OBD to RS232 Interpreter
Almost all new automobiles produced today are
required, by law, to provide an interface from which
test equipment can obtain diagnostic information.
The data transfer on these interfaces follow several
standards, none of which are directly compatible
with PCs or PDAs. The ELM327 is designed to act
as a bridge between these On-Board Diagnostics
(OBD) ports and standard RS232 ports.
The ELM327 builds on improved versions of our
proven ELM320, ELM322, and ELM323 interfaces
by adding seven CAN protocols to them. The result
is an IC that can automatically sense and convert
the most common protocols in use today. There are
a number of other improvements as well – a high
speed RS232 option, battery voltage monitoring, and
customizable features through programmable
parameters, to name only a few.
The ELM327 requires few external components
to make a fully functioning circuit. The following
pages discuss the interface details, and show how to
use the IC to ‘talk’ to your vehicle, then concludes
with two schematics to get you started.
• Supports 12 protocols
• Automatically searches for a protocol
• Fully configurable with AT commands
• RS232 baud rates to 500Kbps
• Voltage input for battery monitoring
• Low power CMOS design
Connection Diagram
(top view)
J1850 Volts
RS232 Tx LED
J1850 Bus+
RS232 Rx LED
Baud Rate
• Diagnostic trouble code readers
• Automotive scan tools
• Teaching aids
RS232 Rx
RS232 Tx
J1850 Bus-
4.00 MHz
Block Diagram
Baud Rate
status LEDs
ISO 15765-4
SAE J1939*
ISO 9141-2
ISO 14230-4
OBD interfaces
Elm Electronics – Circuits for the Hobbyist
SAE J1850
*some support
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Pin Descriptions
MCLR (pin 1)
A momentary logic low applied to this input will reset
the IC. 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 will output a logic
high level when a nominal 8V is required (for J1850
VPW), and will output a low level when 5V is needed
(as for J1850 PWM applications). If this switching
capability is not required for your application, this
output can be left open-circuited.
J1850 Bus+ (pin 4)
This active high output is used to drive the
J1850 Bus+ Line to an active level. Note that this
signal does not have to be used for the Bus- Line (as
was the case for the ELM320), since a separate
J1850 Bus- drive output is provided on pin 14.
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.
Baud Rate (pin 6)
This input controls the baud rate of the RS232
interface. If it is at a high level during power-up or
reset, the baud rate will be set to 38400 (or the
rate that has been set by PP 0C). If at a low level,
the baud rate will be 9600.
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 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 (see the
section on AT Commands).
VSS (pins 8 and 19)
Circuit common must be connected to these pins.
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.
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.
All rights reserved. Copyright 2005, 2006, and 2007 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.
Elm Electronics – Circuits for the Hobbyist
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Pin Descriptions (continued)
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.
RS232Rx (pin 18)
This is the RS232 receive data input. The signal
level is compatible with most interface ICs (when at
idle, the level is normally high), but can be used with
other interfaces as well, since the 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
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 microprocessor, so an external
reset signal is not required. Refer to the Electrical
Characteristics section for further information.
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. If unused, this output can be left
RTS (pin 15)
This active low “Request To Send” input can be used
to interrupt the OBD processing in order to send a
new command. Normally high, the line is brought low
for attention, and should remain so until the Busy
line (pin 16) indicates that the ELM327 is no longer
busy. This input has Schmitt trigger waveshaping.
Busy (pin 16)
This active high output shows the current state of the
ELM327. If it is at a low level, the processor is ready
to receive ASCII commands and characters, but if it
is at a high level, commands are being processed.
RS232Tx (pin 17)
This is the RS232 data transmit output. The signal
level is compatible with most interface ICs (output is
normally high), and there is sufficient current drive to
allow interfacing using only a PNP transistor, if
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 transeiver IC for proper
operation. If you are connecting to an existing CAN
system, the integrity of the entire system might be
jeopardized if a proper interface is not used. See the
Example Applications section for more information.
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. Current capability is suitable for
directly driving most LEDs through current limiting
resistors, or interfacing to other logic for status
reporting. If unused, these pins should be left opencircuited.
Ordering Information
These integrated circuits are 28 pin devices, available in either a 300 mil wide plastic (‘skinny’) DIP format or in a
300 mil SOIC surface mount type of package. To order, add the appropriate suffix to the part number:
300 mil 28 pin Plastic DIP..............................ELM327P
300 mil 28 pin SOIC....................................ELM327SM
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Absolute Maximum Ratings
Storage Temperature....................... -65°C to +150°C
Ambient Temperature with
Power Applied....................................-40°C to +85°C
Voltage on VDD with respect to VSS............ 0 to +7.5V
Stresses beyond those listed here will likely damage
the device. These values are given as a design
guideline only. The ability to operate to these levels
is neither inferred nor recommended.
Voltage on any other pin with
respect to VSS........................... -0.3V to (VDD + 0.3V)
Electrical Characteristics
All values are for operation at 25°C and a 5V supply, unless otherwise noted. For further information, refer to note 1 below.
Supply voltage, VDD
VDD rate of rise
Average supply current, IDD
Input threshold voltage
Schmitt trigger
input thresholds
Maximum Units
see note 2
see note 3
all except Schmitt inputs
see note 4
Output low voltage
current (sink) = 10 mA
Output high voltage
current (source) = 10 mA
Brown-out reset voltage
A/D conversion time
see note 5
1. This integrated circuit is produced with one of Microchip Technology Inc.’s PIC18F2x8x family of devices as
the core embedded microcontroller. For further device specifications, and possibly clarification of those
given, please refer to the appropriate Microchip documentation (available at http://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.
3. Device only. Does not include any load currents.
4. Pins 1, 11, 12, 13, 15 and 18 (only) have internal Schmitt trigger waveshaping circuitry. All other inputs use
standard CMOS circuitry.
5. The typical width of the Busy output pulse while the ELM327 interprets the command, measures the voltage,
scales it and then transmits the result of a mid-range measurement, with the RS232 rate at 38400 baud.
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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 product 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 a PDA with a
terminal program (such as HyperTerminal or ZTerm),
and knowledge of one or two OBD commands, which
we will provide in the following sections…
Communicating with the ELM327
The ELM327 expects to communicate with the
host through an RS232 serial connection. Modern
computers do not usually provide a physical
connection such as this, but 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 ethernet to RS232
devices, or Bluetooth to serial adapters.
No matter how you physically connect to the
ELM327, you will need a way to send and receive
characters. To do this, 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 make
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 be either 9600
baud (if pin 6=0V at power up), or 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 also 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
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 v1.2
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
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.
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 analyzing the entire
string once the complete message has been received.
Commands for the ELM327’s internal use will always
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 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
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
for validity – it only ensures that an even number of
hex digits were received, combined into bytes, then
sent out the OBD port, and it does not know if the
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Communicating with the ELM327 (continued)
message sent to the vehicle was in error.
While processing OBD commands, the ELM327
will continually monitor for an RTS input, or an RS232
input. Either one will interrupt the IC, quickly returning
control to the user, and possibly aborting any initiation,
etc. that was in progress. If you desire to interrupt the
ELM327, that’s fine, but for normal orderly data
transfer, users 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 ‘ATZ’ is equivalent to ‘atz’, and to
‘AtZ’. Also, it ignores space characters and all control
characters (tab, linefeed, etc.) in the input, so they can
be inserted anywhere to improve readability. Another
feature is that sending only a single carriage return
character will always repeat the last command (making
it easier to request updates on dynamic data such as
engine rpm).
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 issued.
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
internal configuration or ‘AT’ commands, and will be
executed upon receipt of a terminating carriage return
character. The ELM327 will usually reply with the
characters ‘OK’ on the successful completion of a
command, so the user knows that it has been
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 may prove useful if you wish to interpret the
values. Also, one should be aware that for the on/off
types of commands, the second character is the
number 1 or 0, the universal terms for on and off.
The following is a description of all of the AT
commands that are recognized by the current version
of the ELM327. Since there are many, a summary
page is provided after this section.
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 the contents of the first header
byte. If it shows that the message uses physical
addressing, the third header byte of the header is used
for the receive address, otherwise (for functional
addressing) the second header byte, increased in
value by 1, will be used. Auto Receive is turned on by
default, and is not used by the J1939 formatting.
[ 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).
[ Automatically set the Receive address ]
Responses from the vehicle will be acknowledged
and displayed by the ELM327, if its internally stored
receive address matches the address that the
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AT Commands (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 new Adaptive Timing feature will automatically
set the timeout value for you, 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 a maximum setting, however. With this new
Adaptive Timing, sampling rates are often doubled or
tripled from those typically experienced with prior
There are three adaptive timing settings that are
available for use. By default, Adaptive Timing option 1
(AT1) is selected, and is the recommended setting.
AT0 is used to disable Adaptive timing (usually used
when experimenting), while AT2 is a more agressive
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 – responses for J1939
use fixed timeouts as set in the standard.
[ 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’
(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.
The length byte represents the actual number of
bytes received, whether they fit into the OBD buffer or
not. This may be useful when viewing long data
streams (with AT AL), as the number accurately
represents the number of bytes received, mod 256.
Note that only the first twelve bytes received are
stored in the buffer.
[ 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. The actual
baud rate (in kbps) will be 4000 divided by this divisor.
For example, a setting of 115.2kbps would require a
divisor of 4000/115.2 or 35. In hexadecimal notation,
35 is written as 23, so the actual command that needs
to be sent would be AT BRD 23.
Since the ELM327 may be able to operate at
much higher rates than some interfaces can support,
the BRD command allows requested rates to be tested
before they are committed to (with automatic fall-back
to the previous baud rate if there are problems). In
use, the command is sent to request a change in the
baud rate, and the ELM327 responds with the familiar
“OK”. After that, an internal timer begins waiting, to
ensure that the controlling computer has sufficient time
to change their baud rate to the new rate. The ELM327
then sends the poweron message at the new baud
rate, and begins waiting while the controlling computer
assesses what has been received. If the AT I message
was received without errors, the controlling computer
sends a carriage return character, and if received by
the ELM327, the rate will be retained. If the controlling
computer sees errors (or worse, nothing), it provides
no response, and switches back to the initial baud
rate. If the ELM327 times out after receiving no
response, or has received something that does not
appear to be a carriage return character, it will revert
back to the former baud rate. A more detailed
discussion of this entire process is provided in the
‘Using Higher RS232 Baud Rates’ section.
Any new baud rate that is set in this manner is
retained across calls to set defaults (AT D), and for
warm starts (AT WS), but will not survive a hardware
reset (a power off/on or a call to AT Z). If you are in the
habit of calling AT Z in your code, we advise using AT
WS instead.
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AT Commands (continued)
BRT hh
[ set Baud Rate Timeout to hh ]
This command allows the timeout used for the
Baud Rate handshake (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 75msec. Note that a value of 00 does not
result in 0 msec - it provides the maximum time of 256
x 5.0 msec.
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
enabled (CAF1), the IC will automatically generate
formatting (PCI) bytes for you when sending, and will
remove them when receiving. This means that you can
continue to issue OBD requests (01 00, etc.) as usual,
without regard to these extra bytes that the CAN
diagnostics systems require. With formatting on, the
trailing (unused) data bytes that are received in a
frame will be removed as well, and only the relevant
ones will be shown. Beginning with v1.2 of the
ELM327, lines with invalid PCI bytes are now ignored,
rather than showing them as ‘<DATA ERROR’s.
Occasionally, long (multi-frame) responses are
returned by the vehicle. To help you analyze these, the
Auto Formatting mode will extract the total data length
and print it on one line. Following this will be each
segment of the message, with the segment number (a
single hexadecimal digit) shown at the beginning of the
line with a colon (':') as a separator.
You may also see the characters 'FC: ' at the
beginning of a line (if you are experimenting). This
represents a Flow Control message that is sent in
response to a multi-line message. 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 whether you have
selected the CAF1 or the CAF0 modes).
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 received data
bytes. No bytes will be hidden from you, and none will
be inserted for you. Similarly, when sending a data
request with formatting off, you must provide all of the
required data bytes exactly as they are to be sent –
the ELM327 will not perform any formatting for you
other than to add some trailing 'padding' bytes to
ensure that the required eight data bytes are sent. This
allows operation in systems that do not use PCI bytes
as ISO 15765-4 does.
Note that turning the display of headers on (with
AT H1) will override some of the CAF1 formatting of
the received data frames, 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 for the ELM327.
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 can be
given any value. Note that this command can be used
to enter 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).
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AT Commands (continued)
CFC0 and CFC1
[ CAN Flow Control off or on ]
The ISO 15765-4 protocol expects a “Flow
Control” message to always be sent in response to a
“First Frame” message. 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. The default setting is CFC1 - Flow Controls
Note that during monitoring (AT MA, MR, or MT),
there are never any Flow Controls sent no matter what
the CFC option is set to.
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 signaled by setting a mask bit to '1',
while a 'don't care' is signaled 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.
bits are set with the AT SH command). Some systems
use several of these bits to assign a priority value to
messages, which is how the command was named.
Any bits provided in excess of the five required will be
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.
[ show the CAN Status ]
The CAN protocol requires that statistics be kept
regarding the number of transmit and receive errors
detected. If there should be a significant number of
them, the device can even go off-line in order not to
affect other data on the bus, should there be a
hardware or software fault. The AT CS command lets
you see both the Tx and the Rx error counts. If the
transmitter should be off (count >FF), you will see
‘OFF’ rather than a specific count.
CV dddd
[ Calibrate the Voltage to dd.dd volts ]
The voltage reading that the ELM327 presents for
an AT RV reading can be calibrated with this
command. The argument (‘dddd’) must always be
provided as 4 digits, with no decimal point (it assumes
that a decimal place is between the second and the
third digits).
To use this calibration feature, simply use a meter
with sufficient accuracy to read the actual input
voltage. If, for example, the ELM327 consistently says
the voltage is 12.2V when you measure 11.99 volts,
simply issue AT CV 1199, and the device will
recalibrate itself for the provided voltage (it should then
read 12.0V due to roundoff). If you use a test voltage
that is less than 10 volts, don’t forget to add a leading
zero (that is, 9.02 volts should be entered as AT CV
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 – it is essentially identical, except
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 set the five most
significant bits in a 29 bit CAN ID word (the other 24
[ 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.
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AT Commands (continued)
[ monitor for DM1s ]
The SAE J1939 Protocol broadcasts trouble codes
periodically as they are detected, using 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
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.
[ Describe the current Protocol ]
The ELM327 is capable of automatically
determining the appropriate OBD protocol to use for
each vehicle that it is connected to. When the IC
connects to a vehicle, however, it returns only the data
requested, and does not report the protocol found. The
DP command is used to determine the current protocol
that the ELM327 is selected for (even if not
connected). If the automatic option is also selected,
the protocol will show the word "AUTO" before it,
followed by the type. Note that the actual protocol
names are displayed, not the numbers used by the
protocol setting commands.
[ 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 (0) or on(1) ]
These commands control whether or not
characters received on the RS232 port are
retransmitted (or echoed) back to the host computer.
To reduce traffic on the RS232 bus, users may wish to
turn echoing off by issuing ATE0. The default is E1 (or
echo on).
[ 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. More
complete details and examples can be found in the
Altering Flow Control Messages section.
FC SH hhh
[ Flow Control Set Header to… ]
The header (or more properly ‘CAN ID’) bytes
used for CAN Flow Control response 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 currently 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 currently only affects Flow Control mode 1.
FC SD [1-5 bytes]
[ Flow Control Set Data to… ]
The data bytes that are sent in a CAN Flow
Control message can be set with this command. The
current version of the software allows one to five data
bytes to be defined, with the remainder of the data
bytes in the message being automatically set to the
default CAN filler byte. Data provided with this
command is only used when Flow Control modes 1 or
2 have been enabled.
H0 and H1
[ Headers off (0) or on(1) ]
These commands control whether or not the
additional (header) bytes of information are shown in
the responses from the vehicle. These are normally
not shown by the ELM327, but can be turned on by
issuing the AT H1 command.
Turning the headers on actually shows more than
just the header bytes – you will see the complete
message as transmitted, including the check-digits and
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AT Commands (continued)
PCI bytes, and possibly the CAN data length code
(DLC) if it has been enabled with PP 29. 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).
Issuing this command causes the chip to identify
itself, by printing the startup product ID string (currently
“ELM327 v1.2”). Software can use this to determine
exactly which integrated circuit it is talking to, without
having to reset the IC.
(IFR) byte is normally the same as the value sent as
the Source (or Tester) Address byte that was in the
header of the request. There may be occasions when
it is desireable 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 defined as the Source Address (usually F1, but it
can be changed by PP 06), 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. AT IFR H is the
default setting.
IB 10
IIA hh
[ Identify yourself ]
[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 96
[set the ISO Baud rate to 9600 ]
Several users have requested this command. It 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 can be restored at any time by issuing
an IB 10 command.
IFR0, IFR1, and IFR2
[ 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 force an IFR byte to
always be sent, no matter what header bytes indicate.
The AT IFR1 command restores the response to
provide the automatic sending of IFRs, as determined
by the ‘K’ bit of the header byte. IFR1 is the default
setting of the ELM327.
[ IFR from Header or Source ]
The value sent in the J1850 In-Frame Response
[ 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:
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, and that this
value is reset to to $33 whenever the defaults, or the
ELM327, are reset.
KW0 and KW1
[ Key Word checks off (0) or on (1) ]
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 those bytes are
not found, the initiation is said to have failed (you
might see “UNABLE TO CONNECT” or perhaps “BUS
INIT: ...ERROR”). This may be because you are trying
to connect to a non-OBD compliant ECU, or perhaps
to an older one.
If you wish to experiment, but do not want the
ELM327 to check the values contained in the key
words, you can turn the checking off with:
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AT Commands (continued)
after which the IC will look for a response, but will not
look at the actual values of the bytes in the response.
This may allow a connection in an otherwise
‘impossible’ situation. Normal behaviour can be
returned with AT KW1, which is the default setting.
Caution should be used with this command, as
you are bypassing the checks that are normally
performed on the keyword bytes. The ELM327 sends
an acknowledgement to the ECU for these bytes, but
that is without considering what the bytes actually are.
You could be incorrectly activating an ISO 9141, or
KWP 2000 protocol, so should be very careful.
tells the ELM327 to always start in an 'Automatic'
protocol search mode, which is the most useful for an
unknown environment. ICs come from the factory set
to this 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.
As mentioned, the default setting for the memory
function is determined by the voltage level at pin 5 at
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.
L0 and L1
[ Linefeeds off (0) or on(1) ]
This option controls the sending of linefeed
characters after each carriage return character. For
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
on will be the default; otherwise it will be linefeeds off.
M0 and M1
[ Memory off (0) or on(1) ]
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
[ Monitor All messages ]
Using this command places the ELM327 into a
bus monitoring mode, in which it displays all messages
as it sees them on the OBD bus. It remains a quiet
monitor on the bus, not sending In Frame Responses
for J1850 systems or Acknowledges for CAN systems.
This continues indefinitely until stopped by activity on
the RS232 input, or the RTS pin.
To stop the monitoring, one can send a single
character then wait for the ELM327 to respond with a
prompt character (‘>’). Alternatively, the RTS input can
be brought to a low level to interrupt the device as
well. Waiting for the prompt is necessary as the
response time is unpredictable, varying depending on
what the IC was doing when interrupted. If for instance
it is in the middle of printing a line, it will first complete
that line then return to the command state, issuing the
prompt character. If it were simply waiting for input, it
would return immediately. Note that the character
which stops the monitoring will always be discarded,
and will not affect subsequent commands.
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 desireable 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, and follow any multi-segment
occurances of them.
Note that the MP command provides no means to
set the first two digits of the requested PGN, and they
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AT Commands (continued)
are always assumed to be 00. For example, the DM2
PGN has an assigned value of 00FECB (see SAE
J1939-73). To monitor for all DM2 messages, you
would issue AT MP FECB, eliminating the 00, since
the ELM327 always assumes that the PGN is
preceeded by these 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
MR hh
[ Monitor for Receiver hh ]
This command also places the IC in a bus
monitoring mode, displaying only 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.
MT hh
[ Monitor for Transmitter hh ]
This command places the IC in a bus monitoring
mode, displaying only messages that were sent by the
transmitter at 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.
[ 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.
Beginning with v1.2, 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.
[ 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.
[ 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 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 – there may be a problem with your
connection. This feature is only available beginning
with v1.2, and is not a provided with any earlier
versions of the ELM327 IC.
PP hh ON
[ 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. Note that all programmable
parameter values are set to ‘FF’ at the factory, so
enabling a programmable parameter before assigning
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AT Commands (continued)
a value to it might result in unexpected behaviour. The
actual time when the value for this parameter becomes
effective is determined by its type. Refer to the
Programmable Parameters section 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 xx SV yy [ Prog. Param. xx: 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, however.
[ 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
value 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 (0) or on(1) ]
These commands control the ELM327’s automatic
display of responses. 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. This is useful if sending
commands blindly when using the IC for a non-OBD
network application, or simulating an ECU in a basic
learning environment. It is not recommended that this
option normally be used, however, as the vehicle may
have difficulty if it is expecting an acknowledgement
byte and never receives one. The default is R1, or
responses on.
[ Read the input Voltage ]
This initiaties 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%.
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).
The value of hex 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.
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. The additional 5 bits needed for a 29 bit system are
provided through the AT CP command (since they
rarely change).
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
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 that 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 is in use by many KWP2000 systems for
(non-OBD) data transfer, so may be useful when
experimenting. Support for 4 byte headers has only
been added with v1.2 of the ELM327 IC, and was not
supported in previous versions.
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AT Commands (continued)
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
is a valid command, and is quite useful for working
with 11 bit CAN systems. It actually results in the
header bytes being internally stored as 00 07 DF.
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 currently valid protocols 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)
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 to automatically try all protocols,
when looking for a valid one. It causes the ELM327 to
sequence through each of the protocols, 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.
If another protocol (other than the Automatic one)
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 familiar
responses 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 require the
one protocol.
[ 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 set
AT SP A3. The default protocol will then be 3, but with
the ability to automatically search for other protocols.
Don't forget to disable the memory function if doing
this, or your neighbour’s protocol could become your
new default. As for AT SP h, an AT SP Ah will save
the protocol information even if the memory option is
off. 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 end up
accepting (acknowledging with an IFR) a message that
was actually meant for another module.
This command does not affect addresses used by
the J1939 protocol. The J1939 routines derive the
required addresses from the header values, as
required by the SAE standard.
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’ from the vehicle. The same
timer setting is also used after a response has been
received, to be sure that no more responses are
coming. The AT ST command allows this time to be
adjusted, in increments of about 4 msec.
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AT Commands (continued)
Setting the timer to a new value simply requires
using the ST command. For example, AT ST 19 will
give a setting of about 100 msec (19 in hex is 25 in
decimal). The ST timer is set to 32 by default (205
msec), but the default setting can be adjusted by
changing PP 03. Note that a value of 00 does not
result in 0 msec – it is a special value that sets the
timer to the default value.
When Adaptive Timing is enabled, the AT ST time
is modified by the actual measured response times,
however, the timer will never be set to a value that is
greater than the AT ST setting.
SW hh
[ Set Wakeup to hh ]
Once a data connection has been made, some
vehicles require that there be data flow every few
seconds, or the connection may time out and ‘go to
sleep.’ The ELM327 will automatically generate
periodic ‘wakeup’ messages in order to maintain this
connection, whenever the user is not requesting any
data. (Currently, only protocols 3, 4, and 5 generate
these messages.) The replies to these messages are
always ignored, and are not visible to the user.
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 thus occurs
when a value of FF (decimal 255) is used. The default
setting provides a nominal delay of 3 seconds between
Note that the value 00 (zero) is treated as a very
special case, and must be used with caution, as it will
stop all periodic messages. This is provided as it may
be convenient in certain circumstances. Issuing
AT SW 00 will not change a prior setting for the time
between wakeup messages, should the protocol be reinitialized.
TP h
[ Try Protocol h ]
[ Try Protocol h with Auto ]
This command is very similar to the AT TP
command above, except that if the protocol that is tried
should fail to initialize, the ELM327 will then
automatically sequence through all of the protocols,
attempting to connect to one of them.
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 bytes that you wish to have sent (one to six of
them), and the ELM327 will then send them as
required, at the rate determined by the AT SW setting.
Note that you do not have to add a checksum byte to
the data – the ELM327 calculates the value and adds
it for you.
[ Warm Start ]
This command causes the ELM327 to perform a
complete reset which is very similar to the AT Z
command, but does not include the power on LED
test. Users may find this a convenient means 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.
[ 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,
and the chip will be put in the idle state, waiting for
characters on the RS232 bus. Any baud rate set with
the AT BRD command will be reset to the default
This command is identical to the SP command,
except that the protocol that you select is not
immediately saved in internal 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.
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AT Command Summary
General Commands
BRD hh
BRT hh
E0, E1
L0, L1
M0, M1
J1850 Specific Commands (protocols 1 and 2)
try Baud Rate Divisor hh
set Baud Rate handshake Timeout
set all to Defaults
Echo Off, or On*
print the version ID
Linefeeds Off, or On
Memory Off, or On
Warm Start (quick software restart)
reset all
repeat the last command
OBD Commands
AT0, 1, 2
H0, H1
MR hh
MT hh
R0, R1
SH xyz
SH xxyyzz
SP h
SR hh
ST hh
TP h
IFRs Off, Auto*, or On
IFR value from Header or Source
ISO Specific Commands (protocols 3 to 5)
IB 10
Set the ISO Baud rate to 10400*
IB 96
Set the ISO Baud rate to 9600
IIA hh
Set the ISO (slow) Init Address to hh
KW0, KW1
Key Word checking Off, or On*
SW hh
Set Wakeup interval to hh x 20 msec
WM [1 - 6 bytes] Set the Wakeup Message
CAN Specific Commands (protocols 6 to C)
Allow Long (>7 byte) messages
Automatically Receive
Adaptive Timing Off, Auto1*, Auto2
perform a Buffer Dump
Bypass the Initialization sequence
Describe the current Protocol
Describe the Protocol by Number
Headers Off*, or On
Monitor All
Monitor for Receiver = hh
Monitor for Transmitter = hh
Normal Length messages*
Protocol Close
Responses Off, or On
Set Header
Set Header
Set Protocol to h and save it
Set Protocol to Auto, h and save it
Set the Receive address
Set Timeout to hh x 4 msec
Try Protocol h
Try Protocol h with Auto search
* = default setting
IFR0, 1, 2
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 Control Off, or On*
CM hhh
set the ID Mask to hhh
CM hhhhhhhh set the ID Mask to hhhhhhhh
CP hh
set CAN Priority (only for 29 bit)
show the CAN Status
(J1939) Monitor for DM1 messages
Flow Control Set the Mode to h
FC SH hhh
FC Set the Header to hhh
FC SH hhhhhhhh FC Set the Header to hhhhhhhh
FC SD [1 - 5 bytes] FC Set Data to [...]
MP hhhh
(J1939) Monitor for PGN hhhh
Misc. Commands
CV dddd
PP xx ON
PP xx SV yy
Calibrate the Voltage to dd.dd volts
disable Prog Parameter xx
all Prog Parameters Off
enable Prog Parameter xx
all Prog Parameters On
for PP xx, Set the Value to yy
print a PP Summary
Read the Voltage
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Reading the Battery Voltage
Before proceding to 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. 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 did not use upper case characters in
this example, mostly out of laziness. 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 to the ELM327. Also note that we have shown a
space character (‘ ’) between the ‘at’ and the ‘rv’. This
is only to separate the commands and make them
more readable. You do not have to add spaces, or if
you wish, you can add many spaces – it does not
affect the internal interpretation of the command.
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 too many changes. Perhaps
in this case, you have connected your multimeter, and
find that it reads 12.47V, and you would like the
ELM327 to read the same. Simply calibrate it to that
voltage using the CV command:
voltage at the input is actually 12.47V. You should not
provide the decimal point in the CV value, as the IC
knows that it should be between the second and the
third digits. To verify that the changes have taken
place, simply read the voltage again:
>at rv
The ELM327 always rounds off the measurement
to one decimal place, so the 12.47V actually appears
as 12.5V. Note that the second decimal place is
always maintained internally for accuracy and use in
the calculations, but it is never displayed.
The ELM327 can 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 used a 9V
battery for your reference, and it is actually 9.32V, then
you must add a leading zero to that when calibrating
the IC:
>at cv 0932
The other AT Commands are used in the same
manner. Simply type the letters A and T, follow that
with the command you want to send, then any
arguments that are required for that command, and
press return (or enter, depending on your keyboard).
You can place space characters as often as you wish
if it improves the readability for you, as they are
ignored by the ELM327.
>at cv 1247
At this point, the internal calibration values have
been changed, and the ELM327 knows that the
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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
single 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 automatically. The initial (default) values for
these extra bytes are usually appropriate for most
requests, but if you wish to change them, there is a
method to do so (see the “Setting the Headers”
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 (seven bytes or 14
hexadecimal digits). Attempts to send more bytes, or
an odd number of hex digits, will result in a syntax
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 haxadecimal 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.
All users will be required to
manipulate the results in some way, though –
combining bytes and dividing by 4 to obtain rpm,
dividing by 2 to obtain degrees of advance, etc., and
may find a software front-end to be of help.
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
ELM327, and is not sent on to the vehicle.
After sending the command, the ELM327 listens
on the OBD bus for messages, looking for ones that
are directed to it. If a message address matches,
those 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 are received, the ELM327 will continue to
reset this timer. Note that the IC will always respond
with something, even if it is to say “NO DATA”
(meaning that there were no messages found that
were addressed to it).
Figure 1. Hex to Decimal Conversion
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Talking to the Vehicle
The OBD standards specify that each request that
is sent to the vehicle must adhere to a set format. The
first byte (known as the ‘mode’) always describes the
type of data being requested, while the second, third,
etc. bytes specify the actual information required
(given by a ‘parameter identification’ or PID number).
These modes and PIDs are described in detail in
documents such as the SAE J1979, or ISO 15031-5
standards, and may also be expanded on by the
vehicle manufacturers.
Normally, one is only concerned with the nine
diagnostic test modes described by J1979 (although
there is provision for more). All of these modes are not
required to be supported by every vehicle, and are
often not. These are the nine modes:
- 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
Within each mode, PID 00 is normally 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. At the prompt, issue the mode 01 PID 00
>01 00
The first time the bus is accessed, you may see a
bus initialization message, and then the response,
which might typically be as follows:
41 00 BE 1F B8 10
(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). This is PID 05 in 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
A typical response would be:
41 0C 1A F8
The returned value (1A F8) is actually a two byte
value that must be converted to a decimal value to be
useful. Converting it, we get a value of 6904, which
seems to be a very high value for engine rpm. 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. In this case, then, the rpm is 1726,
which is much more reasonable.
Hopefully this has shown how typical requests
proceed. It has not been meant to be a definitive guide
on modes and PIDs – this information can be obtained
from the manufacturer of your vehicle, the SAE
(http://www.sae.org/), from ISO (http://www.iso.org/),
or from various other sources on the web.
The 41 signifies a response from a mode 01
request (01+40=41), while the second number (00)
simply 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 supported by this mode
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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 SAE
document J1979 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):
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
Powertrain Codes - SAE defined
“ - manufacturer defined
“ - SAE defined
“ - jointly defined
Chassis Codes - SAE defined
“ - manufacturer defined
“ - manufacturer defined
“ - reserved for future
Body Codes - SAE defined
“ - manufacturer defined
“ - manufacturer defined
“ - reserved for future
Network Codes - SAE defined
“ - manufacturer defined
“ - manufacturer defined
“ - 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 further 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
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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
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
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
The chart at the right provides a quick procedure
on what to do next:
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 not 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!
Ignition Key to ON,
but vehicle not running
to see how many codes
(2nd digit of the 3rd byte)
to see the codes
(ignore the first byte and
read the others in pairs)
to reset the codes
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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 not until your request needs to be sent. If the bus
initiation occurs during the automatic search process,
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:
The three dots appear only as the slow initiation
process is carried out – a fast initiation does not show
them. 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
vehicle’s key to ‘ON’ before attempting to talk to the
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 when 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 though, as the
default settings work well with almost all systems.
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 connecton 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 these,
once a connection is made, simply monitor the OBD
transmit LED – you will see the periodic ‘blips’ created
when the ELM327 sends one. If you are curious as to
the actual contents of the messages, you can then
perform a Buffer Dump to see the bytes. Note that the
ELM327 never obtains or prints a response to any of
the wakeup messages.
The standards state that if there is no activity at
least every five seconds, the data connection may
close. To ensure that this does not happen, and to
provide some margin, the ELM327 will send a wakeup
message after three seconds of inactivity. This time
interval is fully programmable, should you prefer a
different setting (see the AT SW command).
As with the ELM323, the ELM327 allows users to
change the actual wakeup message that is sent. To do
so, simply send the ELM327 a Wakeup Message
command, telling it what you wish the message to be
changed to. 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
From that point forward, every wakeup message
that the ELM327 sends will be as shown above.
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 only
allows for messages of one to six bytes in length). You
should not provide a checksum byte, as it will be
automatically added for you.
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Selecting Protocols
The ELM327 supports several different OBD
protocols (see Figure 2 below). 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 which protocol is used.
For example, if you know that your vehicle uses
the SAE J1850 VPW protocol, you may want the
ELM327 to only try that protocol, and no others. If that
is what you want, simply determine the protocol
number (from the chart below), then use the ‘Set
Protocol’ AT Command as follows:
>AT SP 2
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 current protocol:
Now what happens if your friend has a vehicle that
uses ISO 9141-2? How do you use the ELM327
interface for that vehicle? There are a few choices...
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’ with the protocol number, as follows:
Now, the ELM327 will begin by trying protocol 2,
but will then automatically begin searching for another
protocol should an attempt to connect with protocol 2
fail (as would happen when you try to connect to the
friend’s vehicle). If you have the memory function
enabled when you connect to your friend’s vehicle,
their protocol will then be stored in memory as the new
default protocol, but that is only a minor inconvenience
as the next time you connect to your vehicle, it will try
the friends protocol, fail, and search until it finds yours.
Your protocol will then be resaved as the new default.
Perhaps you have disabled the memory function
(set pin 5 to 0V), and have used AT SP 2 to customize
the IC for your vehicle only, and you don’t want it to
search for others? (In effect, you have turned your
ELM327 into an ELM322 type interface.) In this case,
you might want to use the ‘Try Protocol’ command for
your friend’s vehicle, rather than setting it to something
permanently. You can either say:
>AT TP 3
if you already know that your friend’s vehicle uses
protocol 3, or else you can say:
>AT TP 0
which automatically tries all of the protocols (as
allowed by PP 07) looking for one that responds.
In general, users find that enabling the memory
(setting pin 5 to 5V) and choosing the ‘Auto’ option
(the easiest way is to say AT SP 0) works very well.
After the initial search, the protocol used by your
vehicle becomes the new default mode (so it is tried
first every time), and if the interface is used on another
vehicle, there is only a minor delay while it performs an
automatic search.
SAE J1850 PWM (41.6 Kbaud)
SAE J1850 VPW (10.4 Kbaud)
ISO 9141-2 (5 baud init)
ISO 14230-4 KWP (5 baud init)
ISO 14230-4 KWP (fast init)
ISO 15765-4 CAN (11 bit ID, 500 Kbaud)
ISO 15765-4 CAN (29 bit ID, 500 Kbaud)
ISO 15765-4 CAN (11 bit ID, 250 Kbaud)
ISO 15765-4 CAN (29 bit ID, 250 Kbaud)
SAE J1939 CAN (29 bit ID, 250* Kbaud)
User1 CAN (11* bit ID, 125* Kbaud)
User2 CAN (11* bit ID, 50* Kbaud)
*user adjustable
Figure 2. ELM327 Protocol Numbers
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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.
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
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
3 header bytes
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, and the received data may be
falsely interpreted. To detect errors, the various
protocols all provide some form of check on the
received data, often as simple as a sum calculation (a
‘running total’ is maintained by the receiver as a
message is being processed). This is compared to the
‘running total’ sent by the transmitter, and if they do
not agree, an error has occured. The total is generally
referred to as a ‘checksum’ or a ‘CRC byte’ and is
usually sent at the end of a message. If an error is
detected, the different protocols provide various ways
of handling it.
The OBD data bytes are thus normally
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, as shown in Figure 3 below.
The ISO 15765-4 (CAN) protocol uses a very
similar structure, the main difference really only
relating to the structure of the header. CAN header
bytes are not referred to as that – they are called ‘ID
bits’ instead. The initial CAN standard defined the ID
bits as being 11 in number, and the more recent CAN
up to 7 data bytes
Figure 3. An OBD Message
ID bits (11 or 29)
7 data bytes
Figure 4. A CAN OBD Message
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OBD Message Formats (continued)
standard now allows 29 in total.
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). Note that the ELM327 is not
able to 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 by using advanced techniques such as
physical addressing. The next section provides a
discussion on how to do this…
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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
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
>AT SH A8 48 F1
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 2E
The first byte in the response echos 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
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
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 11 6B
62 11 6B 00 00
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 (0000 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
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Setting the Headers (Cont’d)
>AT CP vv
>AT SH xx yy zz
>AT SH xx yy zz
5 bits only
11 bit ID
29 bit ID
Figure 5. Setting a 29 bit CAN ID
Electronics does not maintain lists of this information,
and cannot provide any further details for you. Mode
22 and others are described in more detail in SAE
document J2190, “Enhanced E/E Diagnostic Test
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 them all to 0’s. If they are all 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. Figure 5 shows how these are
combined for use by the ELM327.
The CAN standard states that for diagnostics, the
priority byte (‘vv’ in the diagram) will always be 1B (this
Figure 6. Setting an 11 bit CAN ID
is the default value used by the ELM327). Since it is
rarely changed, it is assigned separately from the
other header bytes, using the CP command. Changing
of this value is usually only required if experimenting
with J1939 systems.
The next byte (‘xx’) describes the type of message
that this is, and is set to hex DB for functional
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
familiar (J1939 is a CAN standard for use by ‘heavyduty vehicles’ such as trucks and buses). We use
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.
Refer to page 36 for a discussion of the J1939 support
that is provided by the ELM327.
The final header format to discuss is that used in
11 bit CAN systems. They also use a priority/address
structure, but shorten it into roughly three nibbles
rather than three bytes. The ELM327 uses the same
commands to set these values as for other headers,
except that it only uses the 11 least significant (‘rightmost’) bits of the provided header bytes, and ignores
the others (as shown in Figure 6). It quickly becomes
inconvenient to have to enter six digits when only three
are required, so there is a special ‘short’ version of the
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Setting the Headers (Cont’d)
AT SH command that uses only three hex digits. It
actually operates by simply adding the leading zeroes
for you.
The 11 bit CAN standard typically makes
functional requests (ID/header = 7DF), but receives
physical replies (7En). With headers turned on, it is a
simple matter to learn the address of the module that
is replying, 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
>01 05
7E8 03 41 05 46 00 00 00 00
Hopefully this has helped to get you started. As we
often tell those that write – if you are planning to do
some serious experimenting with OBD, you should buy
the relevant standards.
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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
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 periodic
‘wakeup’ messages are not sent while in this mode, so
if you have an ISO 9141 or ISO 14230 bus that had
been initialized previously, it may ‘go to sleep’ while
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
the character you send will be discarded, and will have
no effect on any subsequent commands. The time it
takes to respond to this interrupting character 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
returning to wait for input, so you should always wait
for the prompt character (‘>’), or the Busy line to go
low, before beginning to send a 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 will likely
stop searching at ISO 9141, even if the actual protocol
is ISO 14230. With the Automatic searching enabled,
this should correct itself, however, when the first OBD
request is 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, simply type:
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 all messages that begin with ‘0’ as
the first digit will actually be displayed.
In order to use the AT MR command with CAN 11
bit identifiers, you should always try to use the format
‘AT MR 0x’, where ‘x’ is the digit that you want the
identifiers to begin with. To look for all 2xx’s, use the
command ‘AT MR 02’, and to see all of the 7xx’s, you
should use ‘AT MR 07’.
>AT MT 10
and all messages that contain 10 in the third byte of
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CAN Messages and Filtering
The ELM327 monitoring commands (AT MA, MR
and MT) usually work very well with the ‘slower’
protocols – J1850, ISO 9141 and ISO 14230. The
CAN systems are a different story, however, as they
often have an order of magnitude more information
passing over them. The relatively small 256 byte buffer
that the ELM327 uses for sending can quickly fill up
when data is arriving at 500 Kbps and leaving at
38.4 Kbps, or even 115.2 Kbps.
To help reduce the amount of information seen by
the ELM327, the internal CAN module has a ‘filter’ that
can be used to pass only messages with specific ID
bits. A range of values can be passed when the filter is
used with what is called a ‘mask’ to say which bits are
As an example, consider an application where you
are trying to monitor for 29 bit CAN diagnostic
messages, exactly like the ELM327 does. By
definition, these messages will be sent to the scan tool
at address F1. From ISO 15765-4, you know then that
the ID portion of the reply must be of the form:
18 DA F1 xx
>AT CF 00 00 06 00
The 11 bit ID is stored in the last three locations,
so the 6 would appear where it is shown. Now, to
make that digit relevant, we create the mask:
>AT CM 00 00 0F 00
The system only uses the 11 right-most bits in this
case, so we can be lazy and enter the F as shown (the
first bit of the F will be ignored, and it will be treated as
if we had entered a 7).
Clearly, this can be quite cumbersome if using 11
bit CAN systems routinely. To help with that, the
ELM327 offers some shorter versions of the CF and
CM commands. To use them for the example above,
you need only enter three digit arguments:
>AT CF 600
>AT CM F00
where xx is the address of the module that is sending
the message. To use the filter, then, enter what you
have into it, putting anything in for the unknown portion
(you will see why in a moment). The command to set
the CAN filter is AT CF…
>AT CF 18 DA F1 00
How, you ask, do you tell the ELM327 to ignore
those last two 0’s? You do that with the mask. The
mask is a set of bits that tell the ELM327 which bits in
the filter are relevant. If the mask bit is 1, that filter bit
is relevant, and is required to match. If it is 0, then that
filter bit will be ignored. All bits in the above message
are relevant, except those of the last two digits. To set
the mask for this example then, you would need to use
the CAN Mask command, as follows:
>AT CM 1F FF FF 00
If desired, you can convert the hexadecimal to
binary to see what has been done.
The 11 bit CAN IDs are treated in the same
manner. Recall that they are stored internally in the
right-most 11 bits of the locations used for 29 bit CAN,
which must be considered when creating a filter or
mask. As an example, assume that we wish to display
all messages that have a 6 as the first digit of the 11
bit ID. We need to set a filter to look for 6 in that digit:
As for the full eight digit versions, only the 11 least
significant (right-most) digits are actually used, so you
do not need to take special care with the first bit.
With a little practice, these commands are fairly
easy to master. Initially, try entering the filter and mask
values, then use a command such as AT MA to see
what the results are. The ELM327 knows that you are
trying to filter, and combines the effects of both
commands (it will do that for MR and MT as well). The
MA, MR and MT commands all have the extra benefit
that while they are in effect, the ELM327 will remain
quiet, not sending acknowledgement or error signals,
so anything you do while monitoring should not disrupt
other devices that are on the bus.
Note that if a filter has been set, it will be used for
all CAN messages. Setting filters and masks may
cause standard OBD requests to be ignored though,
and you may begin seeing “NO DATA” responses. If
this happens, and you are unsure of why, you may
want to reset everything to their default values (AT D)
and start over.
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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 must be assembled into one complete message.
One example of this is a request for the serial
number of the vehicle (mode 09, PID 02). This is often
a multiline reply that needs to be joined. In these
situations, you must take care to ensure that all of the
reply has been received and it is in the correct order
before assembling the message. The actual response
usually has a byte that shows the sequence of the
data, to help with this. Here is one example for a
typical SAE J1850 vehicle:
49 02
49 02
49 02
49 02
49 02
Note that all OBD compliant vehicles do not
necessarily provide this information. Many older ones
do not, but as a rule the newer ones do. If your vehicle
does not support this parameter, you will only see a
“NO DATA” response.
The first two bytes (49 and 02) on each line of the
above response do not show any vehicle information.
They only show that this is a response to an 09 02
request. The next byte on each line shows the order in
which the data is to be assembled. Assembling the
remainder of the data in that order, and ignoring the
first few 00’s gives:
31 44 34 47 50 30 30 52 35 35 42 31 32
33 34 35 36
Using an ASCII table to convert these hex digits
gives the following serial number 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:
0: 49 02 01 31 44 34
1: 47 50 30 30 52 35 35
2: 42 31 32 33 34 35 36
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, so that they clearly stand out (0:, 1:, etc.).
CAN systems add this hex digit (it goes from 0 to F
then repeats), to aid in reassembling the data, just as
the J1850 vehicle did.
The first line of this response says that there are
014 bytes of information to follow. That is 14 in
hexadecimal, or 20 in decimal terms, which agrees
with the 6 + 7 + 7 bytes shown on the three lines.
Serial numbers are generally 17 digits long however,
so how do we assemble the number from 20 digits?
The second line shown begins with the familiar 49
02, as this is a response to an 09 02 request. Clearly
they are not part of the serial number. CAN will
occasionally add a third byte to the response which we
see next (the ‘01’), which shows the number of data
items that are to follow (the vehicle can only have one
VIN, so the response says there is only one data item).
That third byte can be ignored, so this 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.
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. We need to turn the headers on to
actually see ‘who’ is doing the talking:
>AT H1
>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 that is valid for them. This type of
response occurs often, and you should be prepared for
A final example shows how similar messages
might occasionally be ‘mixed up’ in a CAN system. We
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Multiline Responses (Cont’d)
ask the vehicle for the Calibration ID (09 04) and are
presented with the following response:
>09 04
0: 49 04
1: 32 38
0: 49 04
2: 00 00
1: 32 38
2: 00 00
01 35 36 30
39 34 39 41 43
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 following
group is very confusing. Why are there two segment
twos? Which ECU do these belong to? The only way
to tell is to turn on the headers, and repeat your
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.
Hopefully this has helped you with some of the more
difficult ones. If you are still having trouble, we urge
you to purchase the relevant standard, and study it.
>AT H1
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. This is the special CAN IDs representing
ECU#1 and ECU#2, respectively. Grouping the
responses from the two ECUs 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
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
From these, the messages can be assembled in
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CAN Message Formats
The ISO 15765-4 (CAN) standard defines several
message types that are to be used with diagnostic
systems. Currently, there are four main ones that may
be used:
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 and what is known as a PCI
(Protocol Control Information) byte. The PCI byte is
always the first byte of them all, 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 always
see it.)
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 a total of
eight data bytes (the data length is not adjustable with
this version of the ELM327). You do not need to set
the Allow Long (AT AL) option in order to do this, 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 descripter 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:
0: 49 02 01 31 44 34
1: 47 50 30 30 52 35 35
2: 42 31 32 33 34 35 36
In this example, the line that begins with 0: is the
First Frame message. The length (014) was actually
extracted from the message by the ELM327 and
printed on the separate line as shown. Following the
First Frame line are two Consecutive Frames as
shown (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 next section.
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
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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
that 8 bytes and CAN has made provision for this,
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 ISO 15765-4 Flow
Control response for you as long as the CAN Flow
Control option is enabled (CFC1), which it is by
Several users have requested that we provide
more flexibility over the data sent in the Flow Control
message, and as of v1.1, we have provided a means
to do this. In order to change how the ELM327
responds when it needs to send a Flow Control
message, you need to change Flow Control ‘modes’.
The default Flow Control mode is number ‘0’. At
any time while you are experimenting, if you should
wish to restore the automatic Flow Control responses
(for ISO 15765-4), simply set the mode to 0:
This will immediately restore the responses to their
default settings.
Mode 1 has been provided for those that need
complete control over their Flow Control messages. To
use it, simply define the CAN ID (header) and data
bytes that you require be sent in response to a First
Frame message. If you try to set the mode before
defining these values, you will get an error:
then you can set the mode:
From this point on, every First Frame message
received will be responded to with your custom
The final mode currently supported allows the user
to set the data bytes which are to be sent, but not the
ID bits. The ID bits (header bytes) in mode 2 are set to
those which were received in the First Frame
message, without change. To use this mode, first
define your data bytes, then activate the mode:
>AT FC SD 00 11 22
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.
Figure 7 summarizes the currently supported
ID Bits
Data Bytes
no values
no values
ID Bits
Data Bytes
ID Bits
Data Bytes
Figure 7. Flow Control Mode Numbers
You must set the headers and data first:
>AT FC SD 00 11 22
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J1939 Support
The SAE J1939 CAN standard is a relatively new
protocol that is being used in many types of heavy
machinery – trucks, agricultural equipment, and
busses, to name a few. It uses a standard CAN
(ISO 11898) physical interface, and defines its own
format for data transfer (although it is very similar to
the ISO 15765 that is used for automobiles).
The ELM327 offers some support for the J1939
standard in Protocol A. (Protocols B and C can be
selected for J1939 formatting if needed, with
Programmable Parameters 2C and 2E). Although this
is not full support as dictated by the standards, the
ELM327 implementation should at least allow you to
begin experimenting.
The default data rate for Protocol A has been set
to 250 kbps, as set by the SAE J1939-11 standard.
Should your application require a different speed, it is
easily changed with PP 2B.
J1939 messages use 29 bit CAN IDs, and up to 8
data bytes for each message (ISO 15765 always uses
8 bytes). Diagnostics on SAE J1939 are defined by the
J1939-73 standard, and the actual data transfer
protocol is described in J1939-21. If you are going to
do a lot of work with J1939, it may be wise to purchase
these standards documents from the Society of
Automotive Engineers (SAE).
The ELM327 handles the sending of messages
with the J1939 protocol in the same way as any of the
other protocols. The header bytes are predefined for
you and all you need do is provide the data bytes that
you wish to send.
For example, assume that you wish to send a
request for engine temperature (PGN 00FEEE). J1939
specifies a reverse order for the data bytes, so when
you tell the ELM327 what to send, you need to switch
the order of the data bytes, and then send them as you
would any other request:
>EE FE 00
The ELM327 then adds the required header (ID)
bytes, etc. and sends the message for you. (It also
configures itself correctly to be able to receive the
response.) Note that the ELM327 sets the header
bytes to EC FF F9 by default (for global requests from
OBD service tool #1), so if you want something else,
you will have to change the headers, using the AT SH
All responses to a request are printed by the
ELM327, whether they are a single CAN message
transmission, 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 (transparently - you won’t
see it happen). If formatting is on (ie CAF1), the output
will resemble the format used to print the ISO 15765-4
multiline responses. Note that if you provide three data
bytes, the ELM327 will assume that you are sending a
PGN, and will look for responses to that. If you send
other than 3 data bytes, it will assume that you are
making a general request, and will only look for
responses to the Source Address (as given in the third
byte of the header).
The SAE J1939 standard predefines several
Diagnostic Messages, and assigns ID values to them.
The first of these (designated ‘DM1’) are reserved for
trouble codes, which are periodically broadcast over
the CAN network. Since these are commonly required,
the ELM327 provides a special command to monitor
for them (aptly named DM1). If you wish to monitor for
all DM1 messages, simply issue:
and the ELM327 will print out all that it finds.
The DM1 message is the only one that is presently
predefined by the ELM327 for you. To monitor for any
other diagnostic message, you need to know it’s PGN
number (in hexadecimal), and monitor for that using
the AT MP command. For example, the DM2 PGN
number is 65227, or 00FECB in hexadecimal. The
ELM327 requires hexadecimal, and assumes that all
PGNs begin with 00 – so to monitor for DM2s, you
need only send:
and the IC will begin looking for all replies to DM2
Please note that the ELM327’s support for the
SAE J1939 standard is only basic (it does not support
address negotiation as described in J1939-73, for
example), and it has not undergone the extensive
testing that our automotive protocols have. We simply
have had a large number of requests for support of the
protocol, and wanted to provide something for you to
begin experimenting with. As always, we are quite
open to your feedback and your suggestions for future
Elm Electronics – Circuits for the Hobbyist
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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
were 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:
This new command performs a software reset,
restoring exactly the same items as the AT Z does, but
it omits the LED test, so it is considerably faster. Also,
it does not affect any baud rates that have been set
with the AT BRD command, so is essential if you are
modifying 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 Programmable
Parameters (which forces them to their default values).
To do so, send the command:
which should disable all of the changes that you have
made. You can then 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 next section
‘Programmable Parameters’, for more details).
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:
It takes approximately one second for the IC to
perform this reset, initialize everything and then test
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Programmable Parameters
The ELM327 contains several programmable
memory locations that retain their data even after
power is turned off. Every time the IC is powered up,
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.
The settings, or parameters, can be altered by the
user at any time using a few simple commands. These
Programmable Parameter 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 try to
make changes.
The following pages list the currently supported
Programmable Parameters for this version of the
ELM327. As an example of how to use a
Programmable Parameter, consider PP 01 (the
printing of headers). If you are constantly powering
your ELM327 and then using AT H1 to turn the
headers on, you may want to change its default
setting, so that they are always on by default. To do
this, simply set the value of PP 01 to 00:
>AT PP 01 SV 00
This changes the value associated with PP 01, but
does not enabled it. To make the change effective, you
must also type:
>AT PP 01 ON
At this point, you have changed the default setting
for AT H1/H0, but you have not changed the actual
value of the current AT H1/H0 setting. From the ‘Type’
column in the table on page 39, you can see that 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.
It is occasionally difficult to know what changes
you have made to the Programmable Parameters. To
help with that, the ELM327 provides a Programmable
Parameter Summary (PPS) command. This simply
prints a list of all of the supported PPs, their current
value, and whether they are on/enabled (N), or
off/disabled (F). For an ELM327 v1.2 IC, with only the
headers enabled (as discussed above), the summary
table would look as follows:
00:FF F
04:01 F
08:FF F
0C:68 F
10:0D F
14:FF F
18:00 F
20:FF F
24:00 F
28:FF F
2C:E0 F
You can see that PP 01 now shows a value of 00,
and it is enabled (oN).
Another example shows how you might change
the CAN filler byte. Some systems use ‘AA’ as the
value to put into unused CAN bytes, while the ELM327
uses ‘00’ by default. To change the ELM327’s
behaviour, simply change PP 26:
>AT PP 26 ON
Again, PP 26 is of type ‘D’, so the above change
will not actually take effect until the AT D command is
issued, or the ELM327 is reset.
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 headers are on when not expected, or
if the CAN response shows data length codes, 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. If you get in too deeply, don’t
forget the ‘all off’ command:
No matter what software you use, you might get
into more serious trouble, should you change the baud
rate, or the Carriage Return character, for example,
and forget what you have set them to. The Carriage
Return value set by PP 0D is the only character that is
recognized by the ELM327 as ending a command, so
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Programmable Parameters (Cont’d)
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 hardware trick.
When the ELM327 first powers up, it looks for a
jumper from pin 28 (the OBD Tx LED) to circuit
common (V DD). If a jumper is in place, it will turn off all
of the PPs for you, restoring the IC to the factory
defaults. To use this feature, simply connect a jumper
to circuit common, then hold the other end of the
jumper to pin 28 while turning the power on. As soon
as you see the RS232 Rx LED begin to flash quickly,
you can remove the jumper – the PPs are off. This
feature should only be used when you get into trouble
too deeply, and it’s your only choice (since putting
jumpers into a live circuit might cause damage if you
put it into the wrong place). As well, it is only available
beginning with version 1.2 of the IC, and should not be
used on any previous versions.
The following is a summary of the currently implemented Programmable Parameters (ELM327 v1.2):
Perform AT MA after powerup or reset
Printing of header bytes (AT H default setting)
Allow long messages (AT AL default setting)
00 = ON
00 = ON
00 = ON
NO DATA timeout time (AT ST default setting)
setting = value x 4.096 msec
00 to FF
Default Adaptive Timing mode (AT AT setting)
00 to 02
OBD Source (Tester) Address. Not used for J1939 protocols.
00 to FF
Last Protocol to try during automatic searches
01 to 0C
Character echo (AT E default setting)
00 = ON
(205 msec)
Linefeed Character
00 to FF
RS232 baud rate divisor when pin 6=1
The baud rate (in kbps) is given by 4000/(this value)
For example, to obtain 500kbps, use a setting of 08 (4000/8 = 500)
08 to FF
(Continued on the next page)
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Programmable Parameters (Cont’d)
Here are some example baud rates, and the divisor to be used for
Baud Rate
PP 0C value
hex (dec)
D0 (208)
68 (104)
45 (69)
23 (35)
11 (17)
08 (8)
Note that the PP 0C value must be provided to the ELM327 as
hex digits only. The decimal values that are shown above in
brackets are for your convenience only.
Carriage Return Character
00 to FF
J1850 voltage settling time
setting = value x 4.096 msec
00 to FF
J1850 Break Signal monitor enable
(reports BUS ERROR if break signal duration limits are exceeded)
00 = ON
Auto search time delay between protocols 1 & 2
setting = value x 4.096 msec
00 to FF
ISO baud rate (AT IB default setting)
00 = 96
FF = 10
(53 msec)
(205 msec)
(see note 2)
ISO wakeup message rate (AT SW default setting)
setting = value x 20.48 msec
00 to FF
Auto search time delay between protocols 4 & 5
setting = value x 4.096 msec
00 to FF
CAN auto formatting (AT CAF default setting)
00 = ON
00 = ON
00 to FF
CAN auto flow control (AT CFC default setting)
CAN filler byte (used to pad out messages)
(2.99 sec)
(no delay)
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Programmable Parameters (Cont’d)
Printing of the CAN data length (DLC) when printing header bytes
Protocol A (SAE J1939) CAN baud rate divisor.
00 = ON
(see note 2)
01 to 40
(250 Kbps)
The protocol A CAN baud rate is determined by this value (rate in
kbps = 500/value). For example, setting this PP to 19 (ie. decimal
25) provides a baud rate of 500/25 = 20 kbps.
Protocol B (USER1) CAN protocol options.
00 to FF
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 bit and 29 bit
b4: reserved for future - leave set at 0
b3: reserved for future - leave set at 0.
b2, b1, and b0 determine the data formatting options, as follows:
b2 b1 b0
Data Format
ISO 15765-4
SAE J1939
Other combinations are reserved for future – results might be
unpredictable if you should select one of them.
Note that b7 is the msb, and b0 is the lsb. For example, the default
value of E0 can be shown as 11100000 in binary, which has b7, b6
and b5 set to 1’s. This means that by default, 11 bit IDs will be
sent, the data length will vary, and the ELM327 will receive
messages with either 11 or 29 bit IDs.
Protocol B (USER1) baud rate divisor. See PP 2B for a description.
01 to 40
(125 Kbps)
Protocol C (USER2) CAN options. See PP 2C for a description.
00 to FF
Protocol C (USER2) baud rate divisor. See PP 2B for a description.
01 to 40
(50 Kbps)
1. The “Type” column indicates when changes to the Programmable Parameter take effect. Possible values
I - the effect is immediate,
D - takes effect after Defaults are restored (AT D, AT Z, AT WS or power off/on)
R - will only become apparent after a Reset (AT Z, AT WS or power off/on)
P - only takes effect after power off/on or AT Z.
2. These are changes since the last version of the IC.
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Using Higher RS232 Baud Rates
The RS232 serial interface has been maintained
throughout the ELM OBD products, largely due to its’
versatility. Older computers and PDAs can use it
directly, as can microprocessors. Newer computers
generally do not have RS232 ports, but they do have
USB and ethernet ports, both of which can be
converted to RS232 with a simple adapter.
Most people will build their ELM327 circuits with
an RS232 interface, mainly because it is relatively
easy and inexpensive to do so. A circuit such as the
one shown in Figure 9 requires very few components
and works extremely well at speeds up to 57600 bps.
Depending on your computer’s RS232 voltages, your
wiring practices, and your choice of components, it
may also work well at speeds as high as 115200 bps.
This is the approximate limit of such a circuit however,
and anything operating at this speed should be
thoroughly tested.
Users that would like to operate at speeds in the
range of 115200 bps (or higher) may wish to look at
some of the single IC solutions that are available.
These include devices such as the ADM232A from
Analog Devices (http://www.analog.com/), or the
popular MAX232 series of ICs from Maxim Integrated
Products (http://www.maxim-ic.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 types of
devices are only rated for operation up to 120 Kbps,
however, so may not be suitable for higher data rates;
be sure to check the manufacturers data sheet before
committing to a design.
An RS232 interface needs relatively large voltage
swings, which are difficult to maintain at higher rates
with large cable capacitances to contend with. (A
typical interface is often limited to about 230400 bps
under ideal conditions.) If you need to operate the
ELM327 at these speeds or higher, it is recommended
that you consider alternatives.
One popular alternative is a USB data connection.
The USB interface is capable of very high data transfer
rates, certainly much higher than the 500 Kbps limit of
the ELM327. Several manufacturers offer special
‘bridge’ circuits that simplify connecting an RS232
device (such as the ELM327) directly to the USB bus.
Examples are the CP2102 from Silicon Labs
(http://www.silabs.com/) or the FT232R from Future
Technology Devices (http://www.ftdichip.com/). If
planning to use the higher baud rates (ie up to 500
Kbaud), these interfaces are essential.
We are often asked about the use of a direct
connection to microprocessor. That is certainly an
option, and one that allows a full speed connection at
essentially zero cost. If you are developing such a
system, you can often simply connect the transmit
output from your microprocessor to the ELM327’s
receive, and the receive input to the ELM327’s
transmit. Provided that they share the same power
supplies, and you use good wiring practices between
them, the connection should work well. You do not
have to convert to RS232 voltage levels in order to use
the RS232 signals.
The default configuration for the ELM327 provides
an RS232 data rate of either 9600 baud, or 38400
baud, depending on the level at pin 6 during power up
or reset. While the 9600 baud rate is not adjustable,
the 38400 one is (beginning with v1.2 of the IC). There
are two ways that the rate can be changed – either
permanently with a Programmable Parameter, or
temporarily with an AT command.
Programmable Parameter ‘0C’ has been provided
to allow you to change the high speed baud rate to
what works best for you. You can change the value
several times if you wish, and each time that you do,
that new value becomes your power on default setting.
The type of storage used for this value is actually
called ‘non-volatile’, as it is not affected by power
cycles or resets. To change the setting requires only a
write to the ELM327’s EEPROM, using the AT PP
If, for example, you would like to have the IC use a
baud rate of 57.6K rather than the factory setting of
38.4K, simply change the value of PP 0C, and then
enable it. PP 0C is actually an internal divisor that is
stored in memory – the baud rate is calculated from
4000 divided by the value of the number that is stored.
To obtain a setting of 57.6, a baud rate divisor of 69 is
required (4000/69 is approximately 57.6). Since 69 in
decimal is 45 in hexadecimal, you tell the ELM327 to
set the value to 45:
>AT PP 0C SV 45
then enable the new value for use:
from that point on (until PP 0C is turned off), the
ELM327 will communicate at the new baud rate.
If you are designing your own circuitry, it is
relatively easy to know what your circuit is capable of,
and then write a value to PP 0C once. Software
developers will not usually know what hardware is
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Using Higher RS232 Baud Rates (Cont’d)
connected to the ELM327 however, so will not know
what the limitations are. For these users, we have
provided the AT BRD command.
The AT BRD command allows a new baud rate to
be tested, and then accepted or rejected depending on
the results of the test. See the chart at the right, which
shows how the command works.
As can be seen, the software first makes a request
for a new baud rate divisor, using this command. For
example, to try the 57600 baud rate that was
previously discussed, the controlling PC would send
the command:
Request for a new
baud rate divisor:
ELM327 responds
with ‘OK’
(if it is supported)
Program switches to
the new baud rate,
and waits for input
If the ELM327 is an older version, it will not
support this command, and will return with the familiar
‘?’ character. If it does support the command, it will
respond with ‘OK’, so the software knows whether to
proceed or not.
Having sent an ‘OK’, the ELM327 then switches to
the new (proposed) baud rate, and then simply waits a
predetermined time (nominally 75 msec). 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 v1.2’) to the PC at the
new baud rate, and then 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, the baud rate then reverts
back to the old rate. Note that the PC might correctly
output the carriage return at this new rate, but the
interface circuitry could corrupt the character, so that
the ELM327 does not see a valid response. Your
software must check for the ‘OK’ response before
assuming that the new rate has been accepted.
Using this method, a program can quickly try
several baud rates, evaluating the connection each
time, and settling on the fastest suitable one. The new
baud rate will stay in effect until reset by an AT Z, or a
Power Off/On. It is not affected by the AT WS (Warm
Start) command.
ELM327 waits for
75msec*, then sends
the AT I string
If the Rx is good,
Program sends a
carriage return
Wait up to 75 msec*
for a carriage return
Print ‘OK’
Revert to the
old baud rate
Print a prompt,
then wait for the
next command
* Note that the 75msec time is adjustable
(with the AT BRT hh command)
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Computer Control
A common question we receive with our OBD
interpreters is “Can I connect the ELM32x circuit
directly to my own circuit, or must I use the RS232
interface shown?” Certainly you may connect directly
to our ICs – you do not need to use an RS232
The ELM327 does not use an inverted RS232 Rx
input (as was the case with the ELM320, ELM322 and
ELM323), so interfacing without RS232 levels is now
even simpler. If you have a microprocessor that uses
the same 5V supply as the ELM327, and has an
internal UART, all you will generally need to do is
connect your processor’s transmit output to the
ELM327’s receive input, and then your receive input to
the ELM327’s transmit output. Communications should
work fine with this connection (and a correct baud rate
setting). The ELM327 is a CMOS device that uses
industry standard levels for all inputs and outputs, so is
easily interfaced to almost any logic circuit.
The ELM327 has a hand-shaking feature that may
be very useful for some interfaces. There is a request
to send (RTS) input, and a Busy output that can be
used to interrupt the ELM327, and to see when it is
ready to receive an input. To use them, set one of your
port pins to normally provide a high output, and
connect it to the RTS input. Use another port pin as an
input to monitor the ELM327 Busy output. When you
want to send a command, simply check the Busy
output first. If it is at a logic high level, then bring your
RTS line low and wait for the Busy line to go low (you
might want to consider using an edge triggered
interrupt for this, 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 waits (indefinitely) for your command.
If you do not use the RTS input on the ELM327,
don’t forget to tie it to a high logic level.
Error Messages
When hardware or data problems occur, the ELM327
will respond with one of the following short messages.
Some of the messages are suppressed during an
automatic search for a protocol, and are only visible if
not in the ‘auto’ mode. Here is a brief description of
The ELM327 provides a 256 byte internal RS232
transmit buffer so that OBD messages can be received
quickly, stored, and sent to the computer or PDA at a
more constant rate. Occasionally (particularly with
CAN systems) the buffer will fill at a faster rate than it
is being ‘emptied.’ Eventually it may become full, and
no more data can be stored (it is lost).
If you are receiving BUFFER FULL messages,
and are using a 9600 baud data rate, give serious
consideration to changing your data rate to 38400
baud. If you still receive BUFFER FULL messages
after that, consider some of the filtering options (the
MR, MT, CF and CM AT Commands).
The ELM327 tried to send the message, or tried to
initialize the bus, and detected too much 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 most often due a wiring
problem that is giving a continuously active input. If
this is an initial trial with your ELM327 circuit, check all
of the voltage levels at the offending OBD input – this
error is very likely due to a wiring problem.
A generic problem has occured. This is most often
from an invalid signal being detected (a pulse that is
longer than a Break signal, etc.) on the bus, but could
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Error Messages (Cont’d)
be 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 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 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
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.
There was a response from the vehicle, but the
information was incorrect or could not be recovered.
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 to ignore the response, even
though one was sent. If you are certain that there
should have been a response, try adjusting the AT ST
time (to be sure that you have allowed sufficient time
to obtain a response), or restoring the CAN filter to its
default setting.
An error was detected in the received CAN data.
This will usually only occur if monitoring a CAN bus,
and the ELM327 is set to an incorrect baud rate, or if
monitoring and there are messages found that that are
not being acknowledged, or that contain bit errors. Try
a different protocol, or a different baud rate.
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. Check all of your connections, and the
ignition, then try the command again.
This is the standard response for a misunderstood
command received on the RS232 input. Usually it is
due to a typing mistake.
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
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 re-sending the command
again – if it was a noise burst, it may be received
correctly the second time.
The IC waited for the period of time that was set
by AT ST, but detected no response from the vehicle.
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Example Applications
The SAE J1962 standard dictates that all OBD
compliant vehicles must provide a standard connector
near the driver’s seat, the shape and pinout of which is
shown in Figure 8 below. The circuitry described here
can be used to connect to this J1962 plug without
modification to your vehicle.
The male J1962 connector required to mate with a
Figure 8. The J1962 Vehicle Connector
vehicle’s connector may be difficult to obtain in some
locations, and you could be tempted to improvise by
making your own connections to the back of your
vehicle’s connector. If doing so, we recommend that
you do nothing that would compromise the integrity of
your vehicle’s OBD network. The use of any connector
which could easily short pins (such as an RJ11 type
telephone connector) is definitely not recommended.
The circuit on page 48 (Figure 9) shows how the
ELM327 might typically be used. Circuit power is
obtained from the vehicle (via OBD pins 16 and 5)
and, after a protecting diode and some capacitive
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 (for visual confirmation that power is present).
We have shown a 78L05 for the regulator as that limits
the current available to about 100mA which is a safe
value for experimenting. The CAN interface is a low
impedance circuit however, and if doing sustained
transmissions on CAN, this type of regulator may shut
down on over-temperature. Should you experience this
problem, you may want to consider using a 1 Amp
version of the regulator.
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
may have a lot of critical information on them, and you
can easily do more harm than good if you fail. It is
strongly recommended that you use a commercial
transceiver chip as shown. The Microchip MCP2551 is
used in our circuit, but most major manufacturers
produce CAN transceiver ICs – look at the NXP
82C251 (NXP was formerly Philips), the Texas
Instruments SN65LBC031, and the Linear Technology
LT1796, to name only a few. Be sure to pay attention
to the voltage limits – depending on the application,
you may have to tolerate 24V, not just 12V.
The next interface shown is for the ISO 9141 and
ISO 14230 connections. 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 leave pin 22 unused.
The ELM327 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. We are often
asked about substitutes for these resistors – if you
need to substitute, you can either go up to 560Ω or
possibly make 510Ω from two resistors (1/4W 240Ω +
270Ω resistors work well), but we do not recommend
using a lower value as it stresses every device on the
bus. Note that 1/2W resistors should be used as a
short at 13.8V causes about 0.4W dissipation.
Data is received from the K Line of the OBD bus
and connected to pin 12 after being reduced by the
R20/R21 voltage divider shown. Because of the
Schmitt trigger input on pin 12, these resistors will give
typical threshold levels of 9.1V (rising) and 4.7V
(falling), providing a large amount of noise immunity
while also protecting the IC.
The final OBD interface shown is 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 7.5V and 5V, which
works well for most vehicles. The two J1850 outputs
are driven by the Q1-Q2 combination for the Bus+,
and Q3 for the Bus-.
The J1850 VPW input uses a resistor divider as
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 singleended one for use by the ELM327. In operation, Q4 is
actually used as the difference amplifier. The Q4-D3
series circuit sets a threshold voltage of about 1V (for
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Example Applications (Cont’d)
noise immunity), while R11 limits the current flow, and
R12 keeps Q4 off when the input is left open-circuited.
Resistor R36 has been added to the circuit of
Figure 9, to help turn transistor Q4 off more rapidly in
certain circumstances. The resistor is generally not
required, but it may be helpful if you are connected to
a very high capacitance J1850 VPW system then force
the ELM327 to operate in the J1850 PWM mode, and
experience some false BUS ERRORs. We show the
resistor as an option and leave the choice whether to
install it up to you.
The voltage monitoring circuitry for the AT RV
command is shown in this schematic connected to pin
2 of the ELM327. The two resistors simply divide the
battery voltage to a safe level for the ELM327, and the
capacitor filters out noise. As shipped, the ELM327
expects a resistor divider ratio as shown, and sets
nominal calibration constants assuming that. If your
application needs a different range of values, simply
choose the resistor values to maintain the input within
the specified 0-5V limit, and then perform an AT CV to
calibrate the ELM327 to your new divider ratio. The
maximum voltage that the ELM327 can show is 99.9V.
A very basic RS232 interface is shown connected
to pins 17 and 18 of the ELM327. This circuit ‘steals’
power from the host computer in order to provide a full
swing of the RS232 voltages without the need for a
negative supply. The RS232 pin 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. The polarity of the ELM327’s RS232 pins
is such that they are compatible with standard
interface ICs (MAX232, etc.), so if you should prefer
such an interface, you can remove all of the discrete
components shown and use one of those.
The four LEDs shown (on pins 25 to 28) have
been provided as a visual means of confirming circuit
activity. They are not essential, but it is nice to see the
visual feedback when experimenting.
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 you
obtain. 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/numbered all of the components and provided
a summary parts list (see 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., 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.
What if you only want to use one of the protocols?
What if you want to use a USB interface? These are
common questions that we receive, and both are
addressed in Figure 11.
There are a few single IC products on the market
that allow you to connect an RS232 system directly to
USB. We have shown the CP2102 by Silicon
Laboratories (http://www.silabs.com) in Figure 11, but
there are others available as well – Future Technology
Devices (http://www.ftdichip.com), for example,
produces several. These ICs provide a very simple
and relatively inexpensive way to ‘bridge’ between
RS232 and USB, and as you can see, require very few
components to support them.
If using 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 of which ones you choose, as some exhibit a
very large capacitance and will affect the transmission
of the USB data. Note also that the circuit as
presented will operate at a 38400 bits per second rate.
If you want to take full advantage of the speed of the
USB interface, you will need to change PP 0C.
Considering the OBD protocol portions of the
circuits in Figures 9 and 11, the differences should be
very apparent. The unused protocols in Figure 11 have
simply had their outputs ignored (left open circuit), and
their inputs wired to a convenient logic level (as CMOS
inputs must never be left floating).
The circuit maintains the status LEDs, and the
J1850 Bus+ circuitry, but the majority of the rest has
been eliminated. The voltage switching circuitry has
been reduced to a single 8V regulator as well, since
there will be no need to switch to 5V. Note that pin 3
has been intentionally left open-circuited as it is not
required by the voltage regulator.
The first time that this circuit is used, it will likely be
set to protocol 0, the default ‘automatic search’ mode
of operation (as shipped from the factory). When you
connect it to the J1850 VPW vehicle, it will first try a
J1850 PWM (protocol 1) connection, fail, and then
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R16 2.2KΩ
R18 2.2KΩ
pin 12
4 (DTR)
6 (DSR)
7 (RTS)
8 (CTS)
R3 470Ω
1 (DCD)
5 (SG)
2 (RxD)
3 (TxD)
4.7KΩ Q1
Figure 9.
An OBD to RS232 Interpreter
R9 10KΩ
J1850 Bus +
J1850 Bus R15
pin 14
*see text, pg 47
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Example Applications (Cont’d)
proceed to try J1850 VPW. If the memory is enabled
(as shown), J1850 VPW will then become the new
default, with no input required by you. This will work
well for most applications, but if the circuit is used on a
vehicle with the key off, for example, then it will again
go searching for a new protocol. In general, you do not
want this to happen every time. It may be only a minor
inconvenience to have to wait while the ELM327
determines that it is “UNABLE TO CONNECT”, but
why go through it if you do not have to? If you know
that you are using the circuit in a J1850 VPW only
application (protocol 2) then you should issue the
command AT SP 2 the very first time that the circuit is
powered. From that point on, it will remain in protocol
2, whether it fails to make a connection or not.
This has provided two examples of how the
ELM327 integrated circuit might be used. Hopefully it
has been enough to get you started on your way to
many more… we hope that you enjoy the journey!
D1 = 1N4001
D2, D3, D4, D5 = 1N4148
L1, L2, L3, L4 = Yellow LED
L5 = Green LED
Q1, Q3, Q5, Q6, Q7, Q9 = 2N3904 (NPN)
Q2, Q4, Q8 = 2N3906 (PNP)
U1 = ELM327
U2 = MCP2551
U3 = 78L05 (5V, 100mA regulator)
U4 = 317L (adj. 100mA regulator)
R32, R33= 100 Ω
R5 = 240 Ω
R1, R2, R3, R4, R27, R28, R29, R30 = 470 Ω
R17, R19 = 510 Ω 1/2W
R16, R18 = 2.2 KΩ
R6, R7, R14, R15, R23, R26, R31 = 4.7 KΩ
R8, R9, R11, R13, R22, R24, R25, R35 = 10 KΩ
R10, R21, R36 = 22 KΩ
R20, R34 = 47 KΩ
R12 = 100 KΩ
C1, C2, C5, C6, C7 = 0.1uF 16V
C3, C4 = 27pF
C8, C9 = 560pF
X1 = 4.000MHz crystal
RS232 Conn = DB9F
IC Socket = 28pin 0.3” (or 2 x 14pin)
Figure 10. Parts List for Figure 9
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Example Applications (Cont’d)
26 25
(type ‘B’
1 (+5)
2 (D-)
3 (D+)
4 (SG)
J1850 Bus +
Figure 11. An OBD (J1850 VPW) to USB Interpreter
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Section Index
Description and Features....................................................................................... 1
Pin Descriptions..................................................................................................... 2
Ordering Information.............................................................................................. 3
Absolute Maximum Ratings....................................................................................4
Electrical Characteristics........................................................................................ 4
Overview................................................................................................................ 5
Communicating with the ELM327...........................................................................5
AT Commands....................................................................................................... 6
AT Command Summary.......................................................................................17
Reading the Battery Voltage................................................................................ 18
OBD Commands.................................................................................................. 19
Talking to the Vehicle........................................................................................... 20
Interpreting Trouble Codes...................................................................................21
Resetting Trouble Codes......................................................................................22
Quick Guide for Reading Trouble Codes............................................................. 22
Bus Initiation.........................................................................................................23
Wakeup Messages...............................................................................................23
Selecting Protocols...............................................................................................24
OBD Message Formats........................................................................................25
Setting the Headers..............................................................................................27
Monitoring the Bus............................................................................................... 30
CAN Messages and Filtering................................................................................31
Multiline Responses............................................................................................. 32
CAN Message Formats........................................................................................ 34
Altering Flow Control Messages...........................................................................35
J1939 Support...................................................................................................... 36
Restoring Order....................................................................................................37
Programmable Parameters.................................................................................. 38
Using Higher RS232 Baud Rates.........................................................................42
Computer Control................................................................................................. 44
Error Messages.................................................................................................... 44
Example Applications........................................................................................... 46
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