ELM327
Setting the Headers (continued)
six digits when only three are required, so there is a
special ‘short’ version of the AT SH command that
uses only three hex digits. It actually operates by
simply adding the leading zeros for you.
The 11 bit CAN standard typically makes
functional requests (ID/header = 7DF), but receives
physical replies (7En). With headers turned on, it is a
simple matter to learn the address of the module that
is replying, 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
Hopefully this has helped to get you started. As we
often tell those that write for help – if you are planning
to do some serious experimenting with OBD, you
should buy the relevant standards.
ELM327DSI
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