Using CAN Extended Addresses
Some vehicles with CAN interfaces use a data
format that is slightly different from what we have
described so far. The data packets look very similar,
except that the first byte is used for the receiver’s (ie
target’s) address. The remaining seven bytes are used
as described previously.
We refer to this type of addressing as ‘CAN
Extended Addressing’, and provide support for it with
the AT CEA commands. Perhaps an example would
better describe how to use them.
Here is a portion of a data transfer that was taken
from a vehicle. For the moment, ignore the first data
bytes on each line and only look at the remaining data
bytes (that are outlined in grey):
7B0 04 02 10 81 00 00 00 00
7C0 F1 02 50 81 00 00 00 00
7B0 04 02 21 A2 00 00 00 00
7C0 F1 10 16 61 A2 01 02 05
7B0 04 30 FF 00 00 00 00 00
7C0 F1 20 DF 01 00 04 09 01
7C0 F1 21 02 05 DF 01 00 04
7C0 F1 22 09 01 00 04 01 00
If you are familiar with the ISO 15765 data format,
you will be able to recognize that the data bytes shown
inside the box seem to conform to the standard. The
rows that begin with 02 are Single Frames, the one
that starts with 10 is a First Frame, while the one with
30 is a Flow Control, and the others are Consecutive
Frames.
The remaining bytes, shown outside the box, are
the standard 11 bit CAN ID, and an extra address
byte. The lines with F1 for the extra address are
directed to the scan tool (all scan tools generally use
F1 as the default address), and the other lines are
being sent to the vehicle’s module (at address 04).
The ELM329 is able to handle these types of
messages, but does require some setup. For example,
if the messages use 11 bit IDs with ISO 15765
formatting, and the baud rate is 50 kbps, then the PB
command to configure protocol B is:
>AT PB 81 0A
OK
Next, we’ll want to receive all messages with an ID
of 7C0, and send with an ID (header) of 7B0:
>AT CRA 7C0
OK
>AT SH 7B0
OK
Notice that there was a flow control message that
was sent in this group, but it’s not quite the same as
the one for OBD systems. For this reason, you’ll need
to define your own flow control with the following three
statements (we won’t show the OK’s any more, to
save space):
>AT FC SH 7B0
>AT FC SD 04 30 FF 00
>AT FC SM1
The final setup statement that you will need is to
tell the ELM329 to send to CAN Extended Address 04:
>
AT CEA 04
Now everything is configured. Next, tell the IC to
use this protocol, and to bypass any initiation (as it is
not standard OBD, and would likely fail):
>AT SP B
>AT BI
That’s all. To exactly reproduce the flow of data
shown, you only need to send the relevant data bytes
and the ELM329 will add the rest:
>10 81
50 81
>21 A2
016
0: 61 A2 01 02 05
0: DF 01 00 04 09 01
1: 02 05 DF 01 00 04
2: 09 01 00 04 01 00
Notice that for some reason, this vehicle has sent
two segment 0’s, but that just means that it doesn’t
exactly follow the ISO 15765 protocol. The above
shows what the responses would look like with
formatting on, and headers off. If you change either,
the data exchange would look more like what we
initially showed.
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