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ELM322P

OBD (VPW) to RS232 Interpreter

Elm Electronics 

ELM322

Talking to the Vehicle

The ELM322 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

vehicle.

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:

01 : show current data

02 : show freeze frame data

03 : show diagnostic trouble codes

04 : clear trouble codes and stored values

05 : test results, oxygen sensors

06 : test results, non-continuously monitored

07 : test results, continuously monitored

08 : special control mode

09 : 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 ELM322 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 with 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 hexadecimal 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:

01 : OBDII (California ARB)

02 : OBD (Federal EPA)

03 : OBD and OBDII

04 : OBD I

05 : not intended to meet any OBD requirements

06 : EOBD (Europe)

Some modes may provide multi-line responses

(09, if supported, can display the vehicle’s serial

number). The ELM322 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 manufacturer of your vehicle, from

the SAE (www.sae.org), ISO (www.iso.org), or from

various other sources on the web.

ELM322DSF

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

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