ISO 11783-9 — the tractor ECU

Part 9 defines the Tractor ECU (TECU) — the tractor’s representative on the bus — and, crucially, its classes. A tractor advertises which facilities it offers and at what class level, so an implement knows up front whether the tractor can do what it needs before it asks.

Why this exists

An implement that needs ground speed, rear-hitch control, and guidance readiness must not blindly send commands and hope. Part 9 makes the tractor publish a contract of capability: “here is what I provide, at this class.” The implement reads it and adapts — or warns the operator that this tractor cannot run this implement fully.

Facilities and classes

   TECU advertisement ──► "I provide:
                            • ground + wheel speed
                            • rear hitch control
                            • rear PTO
                            • guidance readiness"
                              ▲
   implement reads it, then only requests facilities the tractor actually offers

Higher tractor classes provide more facilities (more speed signals, more hitch/PTO control, guidance). machbus models the facility set, the class matrix, and the tractor() preset plus the Implement facilities broadcast that advertises them; maintain-power and guidance-readiness sit here too.

Steering, speed and motion are three separate classes

Base class 1/2/3 says nothing about whether a tractor can be driven over the bus. That is carried by letter addenda, and §4.4.2 gives them separately:

A class 2G tractor steers on command and need not accept a speed command at all. Even with P, §4.4.2.8 says outright that bringing the tractor to a stop (speed 0.0) is optional and “can be determined by an implement via the tractor facilities response message”.

The handshake is not optional for the implement either

Two PGNs, and the second one is the part that surprises people:

• 0xFE09 Tractor Facilities Response — what the TECU has installed.
• 0xFE0A Required Tractor Facilities — what the implement needs.

§4.4.2: “An implement CF can send the required tractor facilities message to the Tractor ECU to enable the transmission of the messages that provide the required facilities. A facility is not required if its corresponding bits are set to 0 in the implement CF required tractor facilities message. The Tractor ECU can then stop the transmission of this implement message to reduce bandwidth.”

So a node that never declares what it needs may simply never receive it. That is why AutoDrive broadcasts the request on a cycle rather than only listening.

The TECU’s other duties

Beyond the advertisement, a TECU is the source of the part-7 status broadcasts (speed, hitch, PTO) and the relay for some tractor-side services. In machbus a tractor node is typically the Implement plugin (for the status/command messages) plus the facilities advertisement, optionally MaintainPower and guidance.

From concept to code

See also

• ISO 11783-7 — implement messages — the signals the TECU produces.
• TIM (AEF) — how an implement borrows the tractor’s facilities under authority.