ELM327P datasheet(52/76 Pages) ELM

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written on the page (if they are written as hex digits),

and the ELM327 will make it work for you. If you do not

want the ELM327 to alter the byte order, the feature

can be disabled (by sending an AT JS command).

The ELM327 always assumes that when you start

making requests of this type, you do not know what

devices are connected to the J1939 bus. That is, by

default the ELM327 sends all requests to the 'global

address' (ie all devices), and then looks for replies.

Often, this works well, but J1939 devices are not

required to respond to such general inquiries, and may

not if they are busy. For this reason, it is usually better

to direct your queries to a specific address, once it is

known.

In order to determine the address to send to, you

may have to monitor the information on the bus for a

while. Make sure that the headers (ID bits) are being

displayed, and note what is shown in the Source

Address position, which is immediately before the data

bytes. In the previous example, this would be 00

(which J1939 defines as the address for engine #1).

As an example, let us assume that it is engine #1 that

you wish to direct your queries to. To do this, you will

want to change the Destination Address from FF (the

global address) to 00 (engine #1).

By default, the ELM327 uses 6 0EAFF F9 for the

ID bits of all requests (or 18 EA FF F9 if you prefer).

That is, it uses a priority of 6, to make a request (EA)

to the global address (FF) by the device at F9 (the

scan tool). Since you only wish to alter the EAFF F9

portion of the ID bits and not the priority, you may do

this with the set header command:

>AT SH EA 00 F9

The priority bits rarely need to be changed, but if

you do need to change them, it is done with the CAN

Priority (AT CP) command.

After making the above change, all data requests

will be directed to the engine, so don’t forget to change

the headers if you wish to again make global requests.

Note that the AT SH command allows you to change

the source (or tester) address at will, so be careful with

this as addresses should really be negotiated using

the method described in J1939-81 and you might

conceivably choose an address that is already in use.

The current version of the ELM327 does not support

J1939-81 address negotiation, so can not obtain an

address for you.

Once the ELM327 has been configured to send all

messages to address 00, repeat the request:

>00FEEE

6 0E8FF 00 01 FF FF FF FF EE FE 00

This response is of the ‘acknowledgement’ type

(E8), which is being broadcast to all (FF) by the device

with address 00. The last three data bytes show the

PGN requested, in reverse byte order, so we know this

is a response to our request. Looking at the other data

bytes, the first is not 00 (which we would expect for a

positive acknowledgement), it is 01 which means

negative acknowledgement. Since all requests to a

specific address must be responded to, the device at

address 00 is responding by saying that it is not able

to respond. That is, retrieve the information using the

MP command.

If the ECU had been able to reply to the request,

the format of the response would have been slightly

different. For example, if a request for engine run time

(PGN 00FEE5) had been made, the response might

have been like this:

>00FEE5

6 0FEE5 00 80 84 1E 00 FF FF FF FF

Notice that the PGN appears in the header for

these types of replies, and the data bytes are those

defined for the SPNs in the PGN.

All responses to a request are printed by the

ELM327, whether they are a single CAN message, or

a multisegment transmission as defined by the

transport protocol (J1939-21). If the responses are

multisegment,

the

ELM327

handles

all

the

negotiation for you. As an example, a multisegment

response to a DM2 request might look like this:

>00FECB

012

7 0EBF9 00 01 04 FF 50 00 04 0B 54

7 0EBF9 00 02 00 00 01 5F 05 02 31

7 0EBF9 00 03 6D 05 03 03 FF FF FF

if the headers are on, and would appear as:

>00FECB

012

01: 04 FF 50 00 04 0B 54

02: 00 00 01 5F 05 02 31

03: 6D 05 03 03 FF FF FF

if the headers are off. Note that multiframe messages

Using J1939 (continued)

52 of 76

ELM327DSH

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