. v1.3a or older), then you might want
to set this time manually (we recommend starting with
AT ST FF for the older ICs). You will not do any harm
if you set the timeout with a newer chip, but the timing
will not be optimal as you will stop the ELM327 from
varying the setting based on the type of message
being received.
The ELM327 also offers one other variation on the
timer setting - the ability to extend the AT ST time by
switching a x5 timer multiplier on (see the JTM5
command description). This may be useful or even
necessary when requesting data that will have a
multiline response, if similar data is already flowing. In
these circumstances, there can only be one message
like this at a time on the bus, so the response to your
ELM327L DSA
request might have to wait while an initial response
completes (and this could take more than the normal
ST time, since broadcast responses must be spaced
at least 50 msec apart). If you know that a reply should
be coming, and you are seeing ‘NO DATA’ responses,
then send AT JTM5 and try it again, as that may be
the problem. Restore the timer multiplier to normal with
AT JTM1.
Once the J1939 protocol is selected, and the
timeout value has been adjusted, the ELM327 is ready
for a command. The first one that we will send is called
a DM1 or ‘diagnostic message 1’, which provides the
currently active diagnostic trouble codes. DM1 is one
of more than 50 predefined diagnostic messages, and
is special in that it is the only one that is broadcast
continually over the bus at regular intervals. The
ELM327 has an AT command that is used to obtain
the DM1 trouble codes:
>AT DM1
If you are connected to a vehicle, you should now
see messages printed at one second intervals. If you
are only connected to a single device (for example,
with a simulator on the bench, or to a device with a
single CAN data port), you may see data with
<RX ERROR printed beside it. This is because the
receipt of the data is not being acknowledged by any
device on the bus (certainly not the ELM327, as it is by
default a completely silent monitor). See our ‘AN05 Bench Testing OBD Interfaces' application note for
more information on this, and some advice on what to
do. If you have a v1.4b or newer chip, you do not have
to take special measures, however. Simply turn off the
silent monitoring with:
>AT CSM 0
and there should be no more RX ERRORs. Once you
have this sorted out, repeat the request. If all goes
well, you should see several replies, similar to this:
00 FF 00 00 00 00 FF FF
00 FF 00 00 00 00 FF FF
You will likely need to stop the flow of data by
pressing any key on the keyboard. This is because the
DM1 command is actually a special form of a
monitoring command, and all monitoring needs to be
stopped by the user. The response means that there
are currently no active trouble codes, by the way.
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ELM327L
Using J1939 (continued)
To see the exact same response, you can also
Monitor for PGN 00FECA (which is the code for DM1):
>AT MP 00FECA
Note that the ELM327 requires that you send hex
digits for all data, as shown above (and as used by all
other protocols). Many of the PGN numbers are listed
in the J1939 standard as both a decimal and a hex
number, so choose the hex version.
You will likely find in your testing that the PGNs
you encounter often begin with a 00 byte as above. To
simplify matters for you, the ELM327 has a special
version of the MP command that will accept a four digit
PGN, and assumes that the missing byte should be
00. An equivalent way to ask for 00FECA is then:
>AT MP FECA
which is a little more convenient.
One feature of the ELM327 is the ability to tell the
IC how many messages to retrieve when monitoring
for PGNs. For example, to see only two responses to
the MP FECA command, send:
>AT MP FECA 2
This saves having to send a character to stop the
flow of data, and also is very convenient when dealing
with multiline messages. While the standard OBD
requests allow you to define how many frames (ie
lines) of information are to be printed with a similar
single digit, the single digit with the MP command
actually defines how many complete messages to
obtain. For example, if the DM1 message is 33 lines
long, then sending AT MP FECA 1 will cause the
ELM327 to show all 33 lines, then stop monitoring and
print a prompt character.
By default, all J1939 messages have the ‘header’
information hidden from view. In order to see this
information (actually the ID bits), you will need to turn
the header display on:
>AT H1
A single response to FECA might then look like:
>AT MP FECA 1
6 0FECA 00 00 FF 00 00 00 00 FF FF
Notice that the ELM327 separates the priority bits
from the PGN information. The ELM327 also uses only
ELM327L DSA
one digit to represent the two extra PGN bits, both of
which may seem unusual if you are used to different
software. We find this a convenient way to show the
actual J1939 information in the header.
If you prefer to see the ID bits separated into bytes
instead, simply turn off the J1939 header formatting
with:
>AT JHF0
Repeating the above request would then result in
a response of this type:
>AT MP FECA 1
18 FE CA 00 00 FF 00 00 00 00 FF FF
The differences are clearly seen. If displaying the
information in this manner, remember that the first
‘byte’ shown actually represents five bits, and of them,
the leftmost three are the priority bits.
The MP command is very useful for getting
information in a J1939 system, but not all information
is broadcast. Some information must be obtained by
making a query for it. Just like the other OBD requests
where you specify the information that you need (with
a mode and a PID), to make a query in a J1939
system, you provide the PGN number and the system
responds with the required data.
For example, to request the current value of the
engine coolant temperature (which is part of PGN
00FEEE), you would send a request for PGN 00FEEE,
and extract the data. To do this, send:
>00FEEE
to which you might receive:
6 0FEEE 00 8C FF FF FF FF FF FF FF
if the headers were on. Note that if you request a PGN
that is already being broadcast, you may very well
receive many replies, as the ELM327 configures itself
to receive anything that is related to the PGN
requested.
If you are familiar with the J1939 standard, you will
be aware that it actually specifies a reverse order for
the sending of the data bytes of a PGN request. That
is, the data bytes for the above request are actually
sent as EE FE 00, and not as 00 FE EE. Since it can
be very confusing to have to reverse some numbers
and not others, the ELM327 automatically handles this
for you, reversing the bytes provided. In this way, you
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ELM327L
Using J1939 (continued)
can directly request PGNs using numbers as they are
written on the page (if they are written as hex digits),
and the ELM327 will make it work for you. If you do not
want the ELM327 to alter the byte order, the feature
can be disabled (by sending an AT JS command).
The ELM327 always assumes that when you start
making requests of this type, you do not know what
devices are connected to the J1939 bus. That is, by
default the ELM327 sends all requests to the 'global
address' (ie all devices), and then looks for replies.
Often, this works well, but J1939 devices are not
required to respond to such general inquiries, and may
not if they are busy. For this reason, it is usually better
to direct your queries to a specific address, once it is
known.
In order to determine the address to send to, you
may have to monitor the information on the bus for a
while. Make sure that the headers (ID bits) are being
displayed, and note what is shown in the Source
Address position, which is immediately before the data
bytes. In the previous example, this would be 00
(which J1939 defines as the address for engine #1).
As an example, let us assume that it is engine #1 that
you wish to direct your queries to. To do this, you will
want to change the Destination Address from FF (the
global address) to 00 (engine #1).
By default, the ELM327 uses 6 0EAFF F9 for the
ID bits of all requests (or 18 EA FF F9 if you prefer).
That is, it uses a priority of 6, to make a request (EA)
to the global address (FF) by the device at F9 (the
scan tool). An EA request is often referred to as a
request using PGN 59904 as the address EA00 in hex
is 59904 in decimal.
Since you only wish to alter the EAFF F9 portion
of the ID bits and not the priority, you may do this with
the three byte ‘set header’ command:
>AT SH EA 00 F9
The priority bits rarely need to be changed, but if
you do need to change them, it may be done with the
CAN Priority (AT CP) command.
After making the above change, all data requests
will be directed to address 00 (the engine), so don’t
forget to change the headers if you wish to again make
global requests. Note that the AT SH command allows
you to change the source (or tester) address at will, so
be careful with this as addresses should really be
negotiated using the method described in J1939-81
and you might conceivably choose an address that is
ELM327L DSA
already in use. The current version of the ELM327
does not support J1939-81 address negotiation, so
can not obtain an address for you.
Once the ELM327 has been configured to send all
messages to address 00, repeat the request:
>00FEEE
6 0E8FF 00 01 FF FF FF FF EE FE 00
This response is of the ‘acknowledgement’ type
(E8), which is being broadcast to all (FF) by the device
with address 00. The last three data bytes show the
PGN requested, in reverse byte order, so we know this
is a response to our request. Looking at the other data
bytes, the first is not 00 (which we would expect for a
positive acknowledgement), it is 01 which means
negative acknowledgement. Since all requests to a
specific address must be responded to, the device at
address 00 is responding by saying that it is not able
to respond. That is, retrieve the information using the
MP command.
If the ECU had been able to reply to the request,
the format of the response would have been slightly
different. For example, if a request for engine run time
(PGN 00FEE5) had been made, the response might
have been like this:
>00FEE5
6 0FEE5 00 80 84 1E 00 FF FF FF FF
Notice that the PGN appears in the header for
these types of replies, and the data bytes are those
defined for the SPNs in the PGN.
All responses to a request are printed by the
ELM327, whether they are a single CAN message, or
a multisegment transmission as defined by the
transport protocol (J1939-21). If the responses are
multisegment, the ELM327 handles all of the
negotiation for you. As an example, a multisegment
response to a DM2 request might look like this:
>00FECB
012
7 0EBF9 00 01 04 FF 50 00 04 0B 54
7 0EBF9 00 02 00 00 01 5F 05 02 31
7 0EBF9 00 03 6D 05 03 03 FF FF FF
if the headers are on, and would appear as:
>00FECB
012
01: 04 FF 50 00 04 0B 54
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ELM327L
Using J1939 (continued)
02: 00 00 01 5F 05 02 31
03: 6D 05 03 03 FF FF FF
if the headers are off. Note that multiframe messages
always send eight bytes of data, and fill in unused byte
positions with FFs.
With the headers off, a multiline response looks
very similar to the multiline responses for ISO15765-4.
The first line shows the total number of bytes in the
message, and the other lines show the segment
number, then a colon, and the data bytes received.
Note that the byte count is a hexadecimal value (ie the
‘012’ shown means that there are 18 bytes of data).
The single line that shows 012 in the above (the
total number of data bytes) is actually a special type of
response, called a ‘Connection Management’ or
‘TP.CM’ message. It has a specific format, but the only
bytes that are typically relevant are those that provide
the total message size in bytes, so that is all that the
ELM327 normally shows. In order to see the other
bytes, you must turn CAN Auto Formatting off (AT
CAF0), and then repeat the request. Note that this will
only show the entire TP.CM message if you have a
version 1.4b or newer ELM327.
This has been a brief description of how to use
the ELM327 in a typical J1939 environment. If you can
monitor for information, make global requests as well
as specific ones, and receive single or multiframe
responses, then you have the tools necessary to
diagnose most vehicle problems.
The FMS Standard
Several European heavy duty truck and bus
manufacturers have joined to form an organization for
standardizing the way in which information is retrieved
from these large vehicles. The result of their work is
the FMS (or Fleet Management Systems) Standard,
and the Bus-FMS Standard.
The FMS standard is based on a subset of the
250 kbps J1939 protocol, which uses only broadcast
messages for the information. In order to not
compromise the integrity of the vehicle’s CAN bus, the
standard also specifies a gateway device to provide
separation between (potentially unskilled) users and
the critical control information on the vehicle.
The information contained in the FMS messages
is defined by PGNs, using the same PGN numbers as
for J1939. The difference is that they only define a
small subset of those specified by J1939.
To monitor the information provided by an FMS
gateway, simply use the AT MP command with the
appropriate PGN number. We should caution that
some information (VIN, software version, etc.) is only
transmitted every 10 seconds, so some patience is
required when waiting for the data.
ELM327L DSA
The FMS standard is completely open, and still
evolving (as of this writing, the most recent update was
version 2.00). For more information, visit the FMS web
sites:
FMS Standard
www.fms-standard.com
Bus FMS Standard
bus-fms-standard.com
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ELM327L
The NMEA 2000 Standard
We are occasionally asked about support for the
NMEA 2000 marine standard. Elm Electronics does
not provide specific support for this protocol, but our
ELM327 integrated circuit is very capable of working
with it.
While the physical connectors may look quite a bit
different than those used for J1939, the CAN interface
and the data format is almost identical to that of the
J1939 standard. NMEA 2000 uses a 250 kbps data
rate, so the easiest way to get started is to select the
ELM327’s predefined protocol A. This is done with the
set protocol to A command:
>AT SP A
When you are finished and want to use the
ELM327 for standard OBDII protocols, don’t forget to
send the AT SP 0 command to reset it.
Many of the PGNs used for NMEA 2000 have
values that are greater than 65535, so the DP bit is
usually set. To monitor for most PGNs then, you can
not use the short version of the MP command. For
example, to monitor for the Engine Parameters PGN
(127488 or hex 1F200), you can not use:
>AT MP 1F200
as the ELM327 actually interprets that as a request for
PGN 1F20, and get 0 replies. To monitor for PGN
1F200, you must send:
>AT MP 01F200
If you keep the above in mind, the ELM327 will
prove to be a handy tool to use while experimenting
with NMEA 2000. It does have a couple of limitations
that must be kept in mind, though. As mentioned with
J1939, it is not capable of address negotiation. Also,
the ELM327 does not currently support the Fast
Packet protocol, which may be an issue for some
users.
For more information on the NMEA 2000 standard,
visit the NMEA web site (http://www.nmea.org).
Periodic (Wakeup) Messages
After an ISO 9141 or ISO 14230 connection has
been established, there needs to be periodic data
transfers in order to maintain that connection, and
prevent it from ‘going to sleep.’ If normal requests and
responses are being sent, that is usually sufficient, but
the ELM327 occasionally has to create its own
messages, to prevent the connection from timing out.
We term these periodic messages that are created
the ‘Wakeup Messages’, as they keep the connection
alive, and prevent the circuitry from going back to an
idle or sleep mode. (Some texts refer to these
messages simply as ‘idle messages.’) The ELM327
automatically creates and sends these for you if there
appears to be no other activity – there is nothing that
you need do to ensure that they occur. To see that
these are being sent, simply watch the OBD transmit
LED – you will see the periodic ‘blips’ as the ELM327
sends each one.
The ELM327 normally sends wakeup messages
after 3 seconds of no activity (this time is adjustable
with the AT SW command). This is within the five
second limit set by the standard.
The default content of these messages vary
depending on the protocol - for ISO 9141, the ELM327
ELM327L DSA
will send 68 6A F1 01 00, and it sends C1 33 F1 3E for
ISO 14230 (KWP). If you would prefer that a different
message be sent, simply use the Wakeup Message
command to define it.
For example, if you would like to send the data
bytes 44 55 with the header bytes set to 11 22 33,
simply send the command:
>AT WM 11 22 33 44 55
and from that point forward, every wakeup message
that the ELM327 sends will be 11 22 33 44 55 (with a
checksum byte following). You do not provide the
checksum byte - it is automatically added for you.
You can change these as often as you want, the
only restriction being that every time you do, you must
provide the complete message – the header bytes and
the data bytes. The current version of the ELM327
allows for messages of one to six bytes total, not
including the checksum.
The ELM327 does not currently support periodic
messages for the CAN protocols. If you require that
function, please consider using our ELM329 CAN
Interpreter product.
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ELM327L
Altering Flow Control Messages
ISO 15765-4 (CAN) provides for only eight data
bytes per frame of data. Of course, there are many
cases where the data which needs to be sent is longer
than 8 bytes, and CAN has made provision for this by
allowing data to be separated into segments, then
recombined at the receiver.
To send one of these multi-line messages, the
transmitter in a CAN system will send a ‘First Frame’
message, and then wait for a reply from the receiver.
This reply, called a ‘Flow Control’ message contains
information concerning acceptable message timing,
etc., and is required to be sent before the transmitter
will send any more data. For ISO 15765-4, the type of
response is well defined, and never changes. The
ELM327 will automatically send this response for you
as long as the CAN Flow Control option is enabled
(CFC1), which it is by default.
In order to provide complete control over the
sending of Flow Control messages and their content,
the ELM327 defines several AT FC commands.
The way in which the ELM327 responds to a First
Frame message is determined by the Flow Control
‘mode’, as set by the AT FC SM command. There are
currently three modes, as shown in the chart. The
default Flow Control mode is number ‘0’, meaning that
the ELM327 will do everything for you.
Flow Control mode 1 is provided for those users
that want complete control over their Flow Control
messages. To use it, you must define both the CAN ID
(header) and the data bytes that you require to be sent
in the response to a First Frame message. Note that if
you try to set the mode before defining these values,
you will get an error:
>AT FC SM 1
?
message that you have defined (7E8 00 11 22 in this
example). Note that we show 11 bits for the ID in this
example, but you may also define 29 bit IDs.
The third mode currently supported allows setting
the data bytes which are to be sent, but not the ID bits.
The ELM327 sets the ID bits in mode 2 from those
which were received in the First Frame message – it
does not change them at all. To use this mode, first
define your data bytes, then activate the mode:
>AT FC SD 00 11 22
OK
>AT FC SM 2
OK
At any time while you are experimenting, if you
should wish to restore the automatic Flow Control
responses, simply set the mode to 0:
>AT FC SM 0
OK
This will immediately restore the responses to their
(ISO 15765-4) default settings.
For most people, there will be little need to
manipulate these ‘Flow Control’ messages, as the
defaults are designed to work with the CAN OBD
standards. If you wish to experiment, these special AT
commands offer that control for you.
The following chart summarizes the currently
supported flow control modes:
FC
Mode
ELM327
Provides
0
ID Bits &
Data Bytes
no values
1
no values
ID Bits &
Data Bytes
2
ID Bits
Data Bytes
You must set the headers and data first:
>AT FC SH 7E8
OK
>AT FC SD 00 11 22
OK
User
Provides
Flow Control Modes
and then you can set the mode:
Note that the ELM327 will only send Flow Control
messages if the protocol’s data format is ISO15765-4.
>AT FC SM 1
OK
From this point on, every First Frame message
received will be responded to with the custom
ELM327L DSA
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ELM327L
Using CAN Extended Addresses
Some vehicles with CAN interfaces use a data
format that is slightly different from what we have
described so far. The data packets look very similar,
except that the first byte is used for the receiver’s (ie
target’s) address. The remaining seven bytes are used
as described previously.
We refer to this type of addressing as ‘CAN
Extended Addressing’, and provide support for it with
the AT CEA command. Perhaps an example would
best describe how to use it.
Here is a portion of a data transfer that was taken
from a vehicle. For the moment, ignore the first data
bytes on each line and only look at the remaining data
bytes (that are outlined in grey):
7B0
7C0
7B0
7C0
7B0
7C0
7C0
7C0
04
F1
04
F1
04
F1
F1
F1
02
02
02
10
30
20
21
22
10
50
21
16
FF
DF
02
09
81
81
A2
61
00
01
05
01
00
00
00
A2
00
00
DF
00
00
00
00
01
00
04
01
04
00
00
00
02
00
09
00
01
00
00
00
05
00
01
04
00
If you are familiar with the ISO 15765 data format,
you will be able to recognize that the data bytes shown
inside the box seem to conform to the standard. The
rows that begin with 02 are Single Frames, the one
that starts with 10 is a First Frame, while the one with
30 is a Flow Control, and the others are Consecutive
Frames.
The remaining bytes, shown outside the box, are
the standard 11 bit CAN ID, and an extra address
byte. Those that use F1 for the extra address are
directed to the scan tool (all scan tools generally use
F1 as the default address), and the other lines are
being sent to the vehicle (at address 04).
The ELM327 is able to handle these types of
messages, but requires some setup first. For example,
assume that the messages use 11 bit IDs, with
ISO 15765 formatting, and the baud rate is 50 kbps.
The PB command to configure protocol B for this is:
>AT PB 81 0A
OK
Next, we’ll need to receive all messages with an
ID of 7C0:
and send with an ID (header) of 7B0:
>AT SH 7B0
OK
Notice that there was a flow control message that
was sent in this group, but it’s not quite the same as
the one for OBD systems. For this reason, you’ll need
to define your own flow control with the following three
statements (we won’t show the OK’s any more, to
save space):
>AT FC SH 7B0
>AT FC SD 04 30 FF 00
>AT FC SM1
The final setup statement that you will need is to
tell the ELM327 to send to CAN Extended Address 04:
>AT CEA 04
Now everything is configured. Next, tell the IC to
use this protocol, and to bypass any initiation (as it is
not standard OBD, and would likely fail):
>AT SP B
>AT BI
That’s all. To exactly reproduce the flow of data
shown, you only need to send the relevant data bytes
and the ELM327 will add the rest:
>10 81
50 81
>21 A2
016
0: 61 A2
0: DF 01
1: 02 05
2: 09 01
01
00
DF
00
02
04
01
04
05
09 01
00 04
01 00
Notice that for some reason, this vehicle has sent
two segment 0’s, but that just means that it doesn’t
exactly follow the ISO 15765 protocol. The above
shows what the responses would look like with
formatting on, and headers off. If you change either,
the data exchange would look more like what we
initially showed.
>AT CRA 7C0
OK
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ELM327L
CAN Input Frequency Matching
Most modern vehicles have a CAN network
connected to pins 6 and 14 of the OBD connector. At
one time, however, the use of these pins was left to
the vehicle manufacturer, and a number of different
systems were connected to them.
In order to prevent the disruption of any connected
systems while the ELM327 is searching for a protocol
(it sends out requests during a search), the ELM327
now performs several tests on these wires. Prior to
firmware version 2.1, the tests simply looked for
activity on the wires but were not frequency selective.
This meant that, for example, vehicles that had a
speedometer signal connected to either pin might be
seen as a valid CAN network, and the ELM327 may
have sent a request on those wires. The new firmware
actually measures the input frequency and requires
that it matches that of the selected CAN protocol
before any test message can be sent.
The diagram below shows how the logic works. It
may seem a little complicated, but what it really says is
that for the default settings, a send is allowed if the
input signal frequency matches the CAN setting (250
or 500 kbps), or if there appears to be no signal. In
addition, if the user is trying a non-standard OBD
frequency, but a standard frequency is received, a
send will not be allowed.
All bits of PP 28 are set to 1 by default (requiring
frequency matching, unless no signal is detected), but
may be changed at any time – see the Programmable
Parameters section for details.
This logic is only used while searching for a valid
protocol. Once a particular protocol is considered to be
active, no further frequency checks are made (as it is
time consuming). Note that if you should use the AT BI
command to bypass the initiation process, this
frequency matching test will also bypassed.
signal is 500 kbps
CAN Rx
(pin 24)
setting is
500 kbps
PP 28, b7 is 0
signal is 250 kbps
Signal
Processing
setting is
250 kbps
allow a
CAN send
PP 28, b6 is 0
signal is not 250 or 500 kbps
setting is not
250 or 500 kbps
input is quiet
PP 28, b0 is 1
Send Logic While Searching for a Protocol
ELM327L DSA
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ELM327L
Programming Serial Numbers
A number of our customers have asked for ways
to uniquely identify a product that uses our ELM327
integrated circuit. While this is often a request for a
means to store a ‘serial number’, people have also
asked for a way to store dates, and version codes, too.
The @2 and @3 commands were created to assist
with this.
If you send the command AT @2 to a new
ELM327 integrated circuit, you will receive an error.
That is, you will see a response that looks like this:
>AT @2
?
In the above dialog, the ELM327 is trying to tell
you that either the chip is very old and does not
support the @2 command, or that nothing has been
programmed into these memory locations yet.
To program characters into the @2 memory, you
must provide exactly 12 characters using the AT @3
command. These characters must be in the ASCII
printable group, in the range from ‘!’ (hex value 21) to
‘_’ (hex value 5F). Typically, an AT @3 command use
would look like:
>AT @3 MYBOARD_9906
OK
This number can never be altered once it is
entered, so you must be sure that you are entering the
values properly. If developing code which does this,
you may find that purchasing an ELM328 IC will save
the expense of trial and error. The ELM328 does not
support OBD protocols however, so is not a viable
option for other uses.
Once the @3 code is set, it will always be
available through the @2 command:
>AT @2
MYBOARD_9906
That’s all there is to using the ELM327 device
identifier.
Saving a Data Byte
The ELM327 provides one memory location that
can be used to save any single byte of information.
This location uses special ‘non-volatile’ EEPROM
memory for storage, so your data is not lost, even if
you should turn the power off.
Typically, this memory location is used by the
controlling software to store the state of flags that were
set by vehicle conditions, by hardware configurations,
or by software choices. By storing them in this type of
memory, the settings will be remembered between
uses of the scan tool.
Storing data is easily done with the Save Data
command - for example, to save the value 7F, simply
send:
Since this single byte of data is stored in the
internal EEPROM array, it is subject to the usual limits
of EEPROM technology - unlimited reads, but typically
only about 1 million writes, with a retention time of 40
years (or more). This should not pose any limits to
ELM327 users that we are aware of.
>AT SD 7F
OK
and data is just as easily retrieved using the Read
Data command:
>AT RD
7F
ELM327L DSA
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ELM327L
The Activity Monitor
The ELM327 contains some firmware that is used
to monitor the OBD input pins (ie pins 11, 12, 13, and
24). These routines continually ‘poll’ the inputs, looking
for any active levels on them. Of course, we don’t want
to look at a pin while we are sending, so there is also
some logic to block those inputs during a send, to filter
the levels, and also to provide timing. Collectively,
these routines are known as the Activity Monitor.
The Activity Monitor software ensures that each
OBD input pin is checked at least once every 4 msec
(and sometimes as often as every few µsec). This may
not be perfect (since a single very short message on
an otherwise quiet bus could be missed), but it does
make sure that the ELM327 detects normal activity on
active busses. The presence or absence of activity can
then be used to cause the ELM327 to go to Low
Power operation, or wake from it. See the next section
for details on that.
If you do not want the Activity Monitor to initiate
Low Power operation, you may set PP 0F so that only
‘ACT ALERT’ reports are provided. Of course, you
may also disable that as well, and make your own
decisions based on what the Activity Monitor is seeing.
To help with this, the ELM327 offers an instruction
that is able to report the current Activity Monitor
Counts. The count is an internal value that represents
the time since OBD activity was detected. The actual
time is given by (AMC value + 1) x 0.65536 seconds.
To use it, send the AMC Command and note the value
returned:
>AT AMC
31
In this case, the value returned is 31 hexadecimal
(ie 49 in decimal), which means that the ELM327 has
not detected any OBD activity in the past 49 x 0.655 =
32.8 seconds. The Activity Monitor Count is limited to
a single byte value (max is FF), and internal logic
prevents it from ‘rolling over’. That is, it will count to FF
and stop there.
After a power on or reset, the Activity Monitor is
initially disabled (to give you time to perform setup
tasks). It will be enabled as soon as you attempt to
send the first OBD message. Also, the monitor is
always disabled while the ELM327 is in a monitoring
mode so that you can use the ELM327 as a reliable
monitor of bus activity (you don’t want it to go to sleep
when it should be monitoring, or to miss anything while
waking from sleep).
Power Control
Often, the ELM327 is connected to a vehicle for
only a short time, so power consumption is not of great
concern. Occasionally, the ELM327 may be connected
for longer times, however, possibly without the engine
running. For those applications, it is often desirable to
be able to put the circuit into a low power ‘standby’
state, and have it return to normal operation when
needed. The power control features of the ELM327
were introduced for this.
There are four ways in which the ELM327 can be
placed into the low power standby mode (these are
shown pictorially in Figure 6). None of them will work
without having the master enable (ie bit 7 of PP 0E)
set to ‘1’, which it is by default.
The first method is with an AT command. You may
simply send:
>AT LP
and the IC will go to the low power mode after a one
second delay (which allows the controlling circuit a
ELM327L DSA
little time to perform some housekeeping tasks).
When in Low Power mode, the ELM327 sets all
outputs to their recessive/off state, sets pin 3 (J1850
voltage control) to a low level, and it reverses the state
of the pin 16 control output. The ELM327 will then
reduce its own power level, and begin monitoring for
inputs that would cause a shift back to full power.
The next method allows automatic switching to the
low power mode when there has been no RS232 input
for a period of time - ie the controlling computer has
stopped for some reason. To enable this method, both
b7 and b5 of PP 0E must be set to ‘1’. The time delay
(either 5 or 20 minutes) is set by b4, and the printing of
a warning is enabled with b3. The warning is handy in
some cases – it is the activity alert message (‘ACT
ALERT’) and is sent 1 minute before the timer is about
to time out. When the timer does time out, you will see
a low power alert warning (‘LP ALERT’), and then 2
seconds later, all of the outputs will change as
described above for the AT LP command.
In a similar way, the Activity Monitor may be used
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ELM327L
Power Control (continued)
to cause the ELM327 to shift to Low Power operation,
if there is no OBD activity for some time. If you follow
the logic path, you will see that b7 and b5 of PP 0F
must both be ‘1’, as well as b7 of PP 0E in order to
allow this switch to occur. The default time that it
allows before switching is set by b4, unless you have
set a value with the AT AMT command. Setting the
Activity Monitor Timeout to 00 blocks all the Activity
Monitor outputs.
The final way to enter the low power mode is by a
low level appearing at the ignition monitor input
(pin 15), with both b2 and b7 of PP 0E set to ‘1’.
The ignition monitor logic inserts a short internal
delay (‘debounce’) to be sure that the low level seen is
a legitimate ‘key off’ and not just some noise. After it is
sure, the ELM327 will then send a low power alert
message (‘LP ALERT’), and 2 seconds later it will
switch to Low Power operation, just the same as was
AT LP
command
1 sec
delay
Go to
Low Power
b7
input is
quiet
b4
RS232
detector
RS232 Rx
(pin 18)
b0*
5 min or 20 min
timer
not
monitoring
b5
b7
all are
quiet
OBD
inputs
print
ACT ALERT
activity
monitor
not
monitoring
Timeout setting
AMT value, if set
else
150 sec if b4*= 1
30 sec if b4*= 0
flash OBD
Tx LED
b3
1 min
remaining
timer
b7
(read with
AT AMC)
1 sec
delay
b5*
AMT ≠ 00
b7*
timer ≥
setting?
2 sec
delay
yes
print
LP ALERT
b3*
sent a
message
Notes
PP 0F bits are shown with an asterisk (*).
All other bits are for PP 0E.
print
ACT ALERT
represents an AND gate
voltage
is low
IgnMon
(pin 15)
represents an OR gate
voltage
monitor
65 msec
debounce
b2
b7
ELM327L DSA
Figure 6. Enabling the Low Power Mode
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ELM327L
Power Control (continued)
described for the other methods.
When connecting to pin 15, care should be taken
to not allow excessive current (ie >0.5 mA) to pass
through the internal protection diodes. Typically all that
is required is a series resistor, but adding a capacitor
helps to filter out ignition noise (note that the Schmitt
input on pin 15 allows the use of large value
capacitors). A second resistor ensures a discharge
path for the capacitor, while raising the threshold
voltage. A circuit like this works well:
47KΩ
+12V switched
by the ignition
16
0.1uF
15
47KΩ
14
Note that the AT IGN command can always be
used to read the level at pin 15, regardless of the
setting of the PP 0E enable bits. This may be used to
advantage if you wish to manually shut down the IC,
using your own timing and criteria. Recall that the
RS232 Rx
(pin 18)
alternate function for pin 15 is the RTS input which will
interrupt any OBD processing that is in progress. If the
ELM327 reports an interrupt with the ‘STOPPED’
message, you can then check the level at pin 15 with
the AT IGN command, and make your own decisions
as to what should be done. For that matter, you don’t
even need to reduce the power based on the input you might possibly do something entirely different.
Having put the ELM327 into Low Power mode,
you will need a method to wake it up. This is done by
‘interrupting’ the IC in ways that are very similar to that
used to put it into Low Power mode.
Figure 7 shows the three ways to ‘wake’ the
ELM327 from low power mode (other than toggling the
power, or pulsing pin 1). Any of these can wake the IC
- they do not have to be the cause of its going to low
power.
The first way is with a low level pulse at the RS232
Rx input (pin 18). The RS232 circuitry is not as
sensitive as normal when in the low power mode, so to
be sure that your input is seen, the ELM327 requires
that the pulse width be at least 128 usec wide. This is
128 µsec min
pulse width
RS232
detector
the input
is active
an input is
now active
OBD
inputs
1 sec
delay
activity
monitor
no input was active
perform a
warm start
(see text)
Go to Full Power
• µP to normal
• pin 16 = b6
the voltage
is now high
IgnMon
(pin 15)
voltage
monitor
b1
1 or 5 sec
timer
the voltage was low
Figure 7. Returning to Normal Operation
ELM327L DSA
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ELM327L
Power Control (continued)
easily accomplished by sending a space or @
character, if the baud rate is 57.6 kbps or less. If using
higher baud rates, you may wish to consider
temporarily shifting to a lower baud rate and sending a
character, or possibly see if your software can
generate a ‘break’ signal for you. The break signal is
not always the same length, but is typically several
msec long (ie much longer than 128 µsec), and can
often be sent by USB through USB to RS232
conversion devices. You will have to experiment if
using other methods (Bluetooth or WiFi, for example).
The second method to have the ELM327 circuit go
back to full power is by having activity appear on the
OBD input pins. At the first sign of an active level, the
circuit will start its wakeup, and within seconds, will be
at full capability. Note that the logic is configured so
that it must see all quiet on the OBD inputs before it
will allow an automatic wakeup. This does not normally
present a problem, and is implemented so that you
can manually put the circuit to low power (AT LP),
even if there is activity on one of the OBD inputs.
Otherwise, the circuit would wake up immediately after
the AT LP.
When connected to protocols 3, 4 or 5 (ISO 91412 or ISO 14230-4), there may be no OBD activity when
the vehicle is first turned on. In this case, you could not
rely on the activity monitor to wake the circuit and you
would have to look for other means.
The final method that is provided to wake up the
circuit is by way of the ignition monitor input and logic.
A low then high level at the IgnMon input will cause the
ELM327 to return to full power operation. Note that
PP 0E bit 2 does not have to be set for the IgnMon to
wake the circuit - the ELM327 always monitors this
pin, and will wake the circuit after the delay that is set
by PP 0E bit 1.
We are often asked if a switched ‘ignition’ voltage
is available at the vehicle’s diagnostic connector for
this purpose. It is not, so you must connect a separate
wire yourself. Often there are convenient places that
you can use for this - possibly the radio power, or the
connection to a convenience outlet.
Note that we present the pin 15 input as if it is an
ignition monitoring circuit, but in fact, it will monitor for
any change in voltage level (as long as the change is
stable for the debounce period). You might consider
connecting to other inputs (such as the 5V from a USB
cable, for example) - just be sure to protect the
ELM327 input circuitry from transients. This usually
just requires a series resistor of 1K to 10K in value.
ELM327L DSA
One alternative is to provide a pullup resistor to
allow waking of the ELM327 with a momentary
pushbutton switch:
+5V
47KΩ
2.2KΩ
16
15
14
If the circuit is in low power mode, a momentary
push of the button will wake it up. This is just one of
the many ways in which you can use this input.
One final note has to do with the changes to the
startup process, beginning with firmware version 2.0.
Initially (with v1.4 and v1.4b), the ELM327 simply used
a warm start command (AT WS) to exit from the low
power mode. The positive feedback that we received
from our ELM329 changes has led us to also change
the wakeup process for the ELM327.
Now, on switching back to full power operation,
the ELM327 performs a warm start, but retains the
following settings:
E0/1
R0/1
CAF0/1
CTM1/5
H0/1
D0/1
CFC0/1
JTM1/5
L0/1
S0/1
CSM0/1
AL/NL
M0/1
AT0/1/2
CEA
IIA
In addition, the ISO/KWP baud rate is retained and
the current protocol is not changed (but it is closed, so
will require initialization).
This has discussed how to have the ELM327 go to
low power operation, and wake from it. While in the
low power mode, the question of ‘just how low is the
power consumption’ arises. We discuss that in the
‘Modifications for Low Power Standby Operation’
section, which is on page 86.
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ELM327L
Programmable Parameters
The ELM327 contains several memory locations
that retain their data even after power is turned off.
Every time the IC is reset, these locations are read
and used to change the default settings for such things
as whether to display the headers, or how often to
send ‘wakeup’ messages. Because they define the
operation of the ELM327, we refer to these memory
locations as the Programmable Parameters.
All of the Programmable Parameters are easily
altered at any time using a few simple commands.
These commands are standard AT Commands, with
one exception: each one requires a two-step process
to complete. This extra step provides some security
against random inputs that might otherwise cause
changes.
As an example of how to change a Programmable
Parameter, consider PP 01 (shown on page 70) which
sets the default state for the AT H command. If you are
constantly powering your ELM327 and then using
AT H1 to turn the headers on, you may want to change
the default setting, so that the headers are always on
by default. To do this, simply set the value of PP 01 to
00:
>AT PP 01 SV 00
OK
This changes the value associated with PP 01, but
does not yet enable it. To make the change effective,
you must also type:
>AT PP 01 ON
OK
At this point, you have changed the default setting
for AT H1/H0, but you have not changed the current
AT H1/H0 setting. From the ‘Type’ column in the table
on page 70, you can see that PP 01 is a type ‘D’
parameter, so the change only becomes effective the
next time that defaults are restored. This could be from
a reset, a power off/on, or possibly an AT D command.
If you send the command AT D, then you should find
that the headers are now displayed by default.
As there are many Programmable Parameters that
you can alter, it might occasionally be difficult to know
what changes you have made to them. To help with
that, the ELM327 provides a Programmable Parameter
Summary (PPS) command. This command simply
prints a list of the entire range of PPs (whether
currently supported or not), their current value, and
whether they are on/enabled (N), or off/disabled (F).
For a version 2.1 ELM327, with only the headers
ELM327L DSA
enabled (PP 01, as discussed above), the summary
table would look like:
>AT PPS
00:FF F
04:01 F
08:FF F
0C:68 F
10:0D F
14:50 F
18:31 F
1C:03 F
20:FF F
24:00 F
28:FF F
2C:E0 F
01:00
05:FF
09:00
0D:0D
11:00
15:0A
19:31
1D:0F
21:FF
25:00
29:FF
2D:04
N
F
F
F
F
F
F
F
F
F
F
F
02:FF
06:F1
0A:0A
0E:9A
12:FF
16:FF
1A:FF
1E:4A
22:FF
26:00
2A:3C
2E:80
F
F
F
F
F
F
F
F
F
F
F
F
03:32
07:09
0B:FF
0F:D5
13:55
17:6D
1B:FF
1F:FF
23:FF
27:FF
2B:02
2F:0A
F
F
F
F
F
F
F
F
F
F
F
F
You can see that PP 01 now shows a value of 00,
and it is enabled (oN), while the others are all off.
Another example shows how you might change
the CAN filler byte. Some systems use ‘AA’ as the
value to send for unused CAN data bytes, while the
ELM327 uses ‘00’ by default. To change the ELM327
in order to have it send AAs instead, simply change
PP 26:
>AT PP 26 SV AA
OK
>AT PP 26 ON
OK
Again, PP 26 is of type ‘D’, so the above change
will not actually take effect until the defaults are
restored, whether it be by an AT D command, or by
resetting the ELM327.
The Programmable Parameters are a great way to
customize your ELM327 for your own use, but you
should do so with caution if using commercial
software. Most software expects an ELM327 to
respond in certain ways to commands, and may be
confused if the carriage return character has been
redefined, or if the CAN response shows a data length
code digit, for example. If you make changes, it might
be best to make small changes and then see the effect
of each, so that it is easier to retrace your steps and
‘undo’ what you have done, should you have to. If you
get in too deeply, don’t forget the ‘all off’ command:
>AT PP FF OFF
No matter what software you use, you might get
into more serious trouble, should you change the baud
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ELM327L
Programmable Parameters (continued)
rate, or the Carriage Return character, for example,
and forget what you have set them to. The Carriage
Return value that is set by PP 0D is the only character
that is recognized by the ELM327 as ending a
command, so if you change its value, you may not be
able to undo your change. In this case, your only
recourse may be to force all of the PPs off with a
special hardware trick.
When the ELM327 first powers up, it looks for a
jumper between pin 28 (the OBD Tx LED output) and
circuit common (V SS). If a jumper is in place, it will turn
off all of the PPs for you, restoring the IC to the factory
default settings. To use this feature, simply connect a
jumper to circuit common (which appears in numerous
places - pins 8 or 19 of the ELM327, pin 5 of an
RS232 connector, one end of most capacitors, or at
the OBD connector), then hold the other end of the
jumper to pin 28 while turning the power on. When you
see the RS232 Rx LED begin to flash quickly, remove
the jumper – the flashing LED means that it worked
and the PPs are all off.
This feature should only be used when you get
into serious trouble, and it’s your only choice (since
putting a jumper into a live circuit might cause damage
if you put it into the wrong place). Be careful if you
need to use it.
Programmable Parameter Summary
The following pages provide a list of the currently
available Programmable Parameters. The character
shown in the ‘Type’ column indicates when changes
will take effect. Possible values are:
I - the effect is Immediate,
PP
00
01
02
03
D - takes effect after Defaults are restored
(AT D, AT Z, AT WS, MCLR or power off/on)
R - takes effect after a Reset
(AT Z, AT WS, MCLR or power off/on)
P - needs a Power off/on type reset
(AT Z, MCLR, or power off/on)
Description
Perform an AT MA command after powerup or reset
Printing of header bytes (AT H default setting)
Allow long messages (AT AL default setting)
Values
Default
Type
00 = ON
FF = OFF
FF
R
(OFF)
00 = ON
FF = OFF
(OFF)
00 = ON
FF = OFF
(OFF)
FF
FF
D
NO DATA timeout time (AT ST default setting)
setting = value x 4.096 msec
00 to FF
04
Default Adaptive Timing mode (AT AT setting)
00 to 02
01
D
06
OBD Source (Tester) Address. Not used for J1939 protocols.
00 to FF
F1
R
07
Last Protocol to try during automatic searches
01 to 0C
09
I
09
Character echo (AT E default setting)
00 = ON
FF = OFF
00
R
(ON)
00 to FF
0A
0A
ELM327L DSA
Linefeed Character
32
D
D
(205 msec)
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R
70 of 94
ELM327L
Programmable Parameter Summary (continued)
PP
0C
Description
Values
RS232 baud rate divisor
baud rate (in kbps) = 4000 ÷ (PP 0C value)
00 to FF
Default
Type
68
P
(38.4)
For example, 500 kbps requires a setting of 08 (since 4000/8 = 500)
Here are some example baud rates, and the divisor to be used:
Baud Rate
(kbps)
PP 0C value
hex
(dec)
19.2
D0
(208)
38.4
68
(104)
57.6
45
(69)
115.2
23
(35)
230.4
11
(17)
500
08
(8)
Notes:
1. The PP 0C value must be provided as hex digits only. The
decimal values (listed above in brackets) are only shown for
your convenience.
2. The ELM327 can only process continuous byte receives at
rates of about 600 kbps maximum. If you need to connect at
a higher rate, add a delay between the bytes to maintain an
average rate of 600 kbps or less.
3. A value of 00 provides a baud rate of 9600 bps.
0D
Carriage Return Character
00 to FF
0E
Power Control options
00 to FF
0D
R
9A
R
(10011010)
Each bit controls an option, as follows:
b7: Master enable
0: off
1: on
if 0, pins 15 and 16 perform as described for v1.0 to v1.3a
(must be 1 to allow any Low Power functions)
b6: Pin 16 full power level
0: low
1: high
normal output level, is inverted when in low power mode
b5: Auto LP control
0: disabled 1: enabled
allows low power mode if the RS232 activity stops
b4: Auto LP timeout
0: 5 mins
no RS232 activity timeout setting
1: 20 mins
b3: Auto LP warning
0: disabled 1: enabled
if enabled, says ‘ACT ALERT’ 1 minute before RS232 timeout
b2: Ignition control
0: disabled 1: enabled
allows low power mode if the IgnMon input goes low
ELM327L DSA
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ELM327L
Programmable Parameter Summary (continued)
PP
0E
Description
Values
Default
Type
D5
D
Power Control options (continued)
b1: Ignition delay
0: 1 sec
1: 5 sec
delay after IgnMon (pin 15) returns to a high level, before
normal operation resumes
b0: reserved for future - leave set at 0
0F
Activity Monitor options
00 to FF
(11010101)
Each bit controls an option, as follows:
b7: monitor master control
must be 1 to allow b3 to b6
0: disabled
b6: allow wake from Low Power
0: no
wakes on shift from no activity to activity
1: enabled
1: yes
b5: Auto LP control
0: disabled 1: enabled
allows low power mode if the OBD activity stops
b4: Auto LP timeout
no OBD activity timeout setting
0: 30 secs
b3: Auto LP warning
0: disabled
if enabled, says ‘ACT ALERT’ on timeout
1: 150 secs
1: enabled
b2: reserved for future - leave set at 1
b1: add exclamation mark
0: no
1: yes
if 1, sends ‘!’ before ACT ALERT and LP ALERT
b0: LP LED
0: disabled 1: enabled
if 1, the OBD Tx LED flashes when in Low Power mode
(one 16 msec flash repeated every 4 seconds)
10
11
12
13
14
15
J1850 voltage settling time
setting (in msec) = (PP 10 value) x 4.096
00 to FF
J1850 Break Signal monitor enable
(reports BUS ERROR if break signal duration limits are exceeded)
00 = ON
FF = OFF
(ON)
00 = invert
FF = normal
(normal)
J1850 Volts (pin 3) output polarity
normal = Low output for 5V, High output for 8V
invert = High output for 5V, Low output for 8V
0D
R
(53 msec)
Time delay added between protocols 1 & 2 during a search
setting (in msec) = 150 + (PP 13 value) x 4.096
00 to FF
ISO/KWP final stop bit width (provides P4 interbyte time)
setting (in µsec) = 98 + (PP 14 value) x 64
00 to FF
ISO/KWP maximum inter-byte time (P1), and also used for
the minimum inter-message time (P2).
00 to FF
00
FF
55
D
R
I
(498 msec)
50
D
(5.2 msec)
0A
D
(21 msec)
setting (in msec) = (PP 15 value) x 2.112
ELM327L DSA
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ELM327L
Programmable Parameter Summary (continued)
PP
16
17
18
19
1A
1B
1C
Description
Values
Default ISO/KWP baud rate (AT IB default setting)
Note: 4800 baud can not be a default - set it with AT IB 48
00 = 96
FF = 10
ISO/KWP wakeup message rate (AT SW default setting)
setting (in msec) = (PP 17 value) x 20.48
00 to FF
ISO/KWP delay before a fast init, if a slow init has taken place
setting (in msec) = 1000 + (PP 18 value) x 20.48
00 to FF
ISO/KWP delay before a slow init, if a fast init has taken place
setting (in msec) = 1000 + (PP 19 value) x 20.48
00 to FF
Protocol 5 fast initiation active time (TiniL)
setting (in msec) = (PP 1A value) x 2.5
00 to FF
Protocol 5 fast initiation passive time (TiniH)
setting (in msec) = (PP 1B value) x 2.5
00 to FF
ISO/KWP outputs used for initiation (b7 to b2 are not used)
00 to FF
b1: L line (pin 22)
b0: K line (pin 21)
Default
Type
FF
R
(10.4K)
92
D
(3.0 sec)
31
I
(2.0 sec)
31
I
(2.0 sec)
0A
D
(25 msec)
0A
D
(25 msec)
0: disabled
0: disabled
03
D
(00000011)
1: enabled
1: enabled
If disabled, an output will remain low during protocol initiations
1D
1E
21
24
25
ISO/KWP P3 time (delay before sending requests)
Ave time (in msec) = (PP 1D value - 0.5) x 4.096
00 to FF
ISO/KWP K line minimum quiet time before an init can begin (W5)
setting (in msec) = (PP 1E value) x 4.096
00 to FF
Default CAN Silent Monitoring setting (for AT CSM)
FF = ON
00 = OFF
(ON)
00 = ON
FF = OFF
(ON)
00 = ON
FF = OFF
(ON)
00
D
FF
D
CAN auto formatting (AT CAF default setting)
CAN auto flow control (AT CFC default setting)
0F
D
(59 msec)
4A
D
(303 msec)
26
CAN filler byte (used to pad out messages)
00 to FF
28
CAN Filter settings (controls CAN sends while searching)
00 to FF
FF
00
00
R
D
D
(11111111)
The bits of this byte control options, as follows:
b7: 500 kbps match
0: ignored
1: required
b6: 250 kbps match
0: ignored
1: required
b5 to b1: reserved for future - leave set to 1
b0: send if bus is quiet
29
ELM327L DSA
0: not allowed 1: allowed
Printing of the CAN data length (DLC) when printing header bytes
(AT D0/D1 default setting)
00 = ON
FF = OFF
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FF
D
(OFF)
73 of 94
ELM327L
Programmable Parameter Summary (continued)
PP
2A
Description
CAN Error Checking (applies to protocols 6 to C)
Values
00 to FF
Default
Type
3C
D
(00111100)
Each bit of this byte controls an option, as follows:
b7: ISO15765 Data Length
0: accept any 1: must be 8 bytes
b6: ISO15765 PCI = 00
0: allowed
1: not allowed
b5: Search after ERR94
0: normal
1: CAN is blocked
b4: Search after LV RESET
0: normal
1: CAN is blocked
b3: Wiring Test
0: bypass
1: perform
Processing 7F xx 78’s:
b2: enabled (CAN & KWP)
0: no
1: yes
b1: valid Modes (xx values)
0: all
1: only 00 to 0F
b0: valid CAN protocols
0: all
1: only ISO15765
2B
Protocol A (SAE J1939) CAN baud rate divisor
baud rate (in kbps) = 500 ÷ (PP 2B value)
01 to 40
02
R
(250 Kbps)
For example, setting this PP to 19 (ie. decimal 25) provides
a baud rate of 500/25 = 20 kbps.
2C
Protocol B (USER1) CAN options.
00 to FF
E0
R
(11100000)
Each bit of this byte controls an option, as follows:
b7: Transmit ID Length
0: 29 bit ID
1: 11 bit ID
b6: Data Length
0: fixed 8 byte 1: variable DLC
b5: Receive ID Length
0: as set by b7 1: both 11 and 29 bit
b4: baud rate multiplier
0: x1
1: x 8/7 (see note 3)
b3: reserved for future - leave set at 0.
b2, b1, and b0 determine the data formatting options:
b2 b1 b0
Data Format
0
0
0
none
0
0
1
ISO 15765-4
0
1
0
SAE J1939
Other combinations are reserved for future updates – results will
be unpredictable if you should select one of them.
2D
Protocol B (USER1) baud rate divisor. See PP 2B for a description.
01 to 40
04
R
(125 Kbps)
2E
Protocol C (USER2) CAN options. See PP 2C for a description.
00 to FF
80
R
(10000000)
2F
Protocol C (USER2) baud rate divisor. See PP 2B for a description.
01 to 40
0A
R
(50 Kbps)
Notes:
1. The ELM327 does not accept decimal digits for the Programmable Parameters - all values are hexadecimal.
2. For Programmable Parameters that describe options in terms of bits, b7 is the msb, and b0 is the lsb.
3. When b4 of PP 2C or PP 2E are set, the CAN baud rate will be increased by a factor of 8/7, but the baud
rate displayed by the AT DP command will still show the base rate (as set by PP 2D or PP 2F). For example,
if you set PP 2C b4 to 1, and then PP 2D to 06, the base frequency will be 83.3 kbps. The AT DP command
will report 83 kbps, but the actual baud rate will be 83.3x8/7 = 95.2 kbps.
ELM327L DSA
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ELM327L
Maximum CAN Data Rates
We are occasionally asked what the maximum
data rate is that the ELM327 can handle. This is often
after someone has tried to monitor all data using the
default settings and has received a ‘BUFFER FULL’
error. It is difficult to say exactly what the maximum
rate is, however, as several factors are involved.
The CAN ‘engine’ inside the ELM327 is actually
configured with one receive register that accepts
messages from the data bus, and another register that
accepts messages from the first. As long as the
firmware empties the second register before the first
register needs it, there should not be any overflow
problems with this component. The ELM327 actually
moves the data quickly to temporary storage, so this is
never a problem.
It would be nice if all the firmware had to do was to
empty the second register, and wait for it to fill again,
but that is not so. It must also check for errors,
possibly queue a CAN response, format the received
message from ECU
11 bit/500 = 220
29 bit/250 = 520
min space
11 bit/500 = 16
29 bit/250 = 32
message
CAN Data
message, convert it to ascii, load it into the RS232
transmit buffer, and then prepare to receive the next
message. These tasks can take a considerable time,
depending on what formatting options you have
chosen, and the baud rate that you select.
The diagram below shows these processes
grouped into blocks. The times shown are typical for
an ISO15765-4 message, and as you can see, vary
with both the length of the CAN message and the CAN
baud rate. All values shown are time measurements in
µsec (microseconds).
When a message arrives, the ELM327 moves
quickly to move the received bytes from the special
CAN registers, so that they do not affect the next
message that arrives. The data is then formatted (as
ASCII bytes) and placed into the RS232 transmit
buffer, for sending to the controlling processor. As long
as messages do not arrive at a rate that is faster than
the ELM327 can process them in, all messages will be
At 40% Bus Loading:
11 bit/500 = 354
29 bit/250 = 828
Rx register is empty and can
accept the next message
Notes:
- all times are in µsec
- times shown are averaged typical
- messages have 8 data bytes, and:
headers on (AT H1),
spaces off (AT S0),
linefeeds off (AT L0)
Moving Data
moves and error checks
500k = 29
250k = 41
message
all data is in the
send buffer
Processing Data
formatting & preparation
11 bit ID = 193
29 bit ID = 210
Sending Results
ELM327L DSA
this is a background task - the ELM327 can
do other things while it is sending RS232
send time depends on message length and baud rate,
but on average is:
38.4k
115.2k
500k
11 bit ID
5180
1740
400
29 bit ID
6475
2175
500
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ELM327L
Maximum CAN Data Rates (continued)
processed. You can see from the figure that even for a
500 kbps message with an 11 bit ID, the ELM327
finishes with time to spare. Since ISO15765-4
specifies that messages must be 8 data bytes in length
(filler bytes are added as needed) these times do
represent the typical situation, with a 40% bus load.
Actually, from these numbers the ELM327 should be
able to handle 100% bus loading (which is not a
practical situation).
Once the ELM327 has placed all of the properly
formatted bytes into the RS232 transmit buffer, it is up
to the controlling computer to fetch them in a timely
fashion. If the bytes are removed too slowly, the buffer
will continue to fill as new OBD messages arrive, and
the buffer will eventually become full. It does not
matter how big the buffer is, if the rate of removing
bytes from the buffer is slower than the rate of putting
them into the buffer, it will eventually fill up. When it is
full, you will see a ‘BUFFER FULL’ message, and you
will have to start over.
The ELM327 transmit buffer is 512 bytes in size
(and the ELM327L’s is 2048 bytes). Considering that
some bytes will be sent while new messages are being
queued, this means that the ELM327 can typically
store:
11 bit/500k
29 bit/250k
38.4k 115.2k 500k
28
38
–
26
56
–
messages in the buffer, if the bus loading is at 40%,
as shown. This storage is more than enough for
almost all OBD requests – the only time that you might
get into trouble is if you are monitoring all messages
on the bus (AT MA) with no filters set. In that case, you
would need to be sure that you are removing bytes as
fast as they are being generated.
The rate at which OBD messages occur depends
on the ‘bus loading’. This is a utilization factor that is
very similar to the duty cycle for a square wave signal.
Ideally, bus loading should be less than about 30%,
but as vehicles become more complex, this is very
difficult to do. Some vehicles are reportedly seeing
70% bus loads.
The above chart does not show any numbers in
the 500k column, as the buffer should never fill up
when bus loading is 40%. As you increase loading,
you will eventually reach the limit, but even with very
busy data busses, we typically see about 150
messages before a BUFFER FULL is reported (the
ELM327L DSA
ELM327L will typically show twice that many).
What can you do to eliminate (or at least delay)
the ‘BUFFER FULL’ messages? You might try a few
things. First, learn to adjust the baud rate with PP 0C.
This will always be of benefit, and may just solve your
problems. Next, we often see log files that show many
‘<DATA ERROR’ messages. These occur because
only some messages on the vehicle’s data bus are
actually in a format that could be OBD data. To
eliminate these error messages, turn off the CAN
formatting (with AT CAF0). To help further, you can
reduce the amount of data that gets sent. Simply
turning off spaces and Linefeeds (AT S0, AT L0) may
be all that you need to do.
When people ask us then, ‘What data rate can the
ELM327 support?’ the answer is not easy to provide,
as it depends on many factors. It depends on the CAN
message content, the data rate, and whether you have
selected filters to eliminate irrelevant messages. The
rate also depends on the RS232 baud rate that you
have chosen, as it may be the limiting factor if there
are a great number of messages being retrieved. If
you are only fetching 10 or 20 messages, however, the
internal buffer takes care of them for you, and the
baud rate that you choose does not matter.
If you are simply fetching PID responses from the
network, there really is no limitation within the
ELM327, and it does not matter what the ‘maximum
CAN data rate’ is. If you are trying to ‘push the
envelope’, monitoring everything that travels on a very
busy CAN data bus, then there will be limitations.
What they are depend to a great deal on what choices
you make. Hopefully this discussion will have helped
to give you the necessary background information to
do so.
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ELM327L
Microprocessor Interfaces
A very common question that we receive is ‘Can I
connect the ELM327 directly to my own circuit, or must
I use the interface shown?’ Certainly you may connect
directly to our ICs, and you do not need to use an
RS232 or USB interface. There are a few items to
consider, however.
The ELM327 is actually a microprocessor that
contains a standard UART type interface, connected to
the RS232 Tx and Rx pins. The logic type is CMOS,
and the Rx pin has Schmitt trigger waveshaping, so
you should be able to connect directly to these pins
provided that the two devices share the same power
supply (VDD), and that they are not physically more
than about 10 to 20 inches apart (CMOS circuits are
subject to latch-up from induced currents, which may
be a problem if you have long leads).
The normal (idle) levels of the ELM327 transmit
and receive pins are at the VDD (logic high) level. Most
microprocessors and RS232 interface ICs expect that
to be the idle level, but you should verify it for your
microprocessor before connecting to the ELM327. The
connections are straightforward - transmit connects to
receive, and receive connects to transmit, as shown
below. Don’t forget to set both devices to the same
baud rate.
The ELM327 also provides a hand-shaking feature
that may simplify the flow of data for you. The interface
consists of two pins - an input an an output. The input
is called ‘request to send’ (RTS), and it is used to
interrupt the ELM327, just the same as tapping a key
on the keyboard when using a terminal program. The
output pin (‘Busy’) is used by the ELM327 to tell your
system that it is processing data.
To use the handshaking feature, set one of your
port pins to normally provide a high output, and
connect it to the RTS input (pin 15). Use another port
pin as an input to monitor the ELM327 Busy output
(pin 16). When you want to send a command, simply
check the Busy output first. If it is at a high logic level,
then either wait for it to go low, or if you need to
interrupt the IC, then bring the RTS line low and wait
for the Busy line to go low. (You might want to
consider using an edge triggered interrupt on the Busy
output, if one is available). When Busy does go low,
restore your RTS line to a high level, and then send
your command to the ELM327. No need to worry
about the ELM327 becoming busy again after you
raise the RTS line at this point – once Busy goes low,
the ELM327 will wait (indefinitely) for your command. If
you do not use the RTS input on the ELM327, it must
be connected to a high logic level, as shown. Note that
the default setting for PP 0E turns these hand-shaking
+5V
your microprocessor
+5V
Tx
Rx
L1-L4
R27-30
470Ω
28
27
26
25
24
23
22
21
20
19
18
17
n.c.
16
Rx
Tx
Busy RTS
11
12
15
327
+5V
1
2
3
4
5
6
7
8
9
C3
27pF
10
to
R21
X1
4.00MHz
ELM327L DSA
+5V
+5V
13
The ELM327 and your
microprocessor should
use the same 5V supply
14
to
R15
C4
27pF
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ELM327L
Microprocessor Interfaces (continued)
signals off, so you will need to change that in order to
use them.
We are often asked about connecting the ELM327
to 3.3V logic, which is quite popular. Many hope that
they can just insert a resistor or two and have it work.
Unfortunately, that is not the case, mostly because the
ELM327 has Schmitt waveshaping on the RS232 Rx
input (pin 18), and so may need as much as 4V for a
high input (although it often works with less). To
ensure that there’s enough voltage, we recommend
using a level translator circuit such as the TXB0102
from Texas Instruments (www.ti.com), the ST2129
from ST Microelectronics (www.st.com), or the Analog
Devices (www.analog.com) ADuM1201 as shown
here. We have been using the ADuM1201 with the
Raspberry Pi lately, because it offers galvanic isolation
(to 2500 Vrms) in addition to the level translation, so
protects the Pi from occasional wiring problems. Note
that for complete isolation you must use separate
commons for the 5V and the 3.3V power supplies. The
one disadvantage with the ‘1201 over the others is that
it draws about 1 mA, which can be an issue if you are
trying to use the low power mode with your ELM327.
+3.3V
your microprocessor
Tx
Rx
8
7
6
5
1
2
3
4
+3.3V
ADuM1201
+5V
+5V
+5V
21
20
19
18
17
n.c.
16
Rx
Tx
Busy RTS
11
12
15
327
7
8
9
10
13
14
Upgrading Versions
A popular question that we receive is “Can I
upgrade my firmware with a file download?”. The
answer to this is no, the ELM327 can not be upgraded
in this way - your integrated circuit must be replaced.
The next question which usually follows is “Can I
simply replace an old ELM327 chip with a new one to
upgrade the firmware?”
The answer to this last question is basically yes.
We say basically because there was a change
between versions 1.3a and 1.4b when we added the
power control feature. This change modified how pins
15 and 16 were used (they took on dual roles), and
that may affect your circuit.
If your circuit board is older and used an earlier
version of our chip (ie pre v1.4b), then you have to
look at what pins 15 and 16 were used for. Almost all
of the early boards followed our example and left pin
16 open, and pin 15 tied to +5V. If your circuit did this,
there is no problem - you may simply replace the old
chip with a new one.
If your circuit board used pins 15 and 16 for
ELM327L DSA
handshaking with a microprocessor or other device,
then you may need to take extra steps. Usually this
only requires turning off the low power control by
setting b7 of PP 0E to 0.
If you are replacing a v1.4b or a v2.0 integrated
circuit with a newer one, then there are no concerns.
Please note that there are several clone products
available that use integrated circuits that mimic the
ELM327 at a software level. They do not necessarily
mimic the hardware. For this reason, you should not
replace a clone chip with a genuine ELM chip, unless
you are absolutely certain that all 28 pins of the
integrated circuits are identical.
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ELM327L
Example Applications
The SAE J1962 standard dictates that all OBD
compliant vehicles must use a standard connector, the
shape and pinout of which is shown in Figure 8 below.
The dimensions and pin connections for this ‘Type A’
connector are fully described in the SAE J1962
standard.
1
8
9
16
Figure 8. The J1962 Vehicle Connector
The circuit that you build with the ELM327 will be
required to connect by way of a matching male J1962
connector. Fortunately these are available from
several sources, easily found with a web search.
Note that before OBDII was adopted, several
vehicles (most notably those made by General Motors)
used a very similar connector (mostly) for their factory
communications. These vehicles typically used what is
known as the ALDL protocol, which the ELM327 does
not support. Check that your vehicle is actually OBDII
compliant before building your ELM327 circuit.
The circuit on page 81 (Figure 9) shows how the
ELM327L might typically be used. Circuit power is
obtained from the vehicle via OBD pins 16 and 5 and,
after a protecting diode and a capacitor for filtering, is
presented to a five volt regulator. (Note that a few
vehicles have been reported to not have a pin 5 – on
these you will use pin 4 instead of pin 5.) The regulator
powers several points in the circuit as well as an LED
(L5) for visual confirmation that power is present. We
have used a 7805 regulator for this circuit as it is very
common, and usually easy to obtain.
The top left corner of Figure 9 shows the CAN
interface circuitry. We do not advise making your own
interface using discrete components – CAN buses
typically have a lot of critical information on them, and
you can easily do more harm than good, so it is
strongly recommended that you use a commercial
transceiver chip as shown. We show a Microchip
MCP2551 in this circuit, but most major manufacturers
produce CAN transceiver ICs – look at the NXP
PCA82C251, the Texas Instruments SN65LBC031,
and the Linear Technology LT1796, to name only a
few. Be sure to pay attention to the voltage limits as
depending on the application, you may have to tolerate
ELM327L DSA
24V, and not just 12V. Also, if you are considering
using the Activity Monitor to wake the ELM327 from a
low power sleep mode, be sure that the transceiver
chip that you pick keeps the receiver functional when
in standby mode (if it does not pass the signals on, the
ELM327 can not see them).
The circuit shown directly below the CAN interface
is used for the ISO 9141 and ISO 14230 signals. We
provide two output lines, as required by the standards,
but depending on your vehicle, you may not need to
use the ISO-L output. (Many vehicles do not require
this signal for initiation, but some do, so it is shown
here.) If your vehicle does not require the L line, simply
do not connect anything to pin 22, and do not install
Q6, R16 or R17.
The ELM327L controls both of the ISO outputs
through NPN transistors Q6 and Q7 as shown. These
transistors have 510Ω pullup resistors connected to
their collectors, as the standard requires. Occasionally,
we are asked about substitutes for these resistors –
the standard specifies 510Ω but in a pinch you might
be able to use 560Ω. A better solution would be to
make 510Ω from 240Ω and 270Ω 1/4W resistors in
series. We do not recommend using a lower value for
the resistance as it stresses every device on the bus.
Note that 1/2W resistors are specified in Figure 10 as
a short at 13.8V causes about 0.4W dissipation.
Be careful if you are designing a circuit that might
monitor other scan tools. Both the ELM327 and the
other scan tool would present 510Ω resistors, so the
vehicle would see 255Ω connected externally. This
would very likely cause data errors, and might even
damage some circuitry. To avoid this, you might wish
to build your circuit in such a way that you can switch
the ELM327’s 510Ω resistors out, and replace them
with a larger value. For example, you might put 10KΩ
resistors in series with the 510Ω ones, and add
removable jumpers or switches across the 10KΩ
resistors. Most people do not need to do this – we only
mention it because of the questions that we do
receive.
The ISO data is both sent and received on the
ISO-K line. Pin 12 of the ELM327 reads this data
through the R20-R21 voltage divider. Because of the
Schmitt trigger input on pin 12, these resistors will give
typical threshold levels of 5.1V (rising) and 4.4V
(falling), which provides a large amount of noise
immunity while also protecting the IC. If you should
connect test equipment to pin 12 (ie in parallel with
R21), the thresholds will increase, so be conscious of
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ELM327L
Example Applications (continued)
what you are doing while testing.
The final OBD interface shown is in the bottom left
corner, and is used for the two J1850 standards. The
J1850 VPW standard needs a positive supply of up to
8V while the J1850 PWM needs 5V, so we have
shown a two level supply that can provide both. This
dual voltage supply uses a 317L adjustable regulator
as shown, controlled by the pin 3 output. With the
resistor values given, the selected voltages will be
about 8.0V and 5.5V, which works well for most
vehicles. The 317L is able to maintain regulation with a
minimum of about 2 mA of current, which is provided
by the 470Ω resistor between the output and adjust
pins. A larger 317 regulator will require more current,
and the resistors would need to be scaled down
proportionally. Once suitable voltages have been
generated, they are driven by the Q1-Q2 combination
for the Bus+, and Q3 for the Bus-.
The J1850 VPW input uses a resistor divider,
similar to that which was used for the ISO input.
Typical threshold voltages with the resistors shown will
be about 4.2V (rising) and 2.2V (falling). The J1850
PWM input is a little different in that it must convert a
differential input into a single-ended one for use by the
ELM327. This is done by connecting Q4 across the
input so that it operates as a difference amplifier. The
Q4-D3 series combination sets a threshold voltage of
about 1V (for improved noise immunity), while R11
limits the current flow, and R12 makes sure that Q4 is
off when it should be. The circuit works well as shown,
but the R14 passive pullup time constant can be easily
affected by stray capacitance - be aware of this if
connecting test equipment to pin 13.
Resistor R10 is the final J1850 component. We
added this to help discharge the data bus more rapidly
when it was found that some vehicles showed higher
capacitance than others. The resistor may not be
required for many vehicles - the choice is yours. If you
should see reports of BUS ERRORs with a J1850
vehicle, it may be this capacitance causing problems
(you will need to ‘scope the signal to be sure).
Moving on, the R25-R26 voltage divider shown
connected to pin 2 is used for the vehicle voltage
monitoring circuitry. The two resistors simply divide the
battery voltage to a safer level for the ELM327, and
the capacitor (C2) helps to filter out noise. With VDD >
4V, the ELM327L expects a resistor divider ratio as
shown, and sets nominal calibration constants
accordingly. If your application needs a different range
of values, simply choose your resistor values to
ELM327L DSA
maintain the input within the ELM327’s VSS to VDD
voltage range, and then perform an AT CV to calibrate
the ELM327 to your new ratio. The maximum voltage
that the ELM327 can display is 99.9V (it’s a software
limit, not hardware).
Four LEDs are shown on pins 25 to 28. These
have been provided as a visual means of confirming
circuit activity. They are not essential, but it is nice to
see the visual feedback.
On the right side of the circuit, the ELM327’s
RS232 pins (17 and 18) are shown connected to an
FTDI USB module. This module makes it very easy to
connect the ELM327 circuit to your computer - all you
need is the VCP Driver software, which is available for
free from the FTDI web site (www.ftdichip.com). The
module pinout matches a 9 pin D-sub connector, so
you can simply solder it in where the RS232 circuitry
used to go. Diode D5 and resistor R32 have been
added to the interface to prevent the USB supply from
backfeeding into the ELM327L.
A 10µF capacitor is shown connected to pin 6 of
the ELM327L. This is required for filtering of the IC’s
internal 3.3V supply. Choose a low ESR (<5Ω)
ceramic or tantalum type.
Finally, the crystal shown connected between pins
9 and 10 is a standard 4.000MHz microprocessor type
crystal. The 27pF crystal loading capacitors shown are
typical only, and you may have to select other values
depending on what is specified for the crystal that you
use. The crystal frequency is critical to circuit operation
and must not be altered.
We often receive requests for parts lists to
accompany our Example Applications circuits. Since
this circuit is more complex than most, we have named
and numbered all of the components and provided a
summary parts list (Figure 10). Note that these are
only suggestions for parts. If you prefer another LED
colour, or have a different general purpose transistor
on hand, etc., then by all means make the change. A
quick tip for those having trouble finding a 0.3” wide
socket for the ELM327: many of the standard 14 pin
sockets can be placed end-to-end to form one 0.3”
wide 28 pin socket. For more help with building and
testing the circuit, see our ‘AN02 - ELM327 Circuit
Construction’ application note.
What if you only want to support one of the
ELM327’s protocols? Well, you may do that if you
wish. Simply remove the OBD interfaces that you do
not require, and connect the rest. Since you must
never leave a CMOS input floating (open-circuited),
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ELM327L
CAN-L
14
6
CAN-H
R22
100Ω
C7
560pF
50V
Vbat
n.c.
R23
100Ω
R24
4.7KΩ
8
C8
560pF
50V
7
6
5
MCP2551
1
U2
2
3
16
Battery
Positive
+5V
4
Vbat
+5V
7805
D1
L5
C1
0.1µF
50V
5
Signal
Ground
C6
0.1µF
U3
+12V
C5
0.1µF
R31
470Ω
Vbat
R19
510Ω
R17
510Ω
15
ISO-L
U5
R16 2.2KΩ
Q6
7
ISO-K
1 (DCD)
4 (DTR)
6 (DSR)
+5V
R18 2.2KΩ
Q7
R20
47KΩ
7 (RTS)
8 (CTS)
R32
4.7KΩ
+5V
ELM327L
pin 12
D4
3 (TxD)
R21
33KΩ
USB
Interface
(mini B)
2 (RxD)
L1-L4
5 (SG)
R27-30
470Ω
Vbat
+5V
+5V
n.c.
28
U1
R25
47KΩ
27
26
25
24
23
22
21
20
19
18
17
16
15
9
10
11
12
13
14
9 (RI)
FTDI
DB9-USB-D5-F
327L
+5V
R26
10KΩ
1
Out
R5
470Ω
(J1962)
317L
Adj
4
5
6
7
8
In
R7
4.7KΩ
C3
27pF
C4
27pF
R1
4.7KΩ
R8
22KΩ
Figure 9.
An OBD to USB Interpreter
(VDD is 5.0V)
Q1
R9
10KΩ
2
J1850 Bus +
R10
22KΩ
10
J1850 Bus R15
4.7KΩ
Q3
D3
ELM327L
pin 14
to
R15
X1
4.00MHz
R2
2.2KΩ
R4
2.2KΩ
Q2
to
R21
+12V
R3 470Ω
R6
10KΩ
D2
3
C9
10µF
U4
OBD
Interface
2
C2
0.1µF
R11
10KΩ
+5V
R14
4.7KΩ
R12
47KΩ
Q4
Q5
R13
10KΩ
ELM327L DSA
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ELM327L
Example Applications (continued)
Semiconductors
D1 = 1N4001
D2, D3, D4 = 1N4148
L1, L2, L3, L4 = Yellow LED
L5 = Green LED
Q1, Q3, Q5, Q6, Q7 = 2N3904 (NPN)
Q2, Q4 = 2N3906 (PNP)
U1 = ELM327L
U2 = MCP2551 or MCP2561
U3 = 7805 regulator (5V 1A)
U4 = 317L adjustable regulator (100 mA)
U5 = FTDI DBP-USB-D5-F usb module
Misc
X1 = 4.000MHz crystal
DB9M connector for OBD cable?
IC Socket = 28 pin 0.3" wide (or 2 x 14pin)
Resistors (1/8W or greater, except as noted)
R22, R23 = 100 Ω
R3, R5, R27, R28, R29, R30, R31 = 470 Ω
R17, R19 = 510 Ω 1/2W
R2, R4, R16, R18 = 2.2 KΩ
R1, R7, R14, R15, R24, R32 = 4.7 KΩ
R6, R9, R11, R13, R26 = 10 KΩ
R10, R8 = 22 KΩ
R21 = 33KΩ
R12, R20, R25 = 47 KΩ
Capacitors (16V or greater, except as noted)
C3, C4 = 27pF
C7, C8 = 560pF 50V
C1 = 0.1uF 50V
C2, C5, C6 = 0.1uF
C9 = 10uF
Figure 10. Parts List for Figure 9
you will need to add a jumper or two on the unused
inputs. See the ‘Unused Pins’ section for advice on
what to connect the pins to.
Some people still prefer to interface their circuits
with standard RS232 circuitry. For these, we offer the
sub-circuits of Figures 11 and 12.
Figure 11 shows a discrete RS232 interface, that
may be connected directly to the ELM327. This circuit
uses a resistor, diode, and capacitor between the two
RS232 signal lines to ‘steal’ power from the host
computer. In this way, the required RS232 negative
voltage is obtained without adding a complicated
power supply to the ELM327 circuit. The RS232
connections shown are for a standard 9 pin connector.
If you are using a 25 pin one, you will need to
compensate for the differences. This circuit works well
at baud rates of 57600 bps or less, but begins to show
some errors at rates of 115200 bps and above.
The circuit of Figure 12 offers another RS232
solution that works well at higher baud rates. It uses a
Maxim product (the MAX3222E) that is capable of
operating at up to a 250 kbps rate (be sure to visit
www.maximintegrated.com for more information).
The MAX3222E RS232 transceiver contains
ELM327L DSA
+5V
RS232
Interface
4.7KΩ
(DB9F)
10KΩ
3 (TxD)
2N3904
10KΩ
+5V
5 (SG)
0.1µF
10KΩ
4.7KΩ
2N3906
2 (RxD)
+5V
1 (DCD)
19
18
17
16
15
327
4 (DTR)
6 (DSR)
7 (RTS)
Figure 11. A Low Speed
RS232 Interface (≤ 57.6 kbps)
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8 (CTS)
82 of 94
ELM327L
Example Applications (continued)
internal charge pump circuitry that generates the
voltages required for RS232 communications, in
addition to the analog interface circuitry needed. All
you have to do is provide a few capacitors, and it does
the rest.
We do caution that the MAX3222E does seem to
place some extra demands on the 5V power supply. It
should work fairly well with the 7805 regulator that we
show in Figure 9, but if you have chosen a lower
current device like the 78L05 or LP2950, you may
experience occasional ‘LV RESET’s. If you do, we
recommend adding two capacitors to the circuit of
Figure 9. We suggest that a 10 µF 50V capacitor be
connected in parallel with C1, and also that a 33 µF
10V unit be installed across C5 (the 5V ‘rail’). This
should eliminate problems, but if it does not, you may
also wish to consider a more powerful regulator (such
as the 7805 that we show in Figure 9) if you do not
already have one installed.
The USB interface of Figure 13 provides another
way to connect the ELM327 to USB systems. It uses a
Silicon Laboratories (www.silabs.com) CP2102 chip to
convert between the ELM327’s serial data and USB.
One of the advantages of going to a USB interface
is the high serial baud rates that you may experience.
In order to use these higher rates, you will have to
program both the USB interface and the ELM327
interface for them.
The CP2102 interface baud rate is actually
configured by driver software. When you set the baud
rate in your terminal program, the software does what
is necessary to configure the CP2102 for operation at
that rate and you do not need to do anything more.
The ELM327 initially only uses a 38,400 bps rate,
however, and must be told to use anything different.
In order to change from the standard 38.4 k baud
rate, you must first set your software to 38.4 kbps, and
power up your ELM327 circuit. Make sure that it is
working, as described in the ‘Communicating with the
ELM327’ section, before you do anything else. When
you are confident that all is well, you can then change
the baud rate. Before doing so, we caution that you
should check to be sure that your software actually
supports the desired rate (as several can not handle
more than about 250 kbps).
The CP2102 chip is able to support a 115.2 kbps
rate natively, and almost all software should be able to
support it as well, so we will use that rate to provide an
example.
First, while connected to the ELM327 at 38.4 kbps,
ELM327L DSA
RS232
Interface
(DB9F)
10
9
11
8
12
7
13
6
14
5
15
4
16
3
17
2
3 (TxD)
0.47µF
2 (RxD)
5 (SG)
0.47µF
+5V
1 (DCD)
0.47µF
18
4 (DTR)
6 (DSR)
0.1µF
1
7 (RTS)
0.1µF
8 (CTS)
+5V
18
17
16
Figure 12. A High Speed
RS232 Interface (≤ 250 kbps)
15
327
5.0V
TVS
+5V
7
1µF
SiLabs
CP2102
6
26 25
0.1µF
USB
Interface
(type ‘B’
connector)
8
1 (+5)
5
2 (D-)
4
3 (D+)
3
4 (SG)
+5V
19
18
17
16
15
327
Figure 13. An Alternative USB Interface
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ELM327L
Example Applications (continued)
3.3V
Interface
we need to change the default rate to 115.2 kbps. No
need to worry that this will affect your communications,
as it will not take effect until the ELM327 has been
reset. To change the data rate, simply change the
value that is stored in Programmable Parameter 0C,
then enable it:
+3.3V
Tx
Rx
3.3V common
>AT PP 0C SV 23
OK
8
7
6
5
1
2
3
4
ADuM1201
+5V
> AT PP 0C ON
OK
That is all that is needed to semi-permanently
change the ELM327. We say semi, because it stays in
effect through power downs, and resets, but you can
change it again if you wish. If you now reset the
ELM327 (send AT Z or power down then up), the
ELM327 will begin operating at the new rate (of
115200 bps, rather than to 38400 bps). Change your
software setting to also be 115200 bps, and you
should be communicating. If you go through the
calculations, you will note that the ELM327 baud rate
is actually off by about 0.8%, but modern UARTs can
typically handle rate errors of a few % without any
problems.
When working with the CP2102, we do caution
that it is very small and difficult to solder by hand, so
be prepared for that. Also, if you provide protection on
the data lines with transient voltage suppressors
(TVS’s), be careful when choosing devices, as some
exhibit a very large capacitance and will affect the
transmission of the USB data.
Our next circuit (Figure 14) shows one way to
interface a 5V ELM327 to circuits that operate at a
different voltage level. We show 3.3V as an example,
but it can actually be anything from 2.7V to 5.5V.
The circuit uses the ADuM1201 iCoupler chip from
Analog Devices (www.analog.com). In addition to
acting as a level translator, this device also provides
isolation (galvanic, to 2500 Vrms) between the two
sides. This is often desired in order to keep the vehicle
circuit completely separate from the computer circuit.
Typically, one might use a standard level shifter IC
to interface to 3.3V – for example the TXB0102 by
Texas Instruments (www.ti.com), or the ST2129 from
ST Microelectronics (www.st.com), but the ADuM1201
offers several other advantages. The main difference
is that it offers isolation, as mentioned, but it also can
be used with 2.7V to 5.5V on either side, it provides a
high output if the input side is unpowered, and it
ELM327L DSA
+5V
+5V
21
20
19
18
17
n.c.
16
Rx
Tx
Busy RTS
11
12
15
327
7
8
9
10
13
14
Figure 14. Connecting to a 3.3V System
typically uses less current and is much faster than
many opto-isolator solutions (the ‘1201 minimum data
is 1 MHz). Of course there are a couple disadvantages
too. It does use current (almost 1 mA), so may be an
issue if trying to reduce sleep current to a minimum,
and it does cost more than devices like the TXB0102.
Many wireless modules (WiFi or Bluetooth®) use
serial interfaces just like what we have shown here for
the RS232 connections. Connecting to them should
not be very difficult if you follow the manufacturers
directions (and perhaps consider using devices like the
ADuM1201 or the TXB0102). If you are considering
using a Bluetooth interface, you might read our ‘AN-04
ELM327 and Bluetooth®’ application note first.
Our final example, Figure 15, shows a complete
3.3V circuit using the ELM327L. It is very similar to
Figure 9, but a 3.3V regulator is used, some resistor
values have been scaled, and Q8 has been added to
keep the pin 3 current to a minimum. Note that instead
of a 1N914/4148 silicon diode for D4, we recommend
using a schottky diode such as the SD103.
This has provided a few examples of how the
ELM327 integrated circuit might typically be used.
Hopefully it has been enough to get you started on
your way to many more. The next section shows how
you might be able to optimize these circuits to reduce
power consumption…
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ELM327L
CAN-L
14
6
CAN-H
3.3V
R22
100Ω
Vbat
R23
100Ω
C7
560pF
50V
R24
4.7KΩ
8
6
5
SN65
HVD234D
C8
560pF
50V
1
U2
Vbat
7
2
3
4
R19
510Ω
3.3V
78M33
D1
C1
0.1µF
50V
5
Signal
Ground
C6
0.1µF
Vbat
In
16
Battery
Positive
3.3V
U3
12V
Out
C
L5
C5
0.1µF
R31
200Ω
R17
510Ω
15
ISO-L
R16
1.5KΩ
U5
Q6
7
ISO-K
R18
1.5KΩ
1 (DCD)
4 (DTR)
6 (DSR)
3.3V
Q7
ELM327L
pin 12
R20
47KΩ
7 (RTS)
8 (CTS)
R32
4.7KΩ
3.3V
D4
3 (TxD)
R21
22KΩ
USB
Interface
(mini B)
2 (RxD)
L1-L4
5 (SG)
R27-30
200Ω
Vbat
3.3V
3.3V
n.c.
28
U1
R25
47KΩ
27
26
25
24
23
22
21
20
19
18
17
16
15
9
10
11
12
13
14
9 (RI)
FTDI
DB9-USB-D3-F
327L
3.3V
R26
4.7KΩ
1
C2
0.1µF
OBD
Interface
Adj
(J1962)
R2
10KΩ
6
7
8
to
R21
C3
27pF
C4
27pF
R1
4.7KΩ
R8
22KΩ
Figure 15.
An OBD to USB Interpreter
(VDD = 3.3V)
Q1
R9
22KΩ
2
J1850 Bus +
R10
22KΩ
10
J1850 Bus R15
3KΩ
Q3
to
R15
Q8
Q2
D2
5
X1
4.00MHz
R3
1.2KΩ
R6
10KΩ
R7
3KΩ
4
In
317L
R5
470Ω
3
C9
10µF
12V
Out
2
D3
ELM327L
pin 14
R11
10KΩ
3.3V
R14
4.7KΩ
R12
47KΩ
Q4
Q5
R13
10KΩ
ELM327L DSA
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85 of 94
ELM327L
Modifications for Low Power Standby Operation
If you wish to semi-permanently install your
ELM327 circuit in a vehicle, current consumption
quickly becomes an issue. A typical ELM327 circuit
draws about as much current as a dome lamp, so can
not be left on for long without the vehicle’s engine
running.
When it is not needed, the ELM327 may be placed
in a low power ‘standby’ mode in which it consumes
very little current. Reducing the integrated circuit
current itself is generally not enough, however, as you
must also consider the current drawn by the other
components as well.
The following discussion takes the circuit of Figure
9, and modifies it slightly in order to highlight our
suggestions for reducing current consumption. The
resulting circuit (Figure 16) is shown following. Note
that portions of the circuit that are the same as Figure
9 are shown in grey, while the changes that we are
making are shown in black.
Consider the circuit of Figure 9 in the Example
Applications section. With 12V to 15V applied to the
‘Battery Positive’ input, the total current used by this
circuit is typically:
base current = 23.9 mA
The same circuit with a standard (ie 5V only)
ELM327 IC would typically use about 30 mA.
Without making any wiring changes, you can
reduce this current by placing the ELM327L into the
low power standby mode. This only requires sending
the Low Power command (AT LP), after which the
current would typically drop to:
current after AT LP = 17.5 mA
At this point, a standard ELM327 IC would use
almost exactly the same current, so it does not really
matter which IC your circuit uses. This reduction in
current is due entirely to the change in the ELM327’s
operating current (it only needs a very small current to
stay in standby mode). But where is the other current
coming from? One obvious load is the LED that shows
that the power is on. The other is the CAN transceiver
IC, U2. By disconnecting the common connections
from R24 and R31, and then returning both to pin 16 of
the ELM327, we can switch the current that these two
use. With this change (which is shown as modification
#1 in Figure 16), the current after AT LP drops further:
There is a considerable current still flowing in the
circuit at this point, but it should mainly be the voltage
regulators that are using it. In the next step, we will
change U3 (a 7805) to an LP2950ACZ-5.0G, and see
how effective that is. While the LP2950 is a good
choice for its lower quiescent current, it does suffer
from stability problems if you do not provide capacitive
loading as shown. Note that the 4.7uF capacitor is
tantalum, while the 2.2µF one is aluminum. At this
point it may also be useful to review our Application
Note ‘AN03 - ELM327 Low Voltage Resets’, as you
may want to use an even larger capacitor on the 5V
side. After changing U3 for an LP2950, the current is
typically:
current after mod #2 = 3.5 mA
If we continue to reduce the load currents beyond
this point, we will quickly get to a point where any
currents injected from external sources (ie. through the
protection diodes at inputs such as pins 2 or 12) will
become significant compared to the load currents. If
these currents should exceed the load current, the Vdd
voltage will rise and damage might result. To prevent
the voltage from rising too high, we recommend that
you add either a zener diode or a transient voltage
suppressor (TVS), directly across the 5V supply, as
shown in the top right corner of Figure 16. Suggestions
for devices to consider are the 1N5232B zener diode
or the SA5.0AG TVS.
Another integrated circuit that is not changing
current during low power mode is the 317L (U4). In
fact, a quick calculation shows that it is likely using
about 2.6 mA when idle, which is very significant. If we
replace this IC with another that uses less current, we
will be close to getting the total circuit current to less
than 1 mA.
Figure 16, modification #3 shows an LP2951ACM
regulator in the circuit, as a replacement for the 317L.
It uses much less current than the 317 during normal
operation, and offers a shutdown control input as well
to further reduce current when it is not required. Note
that the LP2951 circuit needs the ELM327 to provide a
high level at pin 3 for a 5V output, and a low for 8V, so
an inversion is needed. To do this, set PP 12 to 00
with:
>AT PP 12 SV 00
>AT PP 12 ON
current after mod #1 = 8.2 mA
ELM327L DSA
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ELM327L
Modifications for Low Power Standby Operation (continued)
then reset the chip, and the voltages will always be
correct for J1850 from that point on.
Note that the LP2951 regulator also requires a
4.7uF tantalum capacitor for stability (and it helps with
transient capability too).
Leaving U4’s pin 3 solidly connected to circuit
common, we now find that the current is about:
current after mod #3 = 1.1 mA
and, if we tie pin 3 of the LP2951 to pin 16 of the
ELM327, the LP current becomes:
current after mod #4 = 0.9 mA
We can suggest one more change at this point.
The MCP2551 shown draws about 0.3 mA when in
standby mode, and this can be improved upon. By
replacing this chip with the newer MCP2561, the
standby current will be reduced even further. Typically,
you will see a change of at least 0.2 mA, giving:
current after mod #5 = 0.7 mA
The current has been reduced considerably
through the circuit modifications, but why is there still
current flowing? This is due to a number of things,
some that you can change, others which you can not.
The MCP2561 and the ELM327 are never completely
shut off, but are instead in a low power mode, while
the LP2950 regulator is operating normally (as the
MCP2551 and the ELM327 need it). You can not do
anything about that.
There are some currents that you may reduce, by
choice in your design. The R25/R26 resistor pair uses
current, but do you really need to monitor battery
voltage? Similarly, the R20/R21 pair passes current,
but does your application need ISO9141 or ISO14230
support? The OBD Tx LED flashes when in Low
Power mode, but do you need it to? (You may turn it
off with PP 0F b0, but it only uses about 25 µA on
average). All of these little currents eventually add up
to what we’ve measured here.
These few changes that we have shown have
reduced the total current from 24 mA to less than 1 mA
(and power from about 300 mW to 8 mW) - a
considerable savings. We leave any further
improvements to you.
ELM327L DSA
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87 of 94
ELM327L
Modifications for Low Power Standby Operation (continued)
CAN-L
14
2
6
CAN-H
R22
100Ω
C7
560pF
50V
R24
4.7KΩ
8
C8
560pF
50V
7
6
5
5
MCP2561
1
U2
Vbat
Vbat
n.c.
R23
100Ω
2
3
4
D1
+
C1
0.1µF
50V
2.2µF
50V
R17
510Ω
15
ISO-L
+
C5
0.1µF
R18
2.2KΩ
7 (RTS)
8 (CTS)
R32
4.7KΩ
+5V
D4
ELM327L
pin 12
3 (TxD)
R21
33KΩ
L1-L4
(mini B)
5 (SG)
+5V
9 (RI)
+5V
FTDI
DB9-USB-D5-F
Vbat
U1
R25
47KΩ
28
27
26
25
24
23
22
21
20
19
18
17
16
15
9
10
11
12
13
14
327L
+5V
1
C2
0.1µF
2
3
4
5
6
7
8
+12V
C9
10µF
8
7
6
5
U4
47KΩ
3
1
2
3
to
R21
to
R15
4
X1
4.00MHz
LP2951
(J1962)
4.7µF
16V
USB
Interface
2 (RxD)
R27-30
470Ω
OBD
Interface
R31
470Ω
1 (DCD)
4 (DTR)
6 (DSR)
+5V
Q7
R26
10KΩ
4.7µF
10V
L5
U5
Q6
R20
47KΩ
5.0V
TVS
1
R16
2.2KΩ
7
ISO-K
+5V
LP2950
5
Signal
Ground
C6
0.1µF
Vbat
R19
510Ω
16
Battery
Positive
+5V
U3
+12V
C3
27pF
4
C4
27pF
+
R6
10KΩ
22KΩ
Q2
D2
R7
4.7KΩ
4.7KΩ
R1
4.7KΩ
Q1
R9
10KΩ
2
J1850 Bus +
R10
22KΩ
10
J1850 Bus R15
4.7KΩ
Q3
R8
22KΩ
D3
ELM327L
pin 14
R11
10KΩ
+5V
R14
4.7KΩ
R12
47KΩ
Figure 16.
Modifications to reduce power
Q4
Q5
R13
10KΩ
ELM327L DSA
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ELM327L
Error Messages and Alerts
The following shows what the ELM327 will send to
warn you of a condition or a problem. Some of these
messages do not appear if using the automatic search
for a protocol, or if the Programmable Parameter bits
disable them.
?
This is the standard response for a misunderstood
command received on the RS232 input. Usually it is
due to a typing mistake, but it can also occur if you try
to do something that is not appropriate (eg. trying to do
an AT FI command if you are not set for protocol 5).
ACT ALERT or !ACT ALERT
This message occurs as a warning that there has
been no RS232 or OBD activity for some time (see the
Power Control section for details). If allowed, the IC
will be initiating a switch to the Low Power (standby)
mode of operation. If this was initiated by no RS232
activity, sending something within the next minute will
stop the switch to low power. Note that the ‘!’ before
ACT ALERT is printed if PP 0F bit 1 is 1.
BUFFER FULL
activity to do so (it needs a pause in activity in order to
insert the message). Although this could be because
the bus was in fact very busy, it is almost always due
to a wiring problem that is giving a continuously active
input. If this is an initial trial with your new ELM327
circuit, then check all of the voltage levels at the
offending OBD input, as this error is very likely due to
a wiring problem (see our ‘AN02 - ELM327 Circuit
Construction’ for some typical voltages).
BUS ERROR
A generic problem has occurred. This is most
often from an invalid signal being detected on the bus
(for example, a pulse that is longer than a valid Break
signal), but usually is from a wiring error. Note that
some vehicles may generate long pulses as part of
their startup process, so you may see this message as
part of a normal vehicle startup while ‘monitoring all.’
The ELM327L provides an internal 2048 byte
storage area (or buffer) that is used to temporarily
store messages before they are sent to the PC. It fills
at a rate determined by the vehicle, and empties at a
rate determined by the computer connection.
Occasionally (particularly with CAN systems) the
buffer will fill at a faster rate than it is being emptied by
the PC. Eventually it may become full, and no more
data can be stored (it is lost). This error message tells
you that the storage area has filled up.
If you are receiving BUFFER FULL messages,
and you are using a lower PC baud rate, give serious
consideration to changing that rate to something
higher. If you still receive BUFFER FULL messages
after that, you might consider turning the headers and
maybe the spaces off (with AT H0, and AT S0), or
using the CAN filtering commands (AT CRA, or CM
and CF) to reduce the amount of data being sent.
CAN ERROR
BUS BUSY
There was an error in the line that this points to,
either from an incorrect checksum, or a problem with
the format of the message (the ELM327 still shows
you what it received). There could have been a noise
This occurs when the ELM327 tries to send a
message, or to initialize the bus, and detects too much
ELM327L DSA
The CAN system had difficulty initializing, sending,
or receiving. Often this is simply from not being
connected to a CAN system when you attempt to send
a message, but it may be because you have set the
system to an incorrect protocol, or to a baud rate that
does not match the actual data rate. As with BUS
ERRORs, the CAN ERROR might also be the result of
a wiring problem, so if this is the first time using your
ELM327 circuit, review all of your CAN interface
circuitry before proceeding.
DATA ERROR
There was a response from the vehicle, but the
information was incorrect or could not be recovered.
<DATA ERROR
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ELM327L
Error Messages and Alerts (continued)
burst which interfered, possibly a circuit problem, or
perhaps you have the CAN Auto Formatting (CAF) on
and you are looking at a system that is not of the
ISO 15765-4 format. Try resending the command
again – if it was a noise burst, it may be received
correctly the second time.
ERRxx
There are a number of internal errors that might be
reported as ERR with a two digit code following. These
occur if an internally monitored parameter is found to
be out of limits, or if a module is not responding
correctly. If you witness one of these, contact Elm
Electronics for advice.
One error that is not necessarily a result of an
internal problem is ERR94. This code represents a
‘fatal CAN error’, and may be seen if there are CAN
network issues (some non-CAN vehicles may use pins
6 and 14 of the connector for other functions, and this
may cause problems). If you see an ERR94, it means
that the CAN module was not able to reset itself, and
needed a complete IC reset to do so. You will need to
restore any settings that you had previously made, as
they will have been returned to their default values.
Beginning with v1.3a of this IC, an ERR94 will also
block further automatic searches through the CAN
protocols, if bit 5 of PP 2A is a ‘1’ (it is by default). This
is done because most ERR94s will be as a result of
serious CAN wiring problems. Blocking of the CAN
protocols remains in effect until the next power off and
on, or until an AT FE is sent.
FB ERROR
When an OBD output is energized, a check is
always made to ensure that the signal also appears at
the respective input. If there is a problem, and no
active input is detected, the IC turns the output off and
declares that there was a problem with the FeedBack
(FB) of the signal. If this is an initial trial with your
ELM327, this is almost certainly a wiring problem.
Check your wiring before proceeding.
LP ALERT or !LP ALERT
This appears as a warning that the ELM327 is
about to switch to the Low Power (standby) mode of
operation in 2 seconds time. This delay is provided to
allow an external controller enough time to prepare for
the change in state. No inputs or voltages on pins can
ELM327L DSA
stop this action once initiated. Note that the ‘!’ before
LP ALERT is printed if PP 0F bit 1 is 1.
LV RESET
The ELM327 continually monitors the 5V supply to
ensure that it is within acceptable limits. If the voltage
should go below the low limit, a ‘brownout reset’ circuit
is activated, and the IC stops all activity. In rare cases,
a sudden large change in VDD can also trigger a low
voltage reset.
When the voltage returns to normal, the ELM327
performs a full reset, and then prints LV RESET. Note
that this type of reset is exactly the same as an AT Z
or MCLR reset (but it does not print ELM327 v2.1).
An LV RESET will also block automatic searches
through the CAN protocols, if bit 4 of PP 2A is a ‘1’ (it
is by default). This is done because most LV RESETs
seem to occur as a result of CAN wiring problems (the
transceiver is capable of passing very large currents).
Blocking of the CAN protocols is only done until the
next reset (AT Z, WS, etc.) or until an AT FE is sent.
NO DATA
The IC waited for the period of time that was set
by AT ST, and detected no response from the vehicle.
It may be that the vehicle had no data to offer for that
particular PID, that the mode requested was not
supported, that the vehicle was attending to higher
priority issues, or in the case of the CAN systems, the
filter may have been set so that the response was
ignored, even though one was sent. If you are certain
that there should have been a response, try increasing
the ST time (to be sure that you have allowed enough
time for the ECU to respond), or restoring the CAN
filter to its default setting.
<RX ERROR
An error was detected in the received CAN data.
This most often occurs if monitoring a CAN bus using
an incorrect baud rate setting, but it may occur if
monitoring and there are messages found that are not
being acknowledged, or that contain bit errors. The
entire message will be displayed as it was received (if
you have filters set, the received message may not
agree with the filter setting). Try a different protocol, or
a different baud rate.
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ELM327L
Error Messages and Alerts (continued)
STOPPED
UNABLE TO CONNECT
If any OBD operation is interrupted by a received
RS232 character, or by a low level on the RTS pin, the
ELM327 will print the word STOPPED. If you should
see this response, then something that you have done
has interrupted the ELM327. Most people see it
because they have not waited for pin 15 to go high, or
have not waited for the prompt character (‘>’) to be
displayed before sending the next command.
Note that short duration pulses on pin 15 may
cause the STOPPED message to be displayed, but
may not be of sufficient duration to cause a switch to
Low Power operation (as the Low Power circuit
debounces the signal, filtering them out).
If you see this message, it means that the ELM327
has tried all of the available protocols, and could not
detect a compatible one. This could be because your
vehicle uses an unsupported protocol, or could be as
simple as forgetting to turn the ignition key on.
If you are sure that your vehicle uses an OBDII
protocol, then check all of your connections, and the
ignition, then try the command again.
Version History
We are often asked about the differences between
the firmware in the various ELM327 integrated circuits.
The ELM327L started with firmware v2.1:
v2.1
First version available. Supports all ELM327 v2.1
functions and:
- requires a 10µF capacitor on pin 6
- has a 2048 byte RS232 transmit buffer
Ordering Information
ELM327L integrated circuits are 28 pin devices, available in either a 300 mil wide (‘skinny’) Plastic DIP format, or in
a 300 mil (7.50 mm body) SOIC surface mount type of package, or in a 208 mil (5.30 mm body) SSOP surface
mount package. We do not offer a QFN package.
To order, add the appropriate suffix (P, SM, or SS) to the ELM327L part number:
300 mil wide, 28 pin, 0.10” (2.54 mm) pitch, PDIP.............................................................................. ELM327LP
300 mil wide, 28 pin, 0.050” (1.27 mm) pitch, SOIC........................................................................ ELM327LSM
208 mil wide, 28 pin, 0.025” (0.65 mm) pitch, SSOP........................................................................ELM327LSS
ELM327L DSA
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ELM327L
Outline Diagrams
The diagrams at the right show the three package
styles that the ELM327L is available in.
The top diagram shows our ELM327LP product in
what is commonly called a ‘300 mil skinny DIP
package’. It is used for through hole applications.
The middle diagram shows the ELM327LSM
dimensions. This package is also often referred to as
being 300 mil (0.300”) wide, and is called an SOIC
(Small Outline IC) package. We have chosen to simply
refer to it as the SM (surface mount) version.
The bottom drawing shows our smaller surface
mount ELM327LSS. The body width is narrower and
the IC may be referred to as a 208 mil device, or the
equivalent 5.30 mm as shown. This package is often
used when space is at a premium (but is more difficult
to hand solder).
The drawings shown here provide only the basic
dimensions for these ICs. Please refer to the following
Microchip Technology Inc. documentation for more
detailed information:
Package Drawings and Dimensions Specification,
(document name en012702.pdf - 7.5 MB).
Go to www.microchip.com, select ‘Design Support’
then ‘Documentation’ then ‘Packaging Specifications’,
or go directly to www.microchip.com/packaging
PIC18F66K80 Family Data Sheet,
(document name 39977fd.pdf - 5.1 MB).
Go to www.microchip.com, select ‘Design Support’
then ‘Documentation’ then ‘Data Sheets, and search
for 18F25K80.
ELM327LP
2.54
7.24
max
10.92
ELM327LSM
1.27
7.50
10.30
ELM327LSS
0.65
5.30
7.80
Note: all dimensions shown are in mm.
ELM327 is a registered trademark of Elm Electronics Inc.
All rights reserved. Copyright 2005 to 2014 by Elm Electronics Inc.
Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be
given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information
described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these
products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or
systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve
reliability, function, or design.
ELM327L DSA
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ELM327L
Index
A
F
Absolute Maximum Ratings, 7
Activity Monitor, 65
Altering Flow Control Messages, 61
Applications, Example, 79-85
AT Commands, 11
AT Command
Descriptions, 13-29
Summary, 11-13
Alerts, Error Messages and, 89-91
Features, 1
Figure 9, 81
Filter and Mask, CAN, 48
Flow Control Messages, Altering, 61
FMS Standard, 59
Frequency Matching, CAN, 63
H
Headers, setting them, 40-42
Higher RS232 Baud Rates, 51-52
History, Version, 91
B
Battery Voltage, Reading the, 30
Baud Rates, Using Higher RS232, 51-52
Block Diagram, 1
Bus FMS Standard, 59
Bus Initiation, 34
I
Initiation, Bus, 34
Inputs, unused, 6
Interface, Microprocessor, 77-78
Interpreting Trouble Codes, 35
C
CAN Data Rates, Maximum, 75
CAN Extended Addresses, Using, 62
CAN Frequency Matching, 63
CAN Message Types, 45
CAN Receive Filtering, 47
Codes, Trouble,
Interpreting, 35
Resetting, 36
Commands, AT
Descriptions,13-29
Summary, 11-13
Commands, OBD, 31
Communicating with the the ELM327, 9-10
CRA, the Command, 47
J
J1939,
FMS Standard, 59
Messages, 54-55
NMEA 2000, 60
Number of responses, 57
Using, 56-59
K
KeepAlive (Wakeup) Messages, 60
L
Low Power Operation,
Description, 65-68
Modifications, 86-88
D
Data Byte, Saving, 64
Description and Features, 1
E
Electrical Characteristics, 7-8
Error Messages, 89-91
Example Applications
Basic, 79-81
Connecting to 3.3V, 84
Figure 9, 81
Low Power, 86
USB, 81, 83, 85
Extended Addresses, CAN, 62
ELM327L DSA
M
Mask and Filter, CAN, 48
Maximum CAN Data Rates, 75
Maximum Ratings, Absolute, 7
Messages and Filtering, CAN, 47
Messages, Error, 89-91
Message Formats, OBD, 39-40
Message Types, CAN, 45
Microprocessor Interfaces, 77-78
Modifications for Low Power, 86-88
Monitoring the Bus, 49
Multiline Responses, 43-44
Multiple PID Requests, 46
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ELM327L
Index (continued)
N
S
NMEA 2000, Standard, 60
Number of Responses,
J1939, 57
OBDII, 33, 53
Saving a Data Byte, 64
Selecting Protocols, 37-38
Serial Numbers, Programming, 64
Setting the Headers, 40-42
Setting Timeouts (AT & ST commands), 53
Specify the Number of Responses, 33, 53, 57
Summary,
AT Commands, 11-13
Programmable Parameters, 70-74
O
OBD Commands, 31
OBD Message Formats, 39-40
Order, Restoring, 50
Ordering Information, 91
Outline Diagrams, 92
Overview, 9
T
Talking to the Vehicle, 32-33
Timeouts (AT & ST commands), 53
Trouble Codes,
Interpreting, 35
Resetting, 36
P
Pending Response Messages, 46
Periodic Messages, 60
Pin Descriptions, 4-6
Pin 28, resetting Prog Parameters, 70
Power Control,
Description, 65-68
Modifications, 86-88
Programmable Parameters,
general, 69-70
reset with pin 28, 70
Summary, 70-74
types, 70
Programming Serial Numbers, 64
Protocols, list of supported, 37
Protocols, Selecting, 37-38
Q
Quick Guide for Reading Trouble Codes, 36
U
Unused pins, 6
Upgrading Versions, 78
Using J1939, 56-59
Using CAN Extended Addresses, 62
Using Higher RS232 Baud Rates, 51-52
V
Version History, 91
Versions, Upgrading, 78
Voltage, Reading the Battery, 30
W
Wakeup Messages, 60
Wiring Test, 74, 89, 90
R
Reading the Battery Voltage, 30
Reading Trouble Codes, Quick Guide for, 36
Requests, Multiple PID, 46
Resetting,
Prog Parameters, 70
Trouble Codes, 36
Response Pending Messages, 46
Responses, Multiline, 43-44
Restoring Order, 50
RS232 Baud Rates, Using Higher, 51-52
ELM327L DSA
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