ISO 11783-4 — the network layer

Most machines are one CAN segment and can ignore this part. But a big machine — a tractor with its own bus joined to a large implement with its own bus — is two segments bridged by a router. Part 4 defines how a message crosses that boundary safely: which messages forward, in which direction, and how to avoid loops and address clashes between the two sides.

Why this exists

One CAN segment has electrical and load limits. When a machine outgrows them, you split it and join the halves with an interconnect (a network interconnection unit, NIU). The moment you do, two new problems appear: a broadcast could echo back and forth forever (a loop), and two devices on opposite segments might claim the same address. Part 4 is the rulebook that prevents both.

The mental model

   Tractor segment                     Implement segment
   (TECU · VT · GNSS)                   (rate ctrl · sections · sensors)
          │                                      │
          └──────────────►  ┌───────┐  ◄─────────┘
                            │  NIU  │   router: forwards selected PGNs by
                            └───────┘   rule, drops echoes, guards addresses

The NIU is not a dumb repeater. It applies forwarding rules: a given PGN may be allowed to cross in one direction, both, or neither, optionally filtered by source or destination NAME. A loop guard ensures a forwarded frame is not forwarded back.

What machbus models

machbus implements the interconnect in net::niu: forwarding policies per PGN, direction control, NAME-based source/destination filters, and the loop guard. It is the piece that lets a large machine scale without every ECU having to know it lives on a multi-segment network.

For single-segment applications — the common case — none of this is in your way; you build one node on one bus and the network layer is invisible.

From concept to code

See also

• ISO 11783-3 — data link & transport — what is being routed.
• ISO 11783-5 — address claiming — the address clashes the NIU must guard against.