ELM329
Using J1939 (continued)
if the headers are off. Note that multiframe messages
always send eight bytes of data, and fill in unused byte
positions with FFs.
other bytes, you must turn CAN Auto Formatting off
(AT CAF0), and then repeat the request.
This has been a brief description of how to use
the ELM329 in a typical J1939 environment. If you can
monitor for information, make global requests as well
as specific ones, and receive single or multiframe
responses, then you have the tools necessary to at
least diagnose most vehicle problems.
With the headers off, the multiline response looks
very similar to the multiline responses for ISO15765-4.
The first line shows the total number of bytes in the
message, and the other lines show the segment
number, then a colon, and the data bytes following.
Note that the byte count is a hexadecimal value (ie the
‘012’ shown means that there are 18 bytes of data).
The one line that shows the total number of data
bytes is actually called a ‘Connection Management’ or
‘TP.CM’ message. It has a specific format, but the only
bytes that are typically relevant are those that provide
the total message size in bytes. In order to see the
The FMS Standard
Several European heavy duty truck and bus
manufacturers have joined to form an organization for
standardizing the way in which information is retrieved
from these large vehicles. The result of their work is
the FMS (or Fleet Management Systems) Standard,
and the Bus-FMS Standard.
The FMS standard is based on a subset of the
250 kbps J1939 protocol, which uses only broadcast
messages for the information. In order to not
compromise the integrity of the vehicle’s CAN bus, the
standard also specifies a gateway device to provide
separation between (potentially unskilled) users and
the critical control information on the vehicle.
The FMS standard is completely open, and still
evolving (as of this writing, the latest update was
version 2.00, dated November 11, 2010). For more
information, visit the web sites:
FMS Standard
http://www.fms-standard.com
Bus FMS Standard
http://bus-fms-standard.com
The information contained in the FMS messages
is defined by PGNs, using the same PGN numbers as
for J1939. The difference is that they only define a
small subset of those specified by J1939.
To monitor the information provided by an FMS
gateway, simply use the AT MP command with the
appropriate PGN number. We should caution that
some information (VIN, software version, etc.) is only
transmitted every 10 seconds, so some patience is
required when waiting for the data.
ELM329DSB
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
52 of 76
ELM [ ELM ELECTRONICS ]
