AppNote07.pdf

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Application Note
AN07 - Sending Arbitrary CAN Messages
We are often asked about how to send arbitrary
CAN messages. Typically the user wants to send data
bytes of their own choosing and does not want PCI
bytes inserted for them, or to have receive bytes
removed because the ELM IC assumed that they were
ISO 15765 PCI (formatting) bytes. This application
note shows how to send CAN messages of your own
choosing, with our ELM327, ELM327L, ELM329 or
ELM329L products (version 1.3 or newer).
The ELM327 and ELM329 integrated circuits have
evolved over the years, and for this reason it is best to
choose the following settings (even though it may not
be necessary for your particular chip):
AT D, AT WS, or AT Z
If you are about to send a message, but are
unsure of the state of the integrated circuit, it is always
advisable to force a reset to the default settings. How
you do this is typically through one of the above
commands (although you should be aware that the
AT WS and AT Z commands will close any protocols
that you are currently set to).
AT SP
You may wish to choose a protocol, rather than
have the ELM IC look for an active one for you. This is
done with the AT SP command (for example AT SP 8,
to set the IC for 11/250 CAN). Be careful if using the
automatic search on fail option (eg. AT SP A8), as that
may cause the chip to go off looking for a valid
protocol if it does not like a particular response (or lack
of one).
AT AL
This command really only applies to non-CAN
protocols (and protocol 0, when searching). You
should not normally need to use it, but for compatibility
with older ELM327 ICs, it is best to send it once. Note
that the ELM329 and ELM329L ICs do not support the
AT AL command, and will show an error if presented
with it.
AT CEA
Turn off CAN Extended Addressing when trying to
send arbitrary messages (it is off by default), unless
you are absolutely certain of what you are doing. If you
enable extended addressing, it may limit the allowed
number of bytes that the IC will send. The ELM327
v1.3 and v1.3a do not support the CEA commands.
AN07 rev A
AT CAF0
Turn off CAN automatic formatting so that PCI
bytes are not inserted in the messages, or expected in
the responses.
AT V1 or V0
Only you know how many bytes that you wish to
send. ISO 15765 always sends 8 data bytes, inserting
filler (padding) bytes as required in order to make a
total of 8 bytes transmitted. It is the PCI byte that
normally tells how many of the 8 data bytes are to be
used. If you do not wish to have your message padded
out with 00's (i.e. the byte as set by PP 26), then send
AT V1, or ensure that the appropriate Programmable
Parameter bit is set when defining your own protocol.
AT BI
Choosing a protocol does not immediately make it
active (i.e. valid). The ELM ICs will normally look for a
response to a particular request (typically this is 01 00
for ISO 15765) before considering a protocol to be
valid. If a reply is not obtained, and the automatic
search feature is enabled, the IC will likely start
sending search messages using other protocols,
perhaps finding one that might work, or stopping with
an error message. If you do not want to chance this
happening, send the AT BI command in order to
bypass the initiation step, and accept the current
protocol.
AT SH
If you are choosing your own data bytes to send,
you may also want to choose your own ID bits (we call
them header bytes). Since the header values are not
likely to change as often as the data bytes, they are
assigned with a separate AT SH command, and not
with the data bytes. It is not necessary to assign your
own header bytes, but if you do not then the default
OBDII values will be used.
As an example, we will set an ELM327 to use
protocol 8 for sending messages of 1 to 8 bytes in
length. This requires typically sending the following
setup commands at the prompt (we also show the
ELM327 responses, so that you know what to expect):
>AT Z
ELM327 v2.2
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Application Note
AN07 - Sending Arbitrary CAN Messages
>AT SP 8
OK
>AT AL
OK
>AT CAF0
OK
>AT V1
OK
>AT BI
OK
The chip is now ready to send a message of your
choosing. First, assign the header (ID bits) for your
message. We’ll use 777:
>AT SH 777
OK
Now present the data bytes that you wish to send. We
will provide these eight:
>11 22 33 44 55 66 77 88
NO DATA
The ELM327 responds with ‘NO DATA’ because
we had no ECU reply with a message that matched
the receive filters. You must define the receive ID bits
for the ELM IC to look for, using either the CAN
Receive Address command (AT CRA) or the CAN
Filter/CAN Mask commands.
In order to not receive this NO DATA response
while testing, we can turn off the ELM IC’s search for a
response. This is done by either sending AT R0 in the
setup, or by providing a ‘number of responses’ digit
after the data bytes. To send the previous message
but retrieve no replies, we would send:
>11 22 33 44 55 66 77 88 0
>
which eliminates the NO DATA response. Note that in
both of the above cases, the ELM327 transmits exactly
the same CAN message:
777 11 22 33 44 55 66 77 88
For this configuration, the ELM327 will always
send only the data bytes provided, without any extra
filler bytes being added. For example, if you had only
wanted to send the data bytes 11 22 33 44, then at the
prompt, you would provide only those four bytes:
>11 22 33 44
In this case, the ELM327 actually sends the
following:
777 11 22 33 44
for you, since we chose AT V1 to enable a variable
data length. If we had chosen AT V0, then the
message sent would have had eight data bytes, as
follows:
777 11 22 33 44 00 00 00 00
The extra bytes are all 00’s as the filler byte is 00
by default (it is set by PP 26).
Using the above as a guideline, you should now
be able to configure the ELM327 (and ELM327L,
ELM329 or ELM329L) for sending any arbitrary group
of one to eight data bytes. Sending more than eight
bytes is somewhat more complicated, but not
impossible. It involves splitting the data into smaller
chunks that each fit into one message, then sending a
First Frame message, receiving a Flow Control
message, and sending Consecutive Frames. This is
beyond what the average user might require and is left
to the advanced users to work out.
AN07 rev A
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www.elmelectronics.com
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