ELM327P datasheet(20/51 Pages) ELM

20 / 51 page

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:

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 - show ‘pending’ trouble codes

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 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

command:

>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

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

this

mode

(1=supported, 0=not). Although this information is not

very useful for the casual user, it does prove that the

connection is working.

Another example requests the current engine

coolant temperature (ECT). 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.

20 of 51

ELM327

ELM327DSC

Elm Electronics – Circuits for the Hobbyist

ELM327P Datasheet(PDF) 20 Page - ELM Electronics

ELM327P Datasheet(HTML) 20 Page - ELM Electronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

Datasheet Download

Link URL