SAE J1939 basics — CAN for commercial, construction, and agricultural vehicles

A PGN (Parameter Group Number) identifies a message that groups related parameters. Engine speed, vehicle speed, and coolant temperature travel in the same PGN on any manufacturer's vehicle.

An SPN (Suspect Parameter Number) identifies an individual parameter inside a PGN. Resolution, offset, and units are defined in the standard, so receivers can convert to physical values without their own conversion tables.

This is J1939's greatest strength. Coming from passenger-car development, you will likely be surprised to find that you can understand the values without a DBC file.

J1939 uses 29-bit extended identifiers that embed the priority, PGN, and source address. Addresses are 8 bits (0–253), with recommended values per function (engine = 0, transmission = 3, and so on).

Because nodes claiming the same address would collide, an arbitration procedure called address claiming is specified. Each node has a 64-bit NAME, claims an address at startup, and if there is a conflict, the node with the lower NAME wins.

When you add your own ECU to a vehicle, a missing or incomplete address-claim implementation causes hard-to-reproduce faults such as “sometimes it stops communicating.”

A CAN frame carries at most 8 bytes. Longer data is split and sent with the TP (Transport Protocol). There is a point-to-point RTS/CTS method (TP.CM_RTS / TP.CM_CTS) and a broadcast BAM (Broadcast Announce Message) method.

BAM sends one way without waiting for the receiver to be ready, so data can be missed. Handle it with care.

J1939-22 (J1939-FD) is a new data-link layer for CAN FD. It handles up to 64 bytes per frame and introduces a container format that packs several PGNs into one frame.

The key point is that its implementation is separate from classic-CAN J1939. An existing stack cannot simply be reused, so if you plan to move to CAN FD, check this when choosing a stack.

J1939-73 specifies diagnostics. Messages such as DM1 (active faults) and DM2 (previously active faults) express a fault as a pair of SPN and FMI (Failure Mode Identifier).

Emissions regulations add OBD-related requirements (such as the J1939-84 tests). For North America there are further compliance checks such as CARB's Clean Truck Check.

Firmware is usually updated through a bootloader over J1939. Design recovery from power loss during an update, identification of the update target, and an estimate of transfer time first. If there are security requirements, the bootloader must verify image signatures.

ISO 11783 (ISOBUS) extends J1939 for agricultural machinery, adding implement–tractor interconnection, the virtual terminal (VT) and the task controller.

J1587 / J1708 are commercial-vehicle networks that predate J1939 and are still needed when servicing existing vehicles.

NMEA 2000 is a marine network based on J1939.