ELM329
Using J1939 (continued)
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:
formatted output as shown above. Beginning with v2.0,
the ELM329 now displays both the EDP and the DP
bits (the EDP should always be 0 for J1939, but other
protocols do use this bit).
If you prefer to see the ID bits separated into bytes
instead, simply turn off the J1939 header formatting
with:
>AT MP FECA
which is a little more convenient. Please note that the
MP command is very similar to the MA command,
except that it is able to process multiline responses. If
you are simply interested in receiving single line
broadcast messages, then using the CRA and MA
commands may be an option.
>AT JHF0
Repeating the above request would then result in
a response of this type:
Just as the ELM329 allows the number of
ISO 15765 responses to be specified when a request
is made, it also allows you to specify the number of
messages to retrieve when monitoring for PGNs. It is
done in the same way - for example, to specify only
two responses for the MP FECA command, send:
>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.
>AT MP FECA 2
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:
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
ELM329 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:
>00FEEE
to which you might receive:
6 0FEEE 00 8C FF FF FF FF FF FF FF
>AT H1
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 ELM329 configures itself
to receive anything that is related to the PGN
requested.
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 ELM329 separates the priority bits
from the PGN information. The ELM329 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. Note that
version 1.0 of the ELM329 always assumed that the
Extended Data Page (EDP) bit was 0 when printing
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 ELM329 automatically handles this
for you, reversing the bytes provided. In this way, you
ELM329DSB
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
50 of 76
ELM [ ELM ELECTRONICS ]
