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03: 6D 05 03 03 FF FF FF
if the headers are off. Note that multiframe messages
always send eight bytes of data, and fill in unused byte
positions with FFs.
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
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.
Using J1939 (continued)
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