Bedienungsanleitung Elm 327 (Seite 41 von 94) (Englisch)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Setting the Headers (continued)

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

similar (J1939 is a CAN standard for use by ‘heavy-

duty vehicles’ such as trucks and buses). It uses

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.

Pages 53 and 54 provide more details of the J1939

message structure.

Another method to define all 29 CAN ID bits at

once is with the four byte version of the SH command.

Simply provide all 8 nibbles in one command:

>AT SH vv xx yy zz

xx yy zzvv

29 bit ID

As with the AT CP command, only 5 bits are used

from the first byte (the 3 most significant bits are

ignored).

The final header format to discuss is that used by

11 bit CAN systems. They also use a priority and

address structure, but shorten it into roughly three

nibbles rather than three bytes. The ELM327 uses the

same storage locations for these values, so you may

use the methods just discussed to assign 11 bit values

(but only the least significant bits are used for any

messages - the others are ignored).

11 bit CAN ‘headers’ are normally assigned using

a special ‘short’ version of the AT SH command that

uses only three hex digits. Since three digits are

actually 12 bits, the most significant bit is ignored, and

can have any value. As an example of this instruction,

to assign an ID of 7DF, simply send:

>AT SH 7DF

The OK shows that the ELM327 has accepted

your value.

The 11 bit ISO15765-4 CAN standard defines both

functional addressing (ID/header = 7DF) and physical

addressing (7En) for making requests. Generally, you

do not know physical addresses at first, but you do

know (from the standard) that the OBD functional

address is 7DF.

In order to learn the physical addresses, turn the

headers on, and watch what address the replies are

from, 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 00

7E8 06 41 00 BE 3F B8 13 00

>01 05

7E8 03 41 05 46 00 00 00 00

This has just been a quick overview on how to

change headers (and ID bits). Hopefully it has been

enough to at least help to get you started. We do not

recommend experimenting too much without a copy of

the standards though, as some aspects are extremely

difficult to figure out without them.

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