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Elm 327 Manual

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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
OK
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
OK
>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.
… 41 …


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