How does CAN-bus work in a tractor?

From a tangle of wires to a single communication line

In the past, almost every function required its own wire. If a switch wanted to turn on a lamp, a wire literally ran from the switch to that lamp. As more electronic functions were added, the wiring harness grew into an increasingly larger tangle of wires.

CAN bus solves this problem by allowing all computers in the tractor to communicate with each other via the same data line. Instead of using a separate wire for each function, all systems share the same "telephone line".

A telephone line that everyone listens to

You can compare CAN bus to a telephone line to which all computers in the tractor are connected. Think, for example, of the engine computer, the transmission computer, the terminal, and the GPS receiver.

When a computer wants to say something, it sends a CAN message. Such a message starts with a kind of name tag, also called the message ID. This explains what the message is about.

For example, the engine computer might say:

"Message 101: the engine speed is 1,850 rpm."

All computers on the CAN bus receive this message. Most do nothing with it, but systems that need the engine speed, such as the transmission or the terminal, recognize the message ID and use the information.

As a result, the engine computer does not need to communicate separately with each system. A single message on the bus is sufficient.

What does a CAN message look like?

A CAN message consists of several parts, but for the user, three parts are the most important:

• An identification number (ID).

• The data itself (the data).

• A checksum to detect errors.

The identification number explains what the message is about and at the same time determines the priority on the CAN bus. 

The data contains the actual information, such as engine speed, driving speed, oil temperature, or the position of a valve.

At the end of the message, a checksum calculation is sent. Receivers use this to check whether the message was damaged along the way by an interference. If an error is detected, the message is sent again. This ensures remaining communication is reliable, even in an environment with high electrical interference from magnetic fields like in a tractor.

Why are there two wires?

A CAN bus consists of two data wires: CAN-High and CAN-Low.

This might seem redundant, but it is precisely what makes CAN bus so noise-immune. When a message is sent, the voltage on both wires changes at the same time, but in opposite directions. If CAN-High goes slightly up, CAN-Low goes slightly down.

The receiver does not look at the voltage of a single wire, but at the difference between the two wires.

If electrical interference occurs on the cable along the way, for example caused by an alternator, electric motor, or walkie-talkie, it usually affects both wires equally. Because the receiver only looks at the difference, the interference is largely filtered out.

Why are the wires twisted?

Anyone looking at a CAN cable will often see that the two wires are twisted around each other.

This is not done for strength, but to further reduce interference. Because the wires constantly change position relative to sources of interference, any eventual interference is distributed evenly over both wires. This allows the electronics to filter out this noise even better.

That is also the reason why damage or poor repairs to CAN cables can sometimes cause strange electronic malfunctions.

Why are there multiple CAN buses in a tractor today?

When CAN bus was developed, there were far fewer electronic systems than there are today. Modern tractors, however, exchange massive amounts of data.

Think of:

• GPS corrections.

• Engine data.

• Transmission information.

• Hydraulic settings.

• ISOBUS communication with implements.

• Camera feeds and terminals.

A CAN bus can only process a limited amount of messages per second. When too many systems communicate at the same time, the bus becomes full and messages have to wait.

That is why modern tractors often have multiple CAN networks. For example, there could be a separate CAN bus for the engine and transmission, a second one for cab functions, and a third for ISOBUS and implements.

This keeps communication fast and reliable, even when many systems are active simultaneously.

What does a driver notice of this?

In practice, you as a driver usually do not notice anything about the CAN bus. Yet this system ensures that almost all electronic functions in the tractor can work together.

When you enable cruise control, for example, multiple computers communicate with each other at the same time. The engine computer, transmission computer, and terminal continuously exchange data via the CAN bus without you noticing anything.

CAN bus thus essentially forms the nervous system of the modern tractor. Without this system, the huge amount of electronics and automation we are used to today would simply not be possible.

This explanation provides a simplified overview of how the CAN bus works. The technology behind CAN bus is actually much more extensive. This blog is intended as an introduction, so technical details and exceptions have been deliberately omitted to maintain readability.