ELM327P datasheet(36/51 Pages) ELM

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J1939 Support

The SAE J1939 CAN standard is a relatively new

protocol that is being used in many types of heavy

machinery

–

trucks,

agricultural

equipment,

and

busses, to name a few. It uses a standard CAN

(ISO 11898) physical interface, and defines its own

format for data transfer (although it is very similar to

the ISO 15765 that is used for automobiles).

The ELM327 offers some support for the J1939

standard in Protocol A. (Protocols B and C can be

selected

for

J1939

formatting

needed,

with

Programmable Parameters 2C and 2E). Although this

is not full support as dictated by the standards, the

ELM327 implementation should at least allow you to

begin experimenting.

The default data rate for Protocol A has been set

to 250 kbps, as set by the SAE J1939-11 standard.

Should your application require a different speed, it is

easily changed with PP 2B.

J1939 messages use 29 bit CAN IDs, and up to 8

data bytes for each message (ISO 15765 always uses

8 bytes). Diagnostics on SAE J1939 are defined by the

J1939-73 standard, and the actual data transfer

protocol is described in J1939-21. If you are going to

do a lot of work with J1939, it may be wise to purchase

these standards documents from the Society of

Automotive Engineers (SAE).

The ELM327 handles the sending of messages

with the J1939 protocol in the same way as any of the

other protocols. The header bytes are predefined for

you and all you need do is provide the data bytes that

you wish to send.

For example, assume that you wish to send

request for engine temperature (PGN 00FEEE). J1939

specifies a reverse order for the data bytes, so when

you tell the ELM327 what to send, you need to switch

the order of the data bytes, and then send them as you

would any other request:

>EE FE 00

The ELM327 then adds the required header (ID)

bytes, etc. and sends the message for you. (It also

configures itself correctly to be able to receive the

response.) Note that the ELM327 sets the header

bytes to EC FF F9 by default (for global requests from

OBD service tool #1), so if you want something else,

you will have to change the headers, using the AT SH

command.

All responses to a request are printed by the

ELM327, whether they are a single CAN message

transmission, or a multisegment transmission as

defined by the transport protocol (J1939-21). If the

responses are multisegment, the ELM327 handles all

of the negotiation for you (transparently - you won’t

see it happen). If formatting is on (ie CAF1), the output

will resemble the format used to print the ISO 15765-4

multiline responses. Note that if you provide three data

bytes, the ELM327 will assume that you are sending a

PGN, and will look for responses to that. If you send

other than 3 data bytes, it will assume that you are

making a general request, and will only look for

responses to the Source Address (as given in the third

byte of the header).

The SAE J1939 standard predefines several

Diagnostic Messages, and assigns ID values to them.

The first of these (designated ‘DM1’) are reserved for

trouble codes, which are periodically broadcast over

the CAN network. Since these are commonly required,

the ELM327 provides a special command to monitor

for them (aptly named DM1). If you wish to monitor for

all DM1 messages, simply issue:

>AT DM1

and the ELM327 will print out all that it finds.

The DM1 message is the only one that is presently

predefined by the ELM327 for you. To monitor for any

other diagnostic message, you need to know it’s PGN

number (in hexadecimal), and monitor for that using

the AT MP command. For example, the DM2 PGN

number is 65227, or 00FECB in hexadecimal. The

ELM327 requires hexadecimal, and assumes that all

PGNs begin with 00 – so to monitor for DM2s, you

need only send:

>AT MP FECB

and the IC will begin looking for all replies to DM2

requests.

Please note that the ELM327’s support for the

SAE J1939 standard is only basic (it does not support

address negotiation as described in J1939-73, for

example), and it has not undergone the extensive

testing that our automotive protocols have. We simply

have had a large number of requests for support of the

protocol, and wanted to provide something for you to

begin experimenting with. As always, we are quite

open to your feedback and your suggestions for future

upgrades.

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