Bedienungsanleitung Elm 327 (Seite 53 von 94) (Englisch)

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SAE J1939 Messages

The SAE J1939 CAN standard is being used by

many types of heavy machinery – trucks, buses, and

agricultural equipment, to name a few. It uses the

familiar CAN (ISO 11898) physical interface, and

defines its own format for data transfer (which is very

similar to the ISO 15765 standard that is used for

automobiles).

The following will discuss a little of how data is

transferred using the J1939 standard. Considerably

more information is provided in the Society of

Automotive Engineers (SAE) standards documents, so

if you are going to be doing a lot of work with J1939, it

may be wise to purchase copies of them. At minimum,

the J1939-73 diagnostics, the J1939-21 data transfer,

and the J1939-71 vehicle application documents

should be purchased. Another great reference for this

work is the HS-J1939 two book set, also available from

the SAE.

The current version of the J1939 standard allows

only one data rate (250 kbps), but work is underway to

amend the standard so that an alternate rate of

500 kbps will also be allowed. For the purpose of this

discussion, the data rate is not important - it is the

format of the information that we will discuss.

All CAN messages are sent in ‘frames’, which are

data structures that have ID bits and data bytes, as

well as checksums and other items. Many of the J1939

frames are sent with eight data bytes, although there is

no requirement to do so (unlike ISO 15765, which

must always send eight data bytes in each frame). If a

J1939 message is eight bytes or less, it will be sent in

one frame, while longer messages are sent using

multiple frames, just like ISO 15765. When sending

multiple frames, a single data byte is used to assign a

‘sequence number’, which helps in determining if a

frame is missing, as well as in the reassembly of the

received message. Sequence numbers always start

with 01, so there is a maximum of 255 frames in a

message, or 1785 bytes.

One major feature of the J1939 standard is its very

orderly, well defined data structures. Related data is

defined and specified in what are called ‘parameter

groups’. Each parameter group is assigned a

‘parameter group number’, or PGN, that uniquely

defines that packet of information. Often, the

parameter groups consist of eight bytes of data (which

is convenient for CAN messages), but they are not

restricted to this. Many of the PGNs, and the data

within them (the SPNs) are defined in the J1939-71

document, and manufacturers also have the ability to

define their own proprietary PGNs.

The ID portion of a J1939 CAN frame is always 29

bits in length. It provides information as to the type of

message that is being sent, the priority of the

message, the device address that is sending it, and

the intended recipient. Information within the ID bits is

divided roughly into byte size pieces as follows:

Priority

PDU

Format

Destination

Address

Source

Address

8 bits 8 bits 8 bits2 bits3 bits

PDU1 Format

The data structure formed by the 29 bit ID, and the

associated data bytes is called a Protocol Data Unit, or

PDU. When the ID bits have a destination address

specified, as is shown above, it is said to be a PDU1

Format message.

The two bits shown between the Priority and the

PDU Format are known as the Extended Data Page

(EDP), and the Data Page (DP) bits. For J1939, EDP

must always be set to ‘0’, while the DP bit is used to

extend the range of values that the PDU Format may

have. While the DP bit is typically ‘0’ now, that may not

be true in the future.

Not all J1939 information is sent to a specific

address. In fact, one of the unique features of this

standard is that there is a large amount of information

that is being continually broadcast over the network,

with receivers using it as they see fit. In this way,

multiple devices requiring the same information do not

have to make multiple requests to obtain it, information

is provided at regular time intervals, and bus loading is

reduced.

If information is being broadcast over the network

to no particular address, then the Destination Address

field is not required. The eight bits can be put to better

use, possibly by extending the PDU Format field. This

is what is done for a PDU2 Format frame, as shown

here:

Priority

PDU

Format

Group

Extension

Source

Address

8 bits 8 bits 8 bits2 bits3 bits

PDU2 Format

So how does one know if they are looking at a

PDU1 Format frame that contains an address, or a

PDU2 Format frame that does not? The secret lies in

ELM327

53 of 94ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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