J1939 PGN Decoder

How a J1939 identifier is laid out

J1939 runs on CAN with 29-bit extended identifiers, and SAE J1939-21 gives every bit of that identifier a job. Nothing is left for an implementer to choose, which is what allows a node from one manufacturer to make sense of a bus built by another:

• Bits 28–26, priority — three bits, 0 highest. Used for arbitration only.
• Bit 25, EDP and bit 24, DP — the extended data page and data page.
• Bits 23–16, PF — the PDU format, which decides how the next byte is read.
• Bits 15–8, PS — the PDU specific byte: a destination address or a group extension.
• Bits 7–0, SA — the source address of the node that sent the frame.

The parameter group number is assembled from EDP, DP, PF and sometimes PS. Priority and source address are never part of it. That is the point of the split: the PGN identifies what the message is, while the identifier as a whole also says who sent it and how urgently.

The one rule that catches everybody

Whether PS belongs to the PGN depends on PF, and there is no separate flag for it:

• PF below 240 — PDU1, addressed. PS is a destination address. The PGN's low byte is zero.
• PF 240 or above — PDU2, broadcast. PS is a group extension and is part of the PGN.

Getting this backwards is the most common J1939 mistake, and it is self-concealing. A request sent to node 0x21 has the identifier 0x18EA21FE. Reading the middle two bytes as the PGN gives 0xEA21, which appears in no standard and no database, so the message looks like an unknown proprietary group. The actual PGN is 0xEA00 — 59904, the Request message — and 0x21 is simply who it was sent to. Treating the destination as part of the PGN turns one well-known message into 256 imaginary ones.

What priority does and does not do

CAN arbitration is decided bit by bit on the wire, and a dominant bit wins. Because priority occupies the top three bits of the identifier, a frame with priority 3 beats one with priority 6 whenever both start transmitting at once. The loser backs off and retries.

What priority does not do is change which message you are looking at. The same PGN can be sent at different priorities by different nodes, and it is still the same parameter group with the same signal layout. This is why a filter written against a full identifier can silently miss traffic: a node that raises its priority changes its identifier without changing what it is sending.

Addresses, and the two that are not nodes

Every node on a J1939 bus owns an address from 0 to 253, claimed at startup with the Address Claimed message (PGN 60928) and defended if another node claims the same one. Two values are reserved:

• 254, the null address — used as a source by a node that has not won an address. Seeing it as a source means address claiming has failed or has not finished.
• 255, the global address — used as a destination to address every node at once. A PDU1 message sent to 255 reaches the whole bus.

Reading a capture

Most of an ordinary heavy-vehicle capture is a handful of PDU2 messages repeating at fixed rates: engine speed and torque in EEC1, road speed in CCVS1, temperatures in ET1. They are broadcast because every node that cares can use them, and they carry no destination.

The PDU1 traffic is where the conversations are — requests, acknowledgements, transport protocol sessions for messages longer than eight bytes, and address claiming. If you are looking at a bus and trying to work out what talks to what, the addressed messages are the ones to follow.

This decoder names a small set of the parameter groups that fill a normal capture. It is not a J1939 database and does not try to be one; the standard defines thousands of parameter groups and their signal layouts are not freely redistributable. For anything outside the short list, the PGN this gives you is what you look up in your own DBC.

Building an identifier

Going the other way — from a PGN to the identifier you need to transmit — is where the PDU1 rule bites again. A PDU2 PGN carries its group extension, so the identifier follows directly from the PGN, the priority and your own source address. A PDU1 PGN needs a destination supplied separately, because the PGN deliberately does not contain one.

This tool refuses a PDU1 PGN whose low byte is not zero rather than quietly masking it off. A value like 0xEA21 in that field means the caller believes the destination is part of the parameter group, and encoding it anyway would produce a frame that does not mean what they think it means.