OBD (PWM) to RS232 Interpreter
Since the 1996 model year, North American
automobiles have been required to provide an OBD,
or On Board Diagnostics, port for the connection of
test equipment. Data is transferred serially between
the vehicle and the external equipment using this
connection, in a manner specified by the Society of
Automotive Engineers (SAE) standards. In addition
to operating at different voltage levels, these ports
also use a data format that is not compatible with the
standard used for personal computers.
The ELM320 is an 8 pin integrated circuit that is
able to change the data rate and reformat the OBD
signals into easily recognized ASCII characters. This
allows virtually any personal computer to
communicate with an OBD equipped vehicle using
only a standard serial port and a terminal program.
By also enhancing it with an interface program,
hobbyists can create their own custom scan tool.
This integrated circuit was designed to provide a
cost-effective way for experimenters to work with an
OBD system, so a few features such as RS232
handshaking, variable baud rates, etc., have not
been implemented. In addition, this device is only
able to communicate using the 41.6KHz J1850 PWM
protocol that is commonly used in Ford Motor
Company vehicles.
• Low power CMOS design
• High current drive outputs - up to 25 mA
• Crystal controlled for accuracy
• Fully configurable using AT commands
• Standard ASCII character output
• High speed RS232 communications
• 41.6KHz J1850 PWM protocol
Connection Diagram
(top view)
• Diagnostic trouble code readers
• Automotive scan tools
Block Diagram
Timing and
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Pin Descriptions
VDD (pin 1)
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
XT1 (pin 2) and XT2 (pin 3)
A 3.579545 MHz NTSC television colourburst
crystal is connected between these two pins.
Crystal loading capacitors (typically 27pF) will
also normally be connected between each of the
pins and the circuit common (Vss).
OBDIn (pin 4)
The OBD data is input to this pin, with a high
logic level representing the active state (and a
low, the passive). No Schmitt trigger input is
provided, so the OBD signal should be buffered
to minimize transition times for the internal
CMOS circuitry. The external level shifting
circuitry is usually sufficient to accomplish this –
see the Example Application section for a typical
Rx (pin 5)
The computer’s RS232 transmit signal can be
directly connected to this pin from the RS232
line as long as a current limiting resistor
(typically about 47KΩ) is installed in series.
(Internal protection diodes will pass the input
currents safely to the supply connections,
protecting the ELM320.) Internal signal inversion
and Schmitt trigger waveshaping provide the
necessary signal conditioning.
Tx (pin 6)
The RS232 data output pin. The signal level is
compatible with most interface ICs, and there is
sufficient current drive to allow interfacing using
only a single PNP transistor, if desired.
OBDOut (pin 7)
This is the active low output signal which is used
to drive the OBD bus to its active state. Since the
J1850 PWM standard requires a differential bus
signal, the user must create the complement of
this signal to drive the other bus line. See the
Example Application section for more details.
VSS (pin 8)
Circuit common is connected to this pin. This is
the most negative point in the circuit.
Ordering Information
These integrated circuits are available in either the 300 mil plastic DIP format, or in the 208 mil SOIC surface
mount type of package. To order, add the appropriate suffix to the part number:
300 mil Plastic DIP............................... ELM320P
208 mil SOIC..................................... ELM320SM
All rights reserved. Copyright 2001, 2002, 2003 Elm Electronics.
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.
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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........................... -0.6V to (VDD + 0.6V)
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
Maximum Units
see note 2
see note 3
Input low voltage
0.15 VDD
Input high voltage
0.85 VDD
Current (sink) = 8.7mA
Current (source) = 5.4mA
see note 4
see note 5
Output low voltage
Output high voltage
VDD - 0.7
Rx pin input current
RS232 baud rate
1. This integrated circuit is produced with a Microchip Technology Inc.’s PIC12C5XX 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. This specification represents the current flowing through the protection diodes when applying large voltages
to the Rx input (pin 5) through a current limiting resistance. Currents quoted are the maximum that should be
allowed to flow continuously.
5. Nominal data transfer rate when a 3.58 MHz crystal is used as the frequency reference. Data is transferred
to and from the ELM320 with 8 data bits, no parity, and 1 stop bit (8 N 1).
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The following describes how to use the ELM320 to
obtain a great deal of information from your vehicle. To
many, the quantity of information will be overwhelming,
and to others it is not nearly enough.
We begin by discussing just how to talk to the IC,
then how to adjust some options through the use of
‘AT’ commands, and finally go on to actually talk to the
vehicle, obtaining trouble codes and resetting them.
For the more advanced experimenters, there are also
sections on how to use some of the programmable
features of this product as well.
It is not as daunting as it first appears. 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 provide in the
Communicating with the ELM320
The ELM320 relies on a standard RS232 type
serial connection to communicate with the user. The
data rate is fixed at 9600 baud, with 8 data bits, no
parity bit, 1 stop bit, and no handshaking (often
referred to as 9600 8N1). All responses from the IC
are terminated with a single carriage return character
and, by default, a line feed character as well. Make
sure your software is configured properly for the mode
you have chosen.
Properly connected and powered, the ELM320 will
initially display the message:
ELM320 v2.0
In addition to identifying the version of the IC,
receipt of this string is a convenient way to be sure
that the computer connections and the 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 displayed above is the ELM320’s
prompt character. It indicates that the device is in its
idle state, ready to receive characters on the RS232
port. Characters sent from the computer can either be
intended for the ELM320’s internal use, or for
reformatting and passing on to the vehicle’s OBD bus.
Commands for the ELM320 are distinguished from
those to the vehicle by always beginning with the
characters ‘AT’ (as is common with modems), while
commands for the OBD bus must contain only the
ASCII characters for hexadecimal digits (0 to 9 and A
to F). This allows the ELM320 to quickly determine
where the received characters are to be directed.
Whether an ‘AT’ type internal command or a hex
string for the OBD bus, all messages to the ELM320
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 10 seconds, and the ELM320 will print a single
question mark to show that the input was not
understood (and was ignored).
Messages that are misunderstood by the ELM320
(syntax errors) will always be signalled by a single
question mark (‘?’). These include incomplete
messages, invalid hexadecimal digit strings, or
incorrect AT commands. It is not an indication of
whether or not the message was understood by the
vehicle. (The ELM320 is a protocol interpreter that
makes no attempt to assess OBD messages for
validity – it only ensures that an even number of hex
digits were received, combined into bytes, and sent
out the OBD port, so it cannot determine if the
message sent to the vehicle is in error.)
Incomplete or misunderstood messages can also
occur if the controlling computer attempts to write to
the ELM320 before it is ready to accept the next
command (as there are no handshaking signals to
control the data flow). To avoid a data overrun, users
should always wait for the prompt character (‘>’)
before issuing the next command.
Finally, a few convenience items to note. The
ELM320 is not case-sensitive, so ‘ATZ’ is equivalent to
‘atz’, and to ‘AtZ’. The device ignores space characters
as well as control characters (tab, linefeed, etc.) in the
input, so they can be inserted anywhere to improve
readability and, finally, issuing only a single carriage
return character will repeat the last command (making
it easier to request updates on dynamic data such as
engine rpm).
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AT Commands
Several parameters within the ELM320 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 the settings, for example by turning the
character echo off, adjusting the timeout value, or
changing the header addresses. 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 ELM320 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 ELM320 will reply with the characters
‘OK’ on the successful completion of a command, so
the user knows that it has been executed.
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 be provided in pairs. The hexadecimal
conversion chart in the next 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 a number (1 or 0), the universal
terms for on and off, respectively.
The following is a summary of all of the AT
commands that are recognized by the current version
of the ELM320, sorted alphabetically. Users of
previous versions of this product (v1.x) should note
that their ICs will only support the E, H and Z options.
[ Automatically set the Receive address ]
Responses from the vehicle will be acknowledged
and displayed by the ELM320, if its internally stored
receive address matches the address that the
message is being sent to. With the auto receive
mode in effect, the value used for the receive
address will be chosen based on the current header
bytes, and will automatically be updated whenever
the header bytes are changed.
The value that is used for the receive address is
determined based on 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.
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 (echo on).
[ set all to Defaults ]
This command is used to set the E, H, L, and R
options to their default (or factory) settings, as when
power is first applied. Additionally, the Auto Receive
mode (AR) will be selected, data will be transmitted
in the standard formatted way (as if chosen by FD),
the ‘NO DATA’ timeout will be set to its default value,
and the header bytes will be set to the proper values
for the OBDII operation.
[ send Formatted Data ]
This command requests that all responses be
returned as standard ASCII characters which are
readable on virtually any standard terminal program.
Hex digits are shown as two ASCII characters, and
spaces are provided between each byte as a
separator. Also, every line will end with a carriage
return character and (optionally) a linefeed
character, ensuring that every response appears on
a new line. This is the default mode.
H0 and H1
[ Headers off (0) or on(1) ]
These commands control whether or not the header
information is shown in the responses. All OBD
messages have an initial (header) string of three
bytes and a trailing check digit which are normally
not displayed by the ELM320. To see this extra
information, users can turn the headers on by
issuing an ATH1. The default is H0 (headers off).
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AT Commands (continued)
[ Identify yourself ]
Issuing this command causes the chip to identify
itself, by printing the startup product ID string (this is
currently ‘ELM320 v2.0’). Software can use this to
determine exactly which integrated circuit it is talking
to, without resorting to resetting the entire IC.
L0 and L1
[ Linefeeds off (0) or on(1) ]
Whether the ELM320 transmits a linefeed character
after each carriage return character is controlled by
this option. If an ATL1 is issued, linefeed generation
will be turned on, and for ATL0, it will be off. Users
may wish to have this option on if using a terminal
program, but off if using a custom interface (as the
extra characters transmitted will only serve to slow
the vehicle polling down). The default setting is L1
(linefeeds on)
[ Monitor All messages ]
Using this command places the ELM320 into a bus
monitoring mode, in which it displays all messages
as it sees them on the OBD bus. This continues
indefinitely until stopped by activity on the RS232
input. To stop the monitoring, one should send any
single character then wait for the ELM320 to respond
with a prompt character (‘>’). Waiting for the prompt
is necessary as the response time is unpredictable,
varying depending on the IC was doing when
interrupted. If for instance it is in the middle of
printing a line, it will first complete the line then
return to the command state, issuing the prompt
character. If it were simply waiting for input, it would
return immediately. The character which stops the
monitoring will always be discarded, and will not
affect subsequent commands.
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 (i.e. messages which
are found to have that value in their second byte).
Any RS232 activity (single character) aborts the
monitoring, as with the MA command.
MT hh
[ Monitor for Transmitter hh ]
Another monitoring command, which displays only
messages sent by Transmitter address hh. As with
the MA and MR monitoring modes, any RS232
activity (single character) aborts the monitoring.
[ send Packed Data ]
This option is for those that are building a computer
interface and want the fastest data transfer rate
possible while still operating at 9600 baud. When
selected, responses from the vehicle will be
formatted as an initial length byte followed by the
actual response bytes from the vehicle, with no
trailing carriage returns or linefeed characters. The
data will not be altered in any way, except for the
conversion to standard RS232 bytes.
Note that the length byte only represents the total
number of data bytes following, and does not include
itself. Also, if there was a data (checksum) error, the
length byte will have its most significant bit set, so
the user should always check first to see if the length
is greater than 127. (The other 7 bits still provide a
valid byte count if there is an error, so one need only
ignore the msb, or subtract 128 from the value.)
A ‘NO DATA’ response has no data bytes, but still
sends a length byte with value ‘0’.
R0 and R1
[ Responses off (0) or on(1) ]
These commands control the ELM320’s automatic
display of responses. If responses have been turned
off, the IC will not wait for anything to be returned
from the vehicle after sending a request, and will
return immediately to waiting for RS232 commands.
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 (responses on).
SH xx yy zz
[ Set the Header to xx yy zz ]
This command allows the user to control the values
that are sent as the three header bytes in the
message. The value of hex digits xx will be used for
the first or priority/type byte, yy will be used for the
second or target byte, and zz will be used for the
third or source byte. These remain in effect until set
again, or until restored to the default values with the
AT D, or AT Z commands. The default header values
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AT Commands (continued)
are 61 6A F1, as required by the SAE J1979
Diagnostic Test Modes (OBDII) standard.
SR hh
[Set the Receive address to hh ]
Depending on the application, users may wish to
manually set the address to which the ELM320 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.
ST hh
[ Set Timeout to hh ]
After sending a request, the ELM320 waits a preset
time before declaring that there was no response
from the vehicle (the ‘NO DATA’ response).
Depending on the application (and priority of the
request), users may want modify this timeout period
before declaring the request a failure. The ST
command is used to do that.
The actual time used is (approximately) 4 ms x the
byte value passed as the hexadecimal argument.
Passing a value of FF thus results in a maximum
time of about 1020 ms. Values less than 08 will be
ignored and forced to a value of 8, providing a
minimum time of 32ms. The default value is 32
(decimal 50) providing a timeout of 200 ms.
[ 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.
ELM320 AT Commands
D set all to Defaults
I show the ID string
Z reset all
<CR> repeat last command
E1/0 Echo on/off
H1/0 Headers on/off
L1/0 Linefeeds on/off
R1/0 Responses on/off
PD use Packed Data
FD use Formatted Data
ST hh Set Timeout (hh*4ms)
SH xx yy zz Set Header
SR hh Set Rx address
AR Auto Receive
MA Monitor All
MR hh Monitor for Rxer hh
MT hh Monitor for Txer hh
Figure 1. ELM320 AT Commands
AT Command Summary
Figure 1 (at the right) shows all of the ELM320
commands in one convenient chart. In order to help
with the understanding of these, we have grouped
the commands into three functional areas, but this
has no bearing on how the commands are to be
used, it is only for clarity. You may find this chart to
be useful when experimenting with the IC.
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OBD Commands
If the bytes received on the RS232 bus do not
begin with the letters A and T, they are assumed to be
commands for the vehicle’s OBD bus. The bytes will
be tested to ensure that they are valid pairs of
hexadecimal digits and, if they are, will be combined
into bytes for transmitting to the vehicle. Recall that no
checks are made as to the validity of the OBD
command – data is simply retransmitted as received.
OBD commands are actually sent to the vehicle
embedded in a data message. The standards require
that every message begin with three header bytes and
end with a checksum byte, which the ELM320 adds
automatically to every message. The ELM320 powerson expecting to be used for the OBDII mandated
emissions diagnostics, and sets the header bytes
accordingly. If you wish to perform more advanced
functions, these bytes may be changed through the
use of AT commands. To view the extra bytes that are
received with the vehicle’s messages, turn the header
display on by issuing an ATH1 command.
The command portion of most OBD messages is
usually only one or two bytes in length, but can
occasionally be longer as the standard allows for as
many as seven. The current version of the ELM320
will accept the maximum seven command bytes (or 14
hexadecimal digits) per message, while users of
previous versions (v1.x) were limited to only three
command bytes. In either case, attempts to send more
than the maximum number of bytes allowed will result
in a syntax error, with the entire command being
ignored and a single question mark printed.
The use of hexadecimal digits for all of the data
exchange was chosen as it is the most common data
format used in the relevant SAE standards. It is
consistent with mode request listings and is the most
frequently used format for displaying results. With a
little practice, it should not be very difficult to deal in
hex numbers, but some may initially find the table in
Figure 2 or a calculator to be invaluable. All users will
eventually be required to manipulate the results in
some way, though (combine bytes and divide by 4 to
obtain rpm, divide by 2 to obtain degrees of advance,
etc.), and may find a software front-end more helpful.
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 ELM320 on the RS232 bus. The
ELM320 would store the characters as they are
received, and when the third character (the carriage
return) is received, 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 (with a
decimal value of 166). Three header bytes and a
checksum byte would be added, so a total of five bytes
would be sent to the vehicle. Note that the carriage
return character is only a signal to the ELM320, and is
not sent on to the vehicle.
After sending a command, the ELM320 listens on
the OBD bus for any responses that are directed to it.
Each received byte is converted to the equivalent
hexadecimal pair of ASCII characters and transmitted
on the RS232 port for the user. Rather than send
control characters which are unprintable on most
terminals, the digits are sent as numbers and letters
(e.g. the hex digit ‘A’ is transmitted as decimal value
65, and not 10).
If there was no response from the vehicle, due to
no data being available, or because the command is
not supported, a ‘NO DATA’ message will be sent. See
the error messages section for a description of this
message and others.
Figure 2. Hex to Decimal Conversion
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Talking to the Vehicle
The ELM320 cannot be directly connected to a
vehicle as it is, but needs support circuitry as shown in
the Example Applications section. Once incorporated
into such a circuit, you only need to use a terminal
program to send bytes to, and receive them from, the
SAE standards specify that command bytes 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). The
modes and PIDs are described in detail in the SAE
standard documents J1979 and J2190, and may also
be expanded on by the vehicle manufacturers.
Normally, one is only concerned with the nine
diagnostic test modes described in J1979 (although
there is provision for more). Note that it is not a
requirement for all of them to be supported. 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
: test results, continuously monitored
: 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 is required to be supported by all vehicles,
and can be accessed as follows…
Ensure that the ELM320 is properly connected to
your vehicle, and powered. Most vehicles will not
respond without the ignition key in the ON position, so
turn the ignition on, but do not start the vehicle. At the
prompt, issue the mode 01 PID 00 command:
>01 00
A typical response could be as follows:
41 00 BE 1F B8 10
The 41 00 signifies a response (4) from a mode 1
request from PID 00 (a mode 2, PID 00 request is
answered with a 42 00, etc.). The next four bytes (BE,
1F, B8, and 10) represent the requested data, in this
case a bit pattern showing which of PIDs 1 through 32
are supported by this mode (1=supported, 0=not).
Although this information is not very useful for the
casual user, it does serve to show that you are
communicating with the vehicle.
Another example requests the current engine
coolant temperature (ECT). This is PID 05 in mode 01,
and is requested as follows:
>01 05
The response will be of the form:
41 05 7B
This shows a mode 1 response (41) from PID 05,
with value 7B. Converting the hexidecimal 7B to
decimal, one gets 7 x 16 + 11 = 123. This represents
the current temperature in degrees Celsius, with the
zero value offset by 40 degrees to allow operation at
subzero temperatures. To convert to the actual coolant
temperature, simply subtract 40 from the value. In this
case, then, the ECT is 123 - 40 = 83 degrees Celsius.
A final example shows a request for the OBD
requirements to which this vehicle was designed. This
is PID 1C of mode 01, so at the prompt, type:
>01 1C
A typical response would be:
41 1C 01
The returned value (01) shows that this vehicle
conforms to OBDII (California ARB) standards. The
presently defined responses are :
: OBDII (California ARB)
: OBD (Federal EPA)
: not intended to meet any OBD requirements
: EOBD (Europe)
Some modes may provide multi-line responses
(09, if supported, can display the vehicle’s serial
number). The ELM320 will attempt to display all
responses in these cases, but only if it is allowed
sufficient time to process each. There may be
occasions when the vehicle responds too quickly to
allow time for reprocessing, and lines could be lost.
Hopefully this has shown how typical requests
proceed. It has not been meant to be a definitive
source on modes and PIDs – this information can be
obtained from the SAE (http://www.sae.org/), from the
manufacturer of your vehicle, ISO (http://iso.org/), or
from various other sources on the web.
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Interpreting Trouble Codes
Likely the most common use that the ELM320 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:
is only one trouble code here. The response has been
padded with 00’s as is required by the standard, and
the extra 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 first digit of
trouble codes as follows:
If the first hex digit received is this,
Replace it with these two characters
41 01 81 07 65 04
The 41 01 signifies a response to our request, and
the first data byte (81) is the result that we are looking
for. Clearly there would not be 81(hex) or 129(decimal)
trouble codes if the vehicle is 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’) has been turned on by
one of this module’s codes (if there are more than
one), while the other 7 bits provide the actual number
of stored codes. To determine the number of stored
codes, then, one needs to subtract 128 (or 80 hex)
from the number if it is greater than 128, and otherwise
simply read the number of stored codes directly.
The above response then indicates that there is
one stored code, and it was the one that set the MIL or
‘Check Engine’ lamp on. The remaining bytes in the
response provide information on the types of tests
supported by that particular module (see 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
(ATH1) and then look at the third byte of the three byte
header for the address of the module that sent the
Having determined the number of codes stored,
the next step is to request the actual trouble codes
with a mode 03 request:
A response to this could be:
43 01 33 00 00 00 00
The ‘43’ in the above response simply indicates
that this is a response to a mode 03 request. The other
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 there
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’). As for
further examples, if the response had been D016, the
code would be interpreted as U1016, while 1131 would
be P1131.
Had there been codes stored by more than one
module, or more than three codes stored in the same
module, the above response would have consisted of
multiple lines. To determine which module is reporting
each trouble would then require turning the headers on
with an ATH1 command.
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Resetting Trouble Codes
The ELM320 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’ lamp) 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 test results
Clearing of all of this information is not unique to
the ELM320, as it occurs whenever a scan tool is used
to reset your codes. Understand that the loss of this
data could cause your car to run poorly for a short time
while the system recalibrates itself.
To avoid inadvertently erasing stored information,
the SAE specifies that scan tools must verify that a
mode 04 is intended (“Are you sure?”) before actually
sending it to the vehicle, as all trouble code
information is immediately lost when the mode is sent.
Recall, though, that the ELM320 does not monitor the
content of 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 (the ignition on but
the engine not running, etc.) before it will respond to a
mode 04 command.
That is all there is to clearing the codes. Once
again, be very careful not to inadvertently issue an 04!
Error Messages
When hardware or data problems are
encountered, the ELM320 will respond with one of the
following short messages. Here is a brief description of
The ELM320 tried to send the mode command or
request for about 0.5 seconds without success.
Messages are all assigned priorities, which allows
one message to take precedence over another.
More important things may have been going on, so
try re-issuing your request.
An attempt was made to send a message, and the
data bus voltage did not respond as expected. This
could be because of a circuit short or open, so check
all of your connections and try once more.
There was an incomplete message received, and it
was not enough to form a meaningful response. This
may have been caused by the key being turned off,
or a loose connection, for example. Any monitoring
that was in progress will have been aborted.
The error check result (CRC byte) was not as
expected, indicating a data error in the line pointed
to (the ELM320 still shows you what it received).
There could have been a noise burst which
interfered, or a circuit problem. Try re-sending the
There was no response obtained from the vehicle
before a timeout occurred. The mode requested may
not be supported, so the vehicle ignored you, or the
timeout value was too short, or possibly the ignition
key needs to be turned to ‘on’. Try issuing a 01 00
command to be sure that the vehicle is ready to
receive commands, and if that works, try adjusting
the timeout to a longer value using the Set Timeout
AT command.
This is the standard response for a misunderstood
command received on the RS232 bus. Usually it is
due to a typing mistake.
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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
messages. By the same token, you can do a lot of
harm if you are careless, so be very careful.
To see how your vehicle uses the OBD bus, you
will have to enter one of the ELM320’s monitoring
modes. The simplest is the “Monitor All” mode which is
entered into by simply sending the command AT MA
from your terminal program. Once received, the IC will
continually display any information it sees on the OBD
bus, regardless of transmitter or receiver addresses.
Monitoring modes can only be stopped by sending
something over the RS232 connection to the ELM320.
It is not critical what you send - any single character
will interrupt the processor, and return it to the
command mode waiting for an input. Note that the
character you send is discarded and has no effect on
any subsequent commands. The IC will always finish a
task in progress (printing a line, for example) before
returning to wait for input, so always wait for the
prompt character (‘>’) before continuing to issue other
If the headers are not currently displayed, simply
typing ATMA shows only the contents of messages,
not the transmitter and receiver addresses. To show
who is sending to whom, you will need to first turn
headers on (AT H1) before beginning to monitor (AT
MA). Either way, you may end up with an
overwhelming amount of information that you may
want to filter, showing only specific messages.
If, for example, you find that the engine controller’s
address seems to be 10, you may want to restrict the
data displayed to only messages from that ECU. To do
so, you would monitor only for messages transmitted
from address 10, by issuing AT MT 10 from your
terminal program. Only messages with 10 in the third
byte of the header will be displayed. Similarly, you may
wish to only see messages which are being sent to
address 3B. To monitor for these, send AT MR 3B and
only messages with 3B as the second header byte will
be shown.
There are a few limitations to the current
monitoring modes that you should be aware of. First,
there is no internal buffering of OBD messages as
data is being sent on the RS232 connection, so
information may be missed while the IC is busy. If
under computer control, you may want to consider the
‘packed data’ mode to reduce the chance of this. The
second limitation is that the data being printed only
extends up to the End Of Data symbol, and does not
include any In-Frame Response bytes that may be
present. However, for most users this will not be of
Computer Control – Using Packed Data
If a person is simply asking a vehicle for the
current Diagnostic Trouble Codes, speed is normally
not an issue, as data is displayed (essentially) as
quickly as it can be read. If interfaced to a computer,
however, speed may be important.
The packed data mode is a convenient means to
effectively triple the ELM320’s data transfer rate while
maintaining the connection at 9600 baud. Once
entered (with AT PD), all OBD messages will be
returned as a single length byte followed by the actual
data bytes. There are no space characters sent
between bytes, no carriage returns or linefeeds – the
data is retransmitted exactly as received from the
vehicle (except for the change to 9600 baud). While no
longer readable on a terminal, computers will
understand the information just the same, and will gain
speed through both reduced transfer and conversion
times. The ELM320 does not function any differently
when in this mode – if the headers are to be displayed,
they are sent, if in monitoring mode, data is continually
sent, etc. The only difference is in the format in which
the OBD responses are returned to the controlling
Often there is no response from the vehicle for a
particular request. When in the default (formatted data)
mode, this is shown with ‘NO DATA’ being printed, but
while in the packed data mode you will only receive a
single length byte of value 0 (zero).
While rare, errors may occasionally be detected in
the vehicle’s data. Normally, a ‘<DATA ERROR’ would
be printed for this, but in the Packed Data mode, the
checksum (CRC) errors are identified by setting the
most significant bit of the length byte. Because of this,
one should always check the length byte for a value of
128 or greater before processing the remainder of the
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Advanced Data Retrieval – Setting the Headers
Prior to v2.0, the ELM320 used a fixed format for
the message headers, allowing only for the retrieval of
the mandated diagnostic codes, not allowing the user
to change them. The IC is now fully programmable,
however, allowing the headers to be changed and a
great deal more information to be obtained, if your
vehicle supports it. Note that only the OBDII diagnostic
codes have been mandated, so there is no
requirement for all vehicles to support these extra
The diagnostic trouble codes that most people are
familiar with are described by SAE standard J1979
(ISO15031-5). This is really a specific instance of the
many modes allowed by the J2178-4 standard, which
allows for information transfer through what is known
as ‘functional addressing’. For the OBDII mandated
diagnostics, requests are actually made to the
functional address 6A, with whatever processor is
responsible for this function answering the request.
Theoretically many different processors can respond
to a single functional request, each contributing their
insight as to the information requested.
To retrieve some of this extra information, the
function being addressed needs to be known. For
example, consider that you have studied the J2178
standards and want to request that the processor
responsible for Engine Coolant provide the current
Fluid Temperature. You determine that Engine Coolant
is functional address 48, you know that your address
as a scan tool is normally F1, and that since the
ELM320 only supports single IFR responses (type 1),
you choose A1 as the initial priority/type byte.
Combining the above then, it is desirable to set the
three header bytes to A1 48 F1. This is done with the
Set Header command, which would be issued at the
prompt as follows:
>AT SH A1 48 F1
The three header bytes assigned in this manner
will stay in effect until changed with another AT SH
command, a reset, etc. If the default Auto Receive
mode has been selected, the receive address will
automatically be set to 49 (the second byte plus one).
This is consistent with the functional pairs assigned by
J2178-4. If you decide that this is not appropriate for
your case, you can always set the receive address to
what you wish using the AT SR command. For
example, if you wanted to obtain a response that is
being sent to address E2 instead, you would use AT
SR E2 to override the automatic receive mode. Any
receive address selected stays in effect until changed
by another AT SR, or reinstatement of the automatic
Having set the headers, all one needs to do is
issue the secondary ID for fluid temperature (10) at the
prompt. If the display of headers is set to on, the
conversation would typically look like this:
81 49 10 10 2E 41
Ignoring the first three (header) bytes, and the final
check digit, one can see that the response to ID 10 is
the byte 2E. You may find that some requests, being
of a low priority, are not answered immediately,
possibly causing a NO DATA result. In these cases,
you may want to adjust the timeout value, perhaps first
trying the maximum (with AT ST FF).
Using the physical addressing modes described
by the J2190 standard involves an almost identical
process. The main difference is that you must know
the physical address of the device which you want to
speak to. This is always the third byte of a message
sent by any device, so can be determined by
monitoring the headers (for the above response, the
sender’s address is 10). Knowing that you wish to talk
to address 10, that your physical address is F1, and
that for type 1 IFR with physical addressing E4 may be
appropriate for the first byte, you would change the
header bytes using AT SH E4 10 F1. If Auto Receive
is enabled, the receive address will automatically be
set to F1, your physical address (the ELM320 knows
to do this from the first byte). As before, this header
will remain in effect for every message sent until
changed to something else.
One caution to note with physical addressing.
There are modes which initiate the constant sending of
data, and if the ELM320’s timeout is set longer than
the duration between responses, the ELM320 may
return messages forever. In these cases, just like in
the monitoring modes, a single character will have to
be sent to interrupt the process.
Finally, please note that while we have provided
some information on the SAE standards for the
examples, Elm Electronics will only reply to requests
for clarification on our product’s operation, and not on
the standards. It is the customer’s responsibility to
obtain their own information on the relevant standards,
and on their vehicle. Requests to Elm Electronics for
this information will go unanswered.
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Quick Guide for Reading Trouble Codes
If you don’t use your ELM320 for some time, this
data sheet may seem like quite a bit to review when
your ‘Check Engine’ light does eventually come on.
The following provides a quick procedure which may
prove helpful in that case:
Connect using HyperTerminal, ZTerm, etc.,
9600 8N1, and no handshaking
Key on, but vehicle not running
to be sure the IC is reset and responding
to be sure the car is responding
to see how many codes are present
Look at the second digit of the 3rd byte.
to see the codes
Ignore the first byte and read the others in
pairs. The table on page 10 helps.
to reset the codes
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Example Application
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 3 below. The circuitry described here
will be used to connect to this plug without modification
to your vehicle.
The male J1962 connector required to mate with a
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 which 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) would definitely not
be recommended.
The circuit of Figure 4 on the next page shows
how the ELM320 would typically be used. Circuit
power has been obtained from the vehicle (via OBD
pins 16 and 5) and, after some minor filtering, is
presented to a low power (100 mA) 5 volt regulator.
The output of this regulator powers several points in
the circuit as well as an LED (for visual confirmation
that power is present).
The remaining two connections to the vehicle
(OBD pins 2 and 10) are for the differential data
system specified by the J1850 PWM standard. When
no data is being transmitted, both wires are idle with
the transistor drivers off, and the resistive pullup and
pulldown allow voltage levels to float to the supply
levels. Note that the PNP driver transistor and the
2.7KΩ pullup resistor both have series protection
diodes to prevent backfeeds into the ELM320 circuitry.
The ELM320 has only one OBD data output line
(pin 7). It is an active low signal, so must be used to
drive the open-collector ‘Bus +’ signal via the PNP
transistor as shown. By using a portion of this same
signal to drive the NPN transistor for the ‘Bus -’ signal,
one obtains open collector differential drive.
Data is received from the OBD bus, then conerted
and level-shifted by the NPN/PNP transistor pair that
are shown connected to pin 4 of the ELM320. The
NPN transistor detects the differential data signal while
allowing for the presence of common mode voltages,
and the PNP transistor provides the 0 to 5 volt levels
required by OBDIn.
A very basic RS232 interface is shown connected
to pins 5 and 6 of the ELM320. 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 25 pin connector. If you are using a 9 pin, the
connections would be 2(RxD), 5(SG) and 3(TxD).
RS232 data from the computer is directly
connected to pin 5 of the IC through only a 47KΩ
current limiting resistor. This resistor allows for voltage
swings in excess of the supply levels while preventing
damage to the ELM320. A single 100KΩ resistor is
also shown in this circuit so that pin 5 is not left floating
if the computer is disconnected.
Transmission of RS232 data is via the single PNP
transistor connected to pin 6. This transistor allows the
output voltage to swing between +5V and the negative
voltage stored on the 0.1µF capacitor (which is
charged by the computer’s TxD line). Although it is a
simple connection, it is quite effective for this type of
Finally, the crystal shown connected between pins
2 and 3 is a common TV type that can be easily and
inexpensively obtained. The 27pF crystal loading
capacitors shown are only typical, so you may have to
select other values depending on what is specified for
the crystal you obtain.
This completes the description of the circuit. While
it is the minimum required to talk to an OBD equipped
vehicle (it relies on such techniques as using the
internal current limiting of the 78L05 for circuit
protection, for example), it is a fully functional circuit.
As an experimenter, you may want to expand on it,
though, providing more protection from faults and
electrostatic discharge, or providing a different
interface for the RS232 connection to the computer.
Then perhaps a Basic program to make it easier to talk
to the vehicle, and a method to log your findings,
Figure 3. Vehicle Connector
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Notes: - NPN transistors are
2N3904 or similar
‘Power On’
- PNP transistors are
2N3906 or similar
- Diodes are 1N4148,
1N4001, etc.
(Bus +)
(Bus -)
3 (RxD)
7 (SG)
2 (TxD)
Figure 4. Typical OBD to RS232 Interface
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