Why one pair, and what the hall gets wrong

The cords add up

Four devices that must all talk to each other need six cords, one for every pair. Ten need forty-five. Each device added needs a cord to every device already there, and a car with dozens of controllers would be mostly cable.

CAN avoids this by giving every device a tap on one shared pair of wires. Whatever one device puts on the pair, every other device sees. That is the whole idea, and the rest of the course is what it takes to make it work.

No boss, no addresses

Nothing on the pair is in charge. When the pair is free, any device may start a message, and there is no address on the message to say who should read it. Later lessons show how two devices that start together are sorted out, and how each device decides which messages to use.

The hall

In this course the devices are desks in a hall around a rail, and each desk has a Shout key. A shout is the dominant state, which is zero. Quiet is the recessive state, which is one. If any desk shouts, the hall is loud. Nobody can make the hall quieter than somebody else's shout, so the rail is loud whenever anyone wants it to be, and quiet only when everyone is.

What the hall gets wrong

Shout and quiet are only a picture of two logic states. Real CAN is not sound. It is a voltage difference between two wires, and the pair is twisted. A real cable also has delay and reflections, which a room cannot show. The rail down the middle of the bench is real voltage, and later lessons use it for exactly the things the hall cannot do.

A car is where most people meet CAN, but a car has several separate networks joined by gateways, and CAN is used well beyond cars. It is an example here, not the definition.

Common mistakes

• Thinking the hall is how CAN works inside the wire. It is a picture of access and of two states. The wire carries a voltage difference on a twisted pair.
• Looking for the boss. There is none. Any device may start when the pair is free.
• Expecting one network per car. A vehicle usually has several networks joined by gateways, so a wire in one place is not the same bus as a wire in another.