ELM329LP datasheet(36/87 Pages) ELM

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Setting the Header / ID Bits

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 be more specific with your requests.

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 this physical address that

uniquely identifies each module in a vehicle, and

permits you to direct more specific queries to only one

particular module. To direct the queries to a specific

address requires changing the values that the ELM329

uses for the header (ID bits).

The ID bits in an ISO 15765-4 header may follow

one of two different formats - an 11 bit one, and a 29

bit one. First, consider the 29 bit standard, which has a

structure that is very similar to the header structure of

older OBD protocols (J1850, etc.).

There are two ways that you may use to define the

value that the ELM329 uses for a 29 bit header. The

first is to simply provide all of the bits as 4 bytes, or 8

hex digits, using the Set Header command:

>AT SH ww xx yy zz

5 bits

only

29 bit ID

Setting a 29 bit (extended) CAN ID

The ELM329 will ignore the first three bits, leaving

29 that are then used for the messages.

The second way (which is how the ELM327 does

it) is to change the values in two steps. In this method,

the ELM329 splits the 29 bits into a CAN Priority byte

and three header bytes. This makes it a little quicker to

change only one portion of the header (usually, it is the

priority bits that do not change). The two are then

combined by the ELM329 into a 29 bit value that it is

able to use. To set the header in this way, simply use

the CAN Priority and Set Header commands:

>AT SH xx yy zz

>AT CP ww

5 bits only

29 bit ID

Setting a 29 bit (extended) CAN ID

The ISO 15765-4 CAN standard defines each of

the above ‘byte’ values for diagnostics. The priority

byte (‘ww’ in the diagrams) will always be 18 (this is

the default value used by the ELM329). 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 final two bytes are

used in a way that is very similar to other standards –

‘yy’ is the receiver (or Target Address), and ‘zz’ is the

transmitter (or Source Address). For the functional

requests, the receiver is always 33, and the transmitter

is F1 (which is very similar to ISO 14230-4).

The other header structure that the CAN standard

defines uses an 11 bit ID (and is likely the most

common system in use today). The ELM329 uses a

special 3 digit version of the Set Header command in

order to set these bits:

11 bit ID

>AT SH xyz

Setting an 11 bit (standard) CAN ID

In this case, the ELM329 uses the 11 least

significant (‘right-most’) bits of the provided header

bytes, and ignores the most significant bit.

The 11 bit ISO 15765-4 CAN standard typically

makes functional requests (ID/header = 7DF), but

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Elm Electronics – Circuits for the Hobbyist

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