Ground and a twisted pair

Why ground, on a differential bus

The receiver decides each bit from CANH − CANL, and a ground difference between two boards does not change that. But it moves both wires together, measured from the receiving board's ground. The SN65HVD230 works with its bus pins anywhere from −2 V to +7 V of its own GND. Past that it no longer promises to read them, and past −4 V or 16 V it can be damaged.

Two boards on separate supplies have no common ground unless a wire gives them one, and their grounds can sit volts apart. A long cable, a motor on one supply or a different mains socket is enough. The middle screw of the terminal, and the middle of the three pins, are for that wire: GND to GND, alongside CANH and CANL.

On a desk this is easy to miss. Two ESP32 boards on USB cables from the same computer already share its ground, so the bus works without the wire. Then one board moves to a battery and it stops.

Why a twisted pair

The difference cancels noise only if both wires pick up the same amount of it, and they do only if they stay the same distance from every source of it. Twisting them together does that. Two loose wires a few centimetres apart pick up different amounts, and part of the noise survives the subtraction.

So: CANH and CANL on one twisted pair, GND on a third wire of the same cable. Network cable works well, with CANH and CANL on the two wires of one pair.

The shape of the bus

Run one cable past every board, end to end, and keep the short branch from the cable to each board's terminal as short as you can: ISO 11898 allows 0.3 m. That gives the bus exactly two ends, which is where the two 120 Ω resistors go.