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Elm 327 Manual

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57 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
ELM327
can directly request PGNs using numbers as they are
written on the page (if they are written as hex digits),
and the ELM327 will make it work for you. If you do not
want the ELM327 to alter the byte order, the feature
can be disabled (by sending an AT JS command).
The ELM327 always assumes that when you start
making requests of this type, you do not know what
devices are connected to the J1939 bus. That is, by
default the ELM327 sends all requests to the 'global
address' (ie all devices), and then looks for replies.
Often, this works well, but J1939 devices are not
required to respond to such general inquiries, and may
not if they are busy. For this reason, it is usually better
to direct your queries to a specific address, once it is
known.
In order to determine the address to send to, you
may have to monitor the information on the bus for a
while. Make sure that the headers (ID bits) are being
displayed, and note what is shown in the Source
Address position, which is immediately before the data
bytes. In the previous example, this would be 00
(which J1939 defines as the address for engine #1).
As an example, let us assume that it is engine #1 that
you wish to direct your queries to. To do this, you will
want to change the Destination Address from FF (the
global address) to 00 (engine #1).
By default, the ELM327 uses 6 0EAFF F9 for the
ID bits of all requests (or 18 EA FF F9 if you prefer).
That is, it uses a priority of 6, to make a request (EA)
to the global address (FF) by the device at F9 (the
scan tool). An EA request is often referred to as a
request using PGN 59904 as the address EA00 in hex
is 59904 in decimal.
Since you only wish to alter the EAFF F9 portion
of the ID bits and not the priority, you may do this with
the three byte ‘set header’ command:
>AT SH EA 00 F9
The priority bits rarely need to be changed, but if
you do need to change them, it may be done with the
CAN Priority (AT CP) command.
After making the above change, all data requests
will be directed to address 00 (the engine), so don’t
forget to change the headers if you wish to again make
global requests. Note that the AT SH command allows
you to change the source (or tester) address at will, so
be careful with this as addresses should really be
negotiated using the method described in J1939-81
and you might conceivably choose an address that is
already in use. The current version of the ELM327
does not support J1939-81 address negotiation, so
can not obtain an address for you.
Once the ELM327 has been configured to send all
messages to address 00, repeat the request:
>00FEEE
6 0E8FF 00 01 FF FF FF FF EE FE 00
This response is of the ‘acknowledgement’ type
(E8), which is being broadcast to all (FF) by the device
with address 00. The last three data bytes show the
PGN requested, in reverse byte order, so we know this
is a response to our request. Looking at the other data
bytes, the first is not 00 (which we would expect for a
positive acknowledgement), it is 01 which means
negative acknowledgement. Since all requests to a
specific address must be responded to, the device at
address 00 is responding by saying that it is not able
to respond. That is, retrieve the information using the
MP command.
If the ECU had been able to reply to the request,
the format of the response would have been slightly
different. For example, if a request for engine run time
(PGN 00FEE5) had been made, the response might
have been like this:
>00FEE5
6 0FEE5 00 80 84 1E 00 FF FF FF FF
Notice that the PGN appears in the header for
these types of replies, and the data bytes are those
defined for the SPNs in the PGN.
All responses to a request are printed by the
ELM327, whether they are a single CAN message, or
a multisegment transmission as defined by the
transport protocol (J1939-21). If the responses are
multisegment, the ELM327 handles all of the
negotiation for you. As an example, a multisegment
response to a DM2 request might look like this:
>00FECB
012
7 0EBF9 00 01 04 FF 50 00 04 0B 54
7 0EBF9 00 02 00 00 01 5F 05 02 31
7 0EBF9 00 03 6D 05 03 03 FF FF FF
if the headers are on, and would appear as:
>00FECB
012
01: 04 FF 50 00 04 0B 54
Using J1939 (continued)
… 57 …


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