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Elm 327 Manual

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56 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
ELM327
To see the exact same response, you can also
Monitor for PGN 00FECA (which is the code for DM1):
>AT MP 00FECA
Note that the ELM327 requires that you send hex
digits for all data, as shown above (and as used by all
other protocols). Many of the PGN numbers are listed
in the J1939 standard as both a decimal and a hex
number, so choose the hex version.
You will likely find in your testing that the PGNs
you encounter often begin with a 00 byte as above. To
simplify matters for you, the ELM327 has a special
version of the MP command that will accept a four digit
PGN, and assumes that the missing byte should be
00. An equivalent way to ask for 00FECA is then:
>AT MP FECA
which is a little more convenient.
One feature of the ELM327 is the ability to tell the
IC how many messages to retrieve when monitoring
for PGNs. For example, to see only two responses to
the MP FECA command, send:
>AT MP FECA 2
This saves having to send a character to stop the
flow of data, and also is very convenient when dealing
with multiline messages. While the standard OBD
requests allow you to define how many frames (ie
lines) of information are to be printed with a similar
single digit, the single digit with the MP command
actually defines how many complete messages to
obtain. For example, if the DM1 message is 33 lines
long, then sending AT MP FECA 1 will cause the
ELM327 to show all 33 lines, then stop monitoring and
print a prompt character.
By default, all J1939 messages have the ‘header’
information hidden from view. In order to see this
information (actually the ID bits), you will need to turn
the header display on:
>AT H1
A single response to FECA might then look like:
>AT MP FECA 1
6 0FECA 00 00 FF 00 00 00 00 FF FF
Notice that the ELM327 separates the priority bits
from the PGN information. The ELM327 also uses only
one digit to represent the two extra PGN bits, both of
which may seem unusual if you are used to different
software. We find this a convenient way to show the
actual J1939 information in the header.
If you prefer to see the ID bits separated into bytes
instead, simply turn off the J1939 header formatting
with:
>AT JHF0
Repeating the above request would then result in
a response of this type:
>AT MP FECA 1
18 FE CA 00 00 FF 00 00 00 00 FF FF
The differences are clearly seen. If displaying the
information in this manner, remember that the first
‘byte’ shown actually represents five bits, and of them,
the leftmost three are the priority bits.
The MP command is very useful for getting
information in a J1939 system, but not all information
is broadcast. Some information must be obtained by
making a query for it. Just like the other OBD requests
where you specify the information that you need (with
a mode and a PID), to make a query in a J1939
system, you provide the PGN number and the system
responds with the required data.
For example, to request the current value of the
engine coolant temperature (which is part of PGN
00FEEE), you would send a request for PGN 00FEEE,
and extract the data. To do this, send:
>00FEEE
to which you might receive:
6 0FEEE 00 8C FF FF FF FF FF FF FF
if the headers were on. Note that if you request a PGN
that is already being broadcast, you may very well
receive many replies, as the ELM327 configures itself
to receive anything that is related to the PGN
requested.
If you are familiar with the J1939 standard, you will
be aware that it actually specifies a reverse order for
the sending of the data bytes of a PGN request. That
is, the data bytes for the above request are actually
sent as EE FE 00, and not as 00 FE EE. Since it can
be very confusing to have to reverse some numbers
and not others, the ELM327 automatically handles this
for you, reversing the bytes provided. In this way, you
Using J1939 (continued)
… 56 …


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