ELM ELM327 - Using J1939

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ELM327
Using J1939
This section provides a few examples which show
how to monitor an SAE J1939 data bus, and how to
make requests of devices that are connected to it.
To begin, you will need to configure the ELM327
for J1939 operation, at the correct baud rate. Protocol
A is predefined for J1939 at 250 kbps, which is what
most applications require. To use protocol A, send:
>AT SP A
Protocols B and C may also be used with J1939,
if you wish to experiment with other baud rates. To use
them for J1939, the option value (in PP 2C or 2E) must
be set to 42, and the baud rate divisor (in PP 2D or 2F)
must be set to the appropriate value. Perhaps the
simplest way to provide an alternate rate is to use the
AT PB command, that allows you to set both the
options byte (which is always 42), and the baud rate
divisor (which is 500k ÷ the desired baud rate) at the
same time. For example, to set protocol B for J1939
operation at 500 kbps, simply send:
>AT PB 42 01
then send:
>AT SP B
to begin. Note that this setting will not be maintained if
the IC is reset, so if you want a more permanent
setting, you should store the values in PP 2C and 2D.
Once the protocol is set, then make sure that you
have a long timeout value chosen for the data
receives. If you have a version 1.4b IC, you do not
need to do anything, but if you have a previous
version, we recommend that you send:
>AT ST FF
to select a long timeout. You will not do any harm if
you set the timeout with a v1.4b chip, but the timing
will not be optimal, as you will stop the ELM327 from
varying the setting based on the type of message
being received.
The new ELM327 v1.4b also offers one other
variation on the timer setting - the ability to extend the
AT ST time by switching a x5 timer multiplier on and
off (see the JTM5 command). This may be useful
when requesting data that will have a multiline
response while similar data is already flowing. Since
there can be only one message like this at a time on
the bus, the response to your request would have to
wait while the initial response completes (and this
could take more than the normal ST time since
broadcast responses must be spaced at least 50 msec
apart). If you know that a reply should be coming, and
you are seeing ‘NO DATA’ responses, then send
AT JTM5 and try it again, as that may be the problem.
Restore the timer multiplier to normal with AT JTM1.
Once the J1939 protocol is selected, and the
timeout value has been adjusted, the ELM327 is ready
for a command. The first one that we will send is called
a DM1 or ‘diagnostic message 1’, which provides the
currently active diagnostic trouble codes. DM1 is one
of more than 50 predefined diagnostic messages, and
is special in that it is the only one that is broadcast
continually over the bus at regular intervals. The
ELM327 has an AT command that is used to obtain
the DM1 trouble codes:
>AT DM1
If you are connected to a vehicle, you should now
see messages printed at one second intervals. If you
are only connected to a single device (for example,
with a simulator on the bench, or to a device with a
single CAN data port), you may see data with
<RX ERROR printed beside it. This is because the
receipt of the data is not being acknowledged by any
device on the bus (certainly not the ELM327, as it is by
default a completely silent monitor). See our ‘AN05 -
Bench Testing OBD Interfaces' application note for
more information on this, and some advice on what to
do. If you have a v1.4b chip, you do not have to take
special measures, however, simply turn off the silent
monitoring with:
>AT CSM 0
and there should be no more RX ERRORs. Once you
have this sorted out, repeat the request. If all goes
well, you should see several replies, similar to this:
00 FF 00 00 00 00 FF FF
00 FF 00 00 00 00 FF FF
You will likely need to stop the flow of data by
pressing any key on the keyboard. This is because the
DM1 command is actually a special form of a
monitoring command, and all monitoring needs to be
stopped by the user. The response means that there
are currently no active trouble codes, by the way.
To see the exact same response, you can also