ELM329
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ELM329DSC
Elm Electronics – Circuits for the Hobbyist
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Receive Filtering - the CRA command
The ELM329 is always monitoring the CAN data. It
retrieves every message from the CAN bus, and then
decides whether or not to keep it based on criteria that
is established by the ELM329 firmware. This criteria is
always initially set to allow OBDII data to pass, but you
may change it at any time.
Adjusting the criteria normally takes two steps
(see the next section), but there is one AT command
that you can use to make life a little easier. It allows
setting the address (CAN ID) of messages that you
wish to receive, in one simple step.
This command is the ‘CAN Receive Address’ or
CRA command. With it, you can specify a specific
address, or a range of addresses that the ELM329
should accept. For example, if the only messages that
you wish to see are those that have the CAN ID 7E9,
then simply send:
>AT CRA 7E9
and the ELM329 will set the necessary values so that
the only messages that are accepted are the ones with
ID 7E9.
If you do not want an exact address, but would
prefer to see a range of values, for example all the
OBD addresses (those that begin with 7E), then simply
use an ‘X’ for the digit that you do not want the
ELM329 to be specific about. That is, to see all
messages with CAN IDs that start with 7E (7E0, 7E1,
7E2,..., 7EE, and 7EF), then send:
>AT CRA 7EX
and the ELM329 will set the necessary values for you.
This command works exactly the same way for the
29 bit IDs. For example, if you wish to see all
messages that are being sent from the engine (ECU
address 10) to the scan tool (address F1), then you
can send:
>AT CRA XX XX F1 10
and all the settings will be taken care of for you.
If you wish to be more specific and see only the
OBD replies sent by the engine to the scan tool, you
would say:
>AT CRA 18 DA F1 10
and again, the ELM329 makes the necessary changes
for you.
Perhaps you do not care which device is sending
the information, but you want all messages that start
with 18 DA and are being sent to the scan tool. For
this, use the character ‘X’ to tell the ELM329 that you
do not care what value a digit has:
>AT CRA 18 DA F1 XX
and the ELM329 takes care of the details for you.
When working with J1939 data, the ELM329
normally formats the data for you, in order to separate
the priority from the PGN information. This is usually
not a concern when using the CRA command, except
when you are trying to filter for a specific priority. For
example, you might typically see:
>AT MA
3 0FE6C 00 FF FF FF FF FF FF 40 B5
6 0FEEE 00 15 50 FF FF FF FF FF FF
6 0FEF5 00 FE FF FF FF 19 00 23...
The single priority digit out front (the 3 or 6 above)
as well as the leading 0 with the PGN information are
actually part of the first two digits (5 bits) of the ID, and
need to be interpreted as such, in order to use the
CRA command. It may be easier if you turn off the
J1939 header formatting in order to see this:
>AT JHF0
OK
>AT MA
0C FE 6C 00 FF FF FF FF FF FF 40 B5
18 FE EE 00 15 50 FF FF FF FF FF FF
18 FE F5 00 FE FF FF FF 19 00 23...
This more clearly shows the four bytes that need
to be defined for the CRA command to be set. For
example, to search for all 6 0FEF5’s you would
actually send the command:
>AT CRA 18 FE F5 XX
In summary then, the CRA command allows you
to tell the ELM329 what ID codes to look for, and the
letter ‘X’ may be used in it to represent any single digit
that you do not want the ELM329 to be specific about.
This is usually selective enough for most applications,
but occasionally, there is a need to be specific down to
the bit level, rather than to the nibble. For those
applications, you will need to program a separate
mask and filter, as we show in the next section.