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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

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

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