ELM327
JS1A9E39J1S9u3p9pMorets(scaognetisn(uceodn)tinued)
the values assigned to the PDU Format field. If the
PDU Format value begins with ‘F’ (when expressed as
a hexadecimal number), it is PDU2. Any other value
for the first digit means that it is a PDU1 Format frame,
which contains an address.
will repeat it here.
Since the priority (and DP and EDP) values only
rarely change, they may be assigned with the CP
command. By default, the ELM327 uses a priority of 6
(binary 110), and sets the EDP and DP bits both to 0.
The default value for the CP setting is then 110+0+0
(which would be entered as 11000 or 18 in hex). The
values for the other bytes of the J1939 ID are entered
with the AT SH command, as shown in Figure 5.
This has tried to cover the basics of the J1939
message structure, but if you want more information,
you should look at the standards mentioned
previously. One other one that gives good examples of
actual data is J1939-84 which describes the
compliance tests and shows the expected responses.
Even at 250 kbps, J1939 data is transferred at a
rate that is more than ten times faster than the
previous heavy duty vehicle standard (SAE J1708),
and several of the light duty standards. As designers
build more into each system, the amount of
information required continues to grow, however, so
the 500 kbps version of J1939 will be a welcome
addition.
To summarize, PDU1 format frames are sent to a
specific address, and PDU2 frames are sent to all
addresses. To further complicate matters, however,
PDU1 frames may be sent to all addresses. This is
done by sending the message to a special ‘global
address’ which has the value FF. That is, if you see a
PDU1 message (where the first digit of the PDU
Format byte is not an F), and the Destination Address
is FF, then that message is being sent to all devices.
The J1939 recommended practices document
provides a list of addresses that should be used by
devices. It is particularly important to adhere to this list
with the ELM327, as the IC uses a fixed address
method and is not able to negotiate a different one, per
J1939-81. OBD Service Tools should use either F9 or
FA as their address (the ELM327 uses F9). If you wish
to change this, you can use the AT TA (tester address)
command, or simply define it with the header.
You may use either the AT SH or the AT CP and
AT SH commands to assign values for the J1939 ID
bits, just the same as with the other CAN protocols.
How these are used was discussed previously, but we
Use either:
>AT SH vv xx yy zz
or: >AT CP vv and
>AT SH xx yy zz
The values will be used as follows:
vv
xx yy zz
5 bits
only
PDU
Format
Destination
Address
Source
Address
Priority
Figure 5. Setting the J1939 CAN ID
ELM327DSI
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