ELM327P datasheet(49/76 Pages) ELM

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ELM327

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ELM327DSH

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J1939 Support (continued)

the values assigned to the PDU Format field. If the

PDU Format value begins with ‘F’ (when expressed as

a hexadecimal number), it is PDU2. Any other value

for the first digit means that it is a PDU1 Format frame,

which contains an address.

To summarize, PDU1 format frames are sent to a

specific address, and PDU2 frames are sent to all

addresses. To further complicate matters, however,

PDU1 frames may be sent to all addresses. This is

done by sending the message to a special ‘global

address’ which has the value FF. That is, if you see a

PDU1 message (where the first digit of the PDU

Format byte is not an F), and the Destination Address

is FF, then that message is being sent to all devices.

The J1939 recommended practices document

provides a list of addresses that should be used by

devices. It is particularly important to adhere to this list

with the ELM327, as the IC uses a fixed address

method and is not able to negotiate a different one, per

J1939-81. OBD Service Tools should use either F9 or

FA as their address (the ELM327 uses F9). If you wish

to change this, you can use the AT TA (tester address)

command, or simply define it with the header.

The J1939 protocol uses the AT CP and AT SH

commands to assign values for the ID bits, just as the

other CAN protocols do. How these are used was

discussed previously, but we will repeat it here, to be

complete. Since the priority (and DP and EDP) values

only rarely change, they are assigned with the CP

command. By default, the ELM327 uses a priority of 6

(binary 110), and sets the EDP and DP to 0. The

default value for the CP setting is then 110+0+0 (which

would be entered as 11000 or 18 in hex). The values

assigned using the SH command relate directly to the

bytes in the J1939 ID, as shown in Figure 5 below.

This has tried to cover the basics of the J1939

message structure, but if you want more information,

you

should

look

the

standards

mentioned

previously. One other one that gives good examples of

actual

data

J1939-84

which

describes

the

compliance tests and shows the expected responses.

Even at 250 kbps, J1939 data is transferred at a

rate that is more than ten times faster than the

previous heavy duty vehicle standard (SAE J1708),

and several of the light duty standards. As designers

build

into

each

system,

the

amount

information required continues to grow, however, so

the 500 kbps version of J1939 will be a welcome

addition.

SAE J1939 Messages (continued)

Figure 5. Setting the J1939 CAN ID

>AT SH xx yy zz

>AT CP vv

5 bits

only

Priority

PDU

Format

Destination

Address

Source

Address

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