J1939 Basics

A J1939 network consists of ECUs connected by a CAN bus running at 250 k baud rate. Some newer networks might use a 500 k baud rate. A physical ECU can contain one or more logical ECUs called nodes or Controller Applications. This description refers to it as a node or ECU.

J1939 application protocol uses a 29-bit extended frame identifier. The ID is divided into several parts:

• Source Address (8 bits): Determines the address of the node transmitting the frame. By examining the Source Address part of the ID, the receiving session can recognize which node has sent the frame.
• PGN (18 bits): Identifies the frame and defines which signals it contains.
• Priority (3 bits): Priority is used when multiple CAN frames are sent on the bus at exactly the same time. In this case, the CAN frame with the higher priority (lower number) is transmitted before the lower priority frame. The CAN standard defines the CAN frames priority (lower IDs have higher priority). Therefore, the J1939 priority bits are the most significant bits in the ID. This ensures that the ID value with a higher priority is always lower, independent of the PGN and Source Address, as shown in the following figure.

You can send a frame to a global address (all nodes) or a specific address (node with this address). This information is coded inside the PGN, as shown in the following figure.

The PF value in the identifier defines whether the message has a global or specific destination:

• 0–239 (0x00–0xEF): specific destination
• 240–255 (0xF0–0xFF): global destination

In the CAN identifier, this looks like the following (X = don't care):

• 0xXXF0XXXX to 0xXXFFXXXX are messages with global destination (broadcast)
• 0xXX00XXXX to 0xXXEFXXXX are messages with specific destination

For global messages, the PS byte of the ID defines group extension. This extends the number of possible global PGNs to 4096 (0xF000 to 0xFFFF).

For destination-specific messages, PS defines the destination address, so PF defines only 240 destination-specific PGNs (0–239).

DP and EDP bits increase the number of possible PGNs by defining data pages. EDP, however, always is set to 0 in J1939, so only DP can be set to 0 or 1, which doubles the number of PGNs described above. The maximum number of possible PGNs (and so, different messages) in J1939 is 2*(4096 + 240) = 8672.

For node addresses (source address and destination address), the ID reserves 8 bit, which allows values from 0 to 255. Two values have a special meaning:

• 254 is the null address. This means there is no valid address assigned to a node yet.
• 255 is the global address. This allows sending even PGNs with PF 0 to 239 to a global destination.