SAE J1939 networks

State: decoding verified on CSS Electronics' CANedge truck logs against cantools and python-can-j1939. The transport protocol, address claims and DM1 with DTCs are modelled against python-can-j1939 as a peer. No real J1939 ECU. The SAE documents were not read.

When a database is a J1939 database

• DBC: the database says so (ProtocolType "J1939") and its messages are parameter groups: VFrameFormat J1939PG, as CANdb++ writes them, or the older form with the global J1939PGN attribute. Start one with the SAE J1939 template of New CAN Database... (see Templates, exchange and C code).
• ARXML: a J-1939-CLUSTER. Each ECU's address is the TP-ADDRESS of its J-1939-TP-NODE, its NAME the NODE-NAME of its J-1939-NM-NODE, and the groups the transport protocol sends come from J-1939-TP-CONNECTION. A cluster without address claiming gives no NAMEs, said in Problems. 11-bit frames stay plain CAN frames.

Decode and reassemble

• A 29-bit frame decodes by its parameter group number (PDU1 and PDU2, data page, extended data page) whatever its priority, source and destination.
• The trace editor and the CAN Bus panel's Trace show PGN, SA, DA and Pri columns; the filter language has pgn, sa, da and priority; the trace statistics count per PGN and source.
• The transport protocol (BAM, and RTS/CTS with TP.DT, abort reasons named) and ISO 11783's extended transport protocol (ETP) are reassembled live and in traces. The frame that completes a transfer decodes as the group it carried; an aborted or incomplete transfer is reported with its reason, never decoded.

The CAN Bus trace of a truck network with PGN, source and destination address and priority, BAM transfers and a DM1 reassembled at its last packet.

The J1939 Network view

In the Diagnostics panel, shown when a channel carries J1939:

• each node with its address, its NAME with every field decoded and the parameter groups it sends;
• NAMEs that lost an address or could not claim one;
• DM1 and DM2 with lamps and DTCs (SPN, FMI, occurrence count), named by the database's SPN attributes and FMI value table;
• failed transfers.

From a tester address you give (J1939 Tester Address):

• Request a Parameter Group... per node or from all nodes (PGN 59904), with the acknowledgement named;
• Request Address Claims from the null address;
• Clear Active DTCs (DM11), Clear Previously Active DTCs (DM3), Request Previously Active DTCs (DM2).

Requests on a real bus are confirmed; clearing DTCs is confirmed every time, on a virtual bus too.

The J1939 Network view with address claims, node identities and active DM1 trouble codes.

J1939 nodes in the simulation

The rest-bus simulation runs J1939 nodes. A node with a NAME (the NmJ1939 node attributes, or the ARXML NM node):

• claims its NmStationAddress at the start and sends nothing else for 250 ms;
• stops when a lower NAME claims its address;
• sends groups of 9 to 1785 bytes by BAM or, to one node, by RTS/CTS;
• answers requests for its address claim and the groups it sends, and a request for any other group with a negative acknowledgement.

The Simulation view shows how each claim stands. A node without a NAME sends from its address without claiming it.

From an agent

j1939Network, j1939Request, j1939ClearDtcs (confirmed before they send on a real bus, a clear always) and traceJ1939Network.

Not verified: no vendor log carries the transport protocol or a DTC, so TP, ETP and multi-packet DM1 rest on python-can-j1939 and unit tests; ETP on unit tests only. J1939 on CAN FD in ARXML is read as plain CAN FD. Per-PGN statistics are in the J1939 Network view only.