ISO 11783-5 — network management and address claiming

This is the part that makes “hitch any implement to any tractor” actually work. Part 5 turns two strange boxes that both want the same address, with no installer and no central authority into a deterministic, self-healing assignment. It is the first thing every node does after power-on, and nothing above it may speak until it finishes.

Why this exists

SAE J1939 mostly assigns addresses by convention — fine for one manufacturer’s truck. On a farm, a random implement from another vendor, built years apart, may want the same address as something already on the bus. There is no human to resolve it. Part 5 resolves it automatically using the one thing that is guaranteed unique and comparable: the NAME.

NAME arbitration: lowest NAME wins

When two CFs contend for an address, they compare 64-bit NAMEs and the numerically lower one keeps the address. Because NAMEs are unique, the contest always has a winner and the network always converges.

   Both want address 0x80:

   CF-A   NAME = 0x00A0_0000_0000_1234   ┐
   CF-B   NAME = 0x00C0_0000_0000_5678   ┘ compare as 64-bit numbers
                       │
                 0x00A0… < 0x00C0…
                       ▼
        CF-A keeps 0x80   ·   CF-B must move (if self-configurable) or go silent

This is the same “lowest number wins” rule as CAN arbitration — applied to identity instead of priority.

The state machine

   POWER ON
      │   have NAME + preferred address; may not send app traffic yet
      ▼
   CLAIMING ── broadcast "I claim 0x80, NAME = N" ──►  bus
      │
      │   wait the contention window, then:
      │
      ├─ no contender ──────────────►  CLAIMED   (may now send app traffic)
      ├─ contender, higher NAME ────►  CLAIMED   (the contender yields)
      └─ contender, lower NAME ─────►  lose 0x80:
            ├─ self-configurable ─►  claim another address (→ CLAIMING)
            └─ not ───────────────►  SILENT (cannot operate)

Rules machbus enforces:

• No application traffic before CLAIMED. The session core refuses app sends until the claim completes — this is why “drive the claim loop first” is the cardinal rule.
• Request for Address Claim. Any node can ask everyone to re-announce; a CF replies with its current claim.
• Address violation → diagnostic. Detecting another node on your claimed address is surfaced as a DTC (SPN derived from the offending address).
• Loss mid-run. A lower-NAME newcomer can take your address while you are running; a self-configurable CF moves, and the change is reported on the event stream.
• Convergence. Because the tie-break is numeric and NAMEs are unique, the network reaches a stable assignment with no oscillation.

NAME management (dynamic identity)

A further wrinkle: a CF can be commanded to adopt a new NAME (NAME management). The node applies the new identity and re-claims. machbus models this with the NameManagement plugin, which answers the management requests, adopts a commanded NAME, and triggers a fresh claim.

From concept to code

See also

• ISO 11783-1 — the NAME — the identity this part arbitrates on.
• The networking foundation — the overview that ties parts 1–5 together.