The standards, end to end

This section is the story of how an ISOBUS machine works — not a clause-by-clause recital, but the narrative an engineer needs to hold in their head. By the end you should be able to look at any frame on a tractor bus and know which layer produced it, why, and what happens next.

We build the picture from the wire up: a single twisted pair, then the language spoken on it, then the agreement that lets a tractor and a stranger’s implement cooperate the first time they are bolted together.

Throughout, standards are named only at the part level — “ISO 11783-6 (Virtual Terminal)”, “SAE J1939”, “NMEA 2000”. The explanations are written from how machbus implements the behavior; consult the official documents for normative wording.

The one-paragraph version

A tractor and an implement are two computers that have never met. They share two copper wires. CAN lets them put bits on those wires without electrocuting each other’s messages. SAE J1939 turns those bits into named messages (PGNs) sent between addresses. ISO 11783 takes J1939 and adds everything farming needs: a way to claim an address by identity so two strange devices never collide, a way to move data bigger than 8 bytes, a screen-sharing protocol so an implement can draw its own controls on the tractor’s terminal, a job/data protocol so a field computer can run a prescription, plus diagnostics, files, guidance, and safety interlocks. AEF is the club that certifies all of this actually interoperates; NMEA 2000 is the cousin protocol that carries the GPS fix.

The layer cake

Everything below sits on the layer above’s shoulders. Read it bottom-to-top: each layer only worries about its own job and trusts the one beneath it.

   ┌────────────────────────────────────────────────────────────────────┐
   │ application services   VT · TC · File Server · Sequence Control ·  │
   │                        TIM · diagnostics · guidance · GNSS feed    │
   │                        (ISO 11783-6, -7, -9 … -14)                 │
   ├────────────────────────────────────────────────────────────────────┤
   │ messages & transport   PGN requests/acks · TP / ETP · Fast Packet ·│
   │                        interconnect routing                        │
   │                        (ISO 11783-3, -4 · SAE J1939)               │
   ├────────────────────────────────────────────────────────────────────┤
   │ identity & addressing  NAME · address claim · who-may-talk         │
   │                        (ISO 11783-5)                               │
   ├────────────────────────────────────────────────────────────────────┤
   │ naming                 29-bit ID = priority + PGN + source + dest  │
   │                        (SAE J1939)                                 │
   ├────────────────────────────────────────────────────────────────────┤
   │ wire                   250 kbit/s CAN, 29-bit extended frames      │
   │                        (ISO 11783-2, ISO 11898)                    │
   └────────────────────────────────────────────────────────────────────┘

For a one-screen index of every part — what it does, where machbus implements it, and which chapter covers it — see Standards capability map.

The deep-dive chapters in this section follow these bands:

• The networking foundation — the bottom four bands: CAN, J1939 naming, address claiming, and transport. This is the spine; nothing else works until a node has claimed an address and can move data.
• The Virtual Terminal — ISO 11783-6, the screen-sharing protocol and the most elaborate application service.
• The Task Controller and the data dictionary — ISO 11783-10 and -11: documented work, process data, and the DDI vocabulary.
• Implement control, the tractor ECU, and the rest — ISO 11783-7/-9 plus diagnostics (-12), File Server (-13), Sequence Control (-14), and TIM.
• Positioning: NMEA and GNSS — how the fix gets onto the bus.

Who is on the bus

A useful mental model before the protocols: an ISOBUS network is a small town of control functions (CFs). Each CF is one participant with one job and one identity. A physical box (an ECU) may host several CFs. The cast of characters on a typical tractor-plus-implement bus:

   The bus — control functions side by side:

   ┌─────────────────┐ ┌─────────────────┐ ┌─────────────────┐ ┌─────────────────┐ ┌─────────────────┐
   │ Tractor         │ │ Virtual         │ │ Task            │ │ Implement       │ │ GNSS            │
   │ ECU (TECU)      │ │ Terminal        │ │ Controller      │ │ ECU             │ │ receiver        │
   │ speed · hitch   │ │ the screen,     │ │ runs the job,   │ │ VT + TC client, │ │ position,       │
   │ PTO · GNSS      │ │ soft keys       │ │ logs work       │ │ the actuators   │ │ COG / SOG       │
   └─────────────────┘ └─────────────────┘ └─────────────────┘ └─────────────────┘ └─────────────────┘

The tractor side mostly serves (it has the screen, the speed, the hitch). The implement side mostly requests (it borrows the screen, asks for the job, reports what it did). Many roles come in client/server pairs — a VT server (the terminal) and VT clients (each implement), a TC server and TC clients. machbus implements both halves of each pair.

A machine waking up: the first ten seconds

The single most important sequence to internalize is what happens when you turn the key. Every protocol above the wire depends on it.

 t0  Power on. The CF has a NAME (its 64-bit identity) and a *preferred* address,
     but is not yet allowed to send application traffic.

 t1  It broadcasts an Address Claim for its preferred address, putting its NAME
     on the wire as the tie-breaker.

 t2  Silence for the contention window?  → the address is ours. Claimed.
     Someone else claims the same address? → the *lower NAME wins*. The loser
     either grabs another address (if self-configurable) or goes quiet.

 t3  Now addressed, the CF starts its periodic chores: a TECU broadcasts wheel
     speed and hitch state; a CF may begin its heartbeat.

 t4  An implement's VT client goes looking for a terminal: "any VT out there?"
     It discovers the VT's address and version.

 t5  The VT client uploads its *object pool* — the entire description of its UI —
     in one big transport-protocol transfer, then asks the VT to activate it.

 t6  In parallel, the TC client uploads its *device description* (the DDOP) to the
     Task Controller and the two begin trading process data.

 t7  Steady state: status broadcasts tick along, the operator presses soft keys,
     setpoints flow down, as-applied values flow up, and the GNSS receiver feeds
     position the whole time.

machbus mirrors exactly this order. In the session facade you plug the subsystems you need, start() the claim, and drive poll(); address claim runs first and the application plugins only act once an address is held. See The session facade.

Why identity-based addressing is the clever bit

J1939 alone assigns addresses more or less by convention. That is fine for a truck built by one manufacturer. It falls apart the moment a random implement from a different manufacturer is hitched to a tractor it has never seen — both might want the same address.

ISO 11783-5 (network management) solves this with NAME-based arbitration. Every CF carries a 64-bit NAME encoding what it is (manufacturer, device class, function, instance, and a self-configurable flag), not where it sits. When two CFs want the same address, the numerically lower NAME wins the address and the other moves. Because NAMEs are unique by construction, the network always converges — no human, no DIP switches, no central authority. This is the property that makes “hitch any implement to any tractor” actually work, and it is why address claim is the first thing every node does.

   Conflict on address 0x80:

   CF-A  NAME = 0x00A0_0000_0000_1234   ─┐
   CF-B  NAME = 0x00C0_0000_0000_5678   ─┤  compare NAMEs
                                          ▼
                 0x00A0… < 0x00C0…  →  CF-A keeps 0x80
                 CF-B is self-configurable → claims 0x81 instead

Two sizes of message, and why transport exists

A raw CAN frame carries at most 8 bytes. Plenty for “engine speed = 1500 rpm”; hopeless for “here is a 40 KB object pool describing my user interface.” ISO 11783 inherits J1939’s answer and extends it:

• Single frame — ≤ 8 bytes, one shot.
• Transport Protocol (TP) — up to 1785 bytes, broken into 7-byte packets with flow control (a destination can say “send me packets 1–16, then pause”).
• Extended Transport Protocol (ETP) — megabyte-scale transfers for big object pools and files.
• Fast Packet — NMEA 2000’s lighter multi-frame scheme for things like a GNSS position record.

The deep dive covers the handshakes. The key idea: every layer above transport gets to pretend messages are arbitrarily large; transport quietly chops and reassembles.

The application services, in one breath

Once a node can claim an address and move data, the farming-specific services are just well-defined conversations on top:

Each has a dedicated codec and a session plugin in machbus. The deep-dive chapters tell each story properly.

Where AEF and NMEA fit

• AEF (Agricultural Industry Electronics Foundation) does not invent wire formats; it defines functionalities and a certification process that proves two vendors’ boxes actually interoperate. TIM (Tractor Implement Management) is an AEF-driven capability layered on the ISO messages. machbus models the functionality advertisement and the TIM authority guard, but ships no certification — see Conformity first.
• NMEA 2000 is a separate but CAN-compatible standard. On an ISOBUS machine it is how the GNSS receiver publishes position, course, speed, and attitude. machbus decodes the relevant PGNs so guidance and TC-GEO have a fix to work with.

How to read the rest of this section

The four deep-dive chapters are written to be read in order but stand alone. Each follows the same arc: the field problem, the mental model, the message anatomy, the lifecycle (with diagrams), how machbus expresses it, and the failure modes that bite in practice.

• The networking foundation
• The Virtual Terminal
• The Task Controller and the data dictionary
• Implement control, the tractor ECU, and the rest
• Positioning: NMEA and GNSS

If you would rather learn by building, jump to The session facade and come back here when a frame surprises you.