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ELM329L DSA

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ELM329L

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. 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.

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 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

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:

>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 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

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 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 ELM329 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 ELM329 automatically handles this

for you, reversing the bytes provided. In this way, you

Using J1939 (continued)

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