The two ends

An edge that has nowhere to go

A change of voltage travels along the pair as a wave. At the end of a wire with nothing connected, the wave has nowhere to go and part of it is reflected back, so edges bounce up and down the rail and arrive again late. At low speeds the bounces die away before the receiver looks at the bit; as bits get shorter they can land on it, and a bus without ends can fail, sooner as the speed goes up.

A block at each end

The twisted pair has a characteristic impedance of about 120 ohm. A 120 ohm resistor across the two wires at each end matches it, so the edge arriving at the end is absorbed instead of reflected. The blocks may sit on the cable or inside the two end devices. CiA 102 suggests 124 ohm at both ends for rails shorter than about 40 metres.

Measuring it with the power off

Two resistors of 120 ohm across the same pair are in parallel, so the meter between CAN high and CAN low reads about 60 ohm. With one block it reads about 120 ohm, and with none it reads very high, because only the receivers' inputs are left. That is the quickest check of a rail, and it tells you how many ends are fitted. The driver is specified for a 60 ohm load.

Not in the middle

A third block in the middle gives 40 ohm. Some boards ship with a block of their own already fitted. Six of those on one rail make 20 ohm, which is outside what the driver is designed for. The datasheet gives no figures there, so leave two.

A line, not a star

Branches off the rail have no block at their tips, so they reflect. ISO 11898 recommends no more than about 0.3 metres at 1 Mbit/s, and slower rates allow longer ones. Wire the rail as one line with a device tapped on at each point, not as a star with all the cables meeting in one place.

Common mistakes

• Fitting a block on every board. Blocks belong at the two ends of the line. Several fitted blocks in parallel pull the total well below 60 ohm.
• Measuring with the power on. The reading is only meaningful with the supply off, otherwise the drivers and receivers are part of the circuit.
• Wiring a star. Long branches reflect, and the bit rate you can run falls with them. Keep every branch short or make the rail pass each device in turn.