CAN is a differential transmission line, and like any high-speed transmission line it must be terminated at its characteristic impedance to absorb signal energy at the ends; otherwise the signal reflects back along the bus. The convention is a 120 Ω resistor across CAN_H/CAN_L at each physical end of the bus (two total), which matches the ~120 Ω characteristic impedance of typical twisted-pair CAN cable, and the two in parallel measure ~60 Ω across the bus with power off, a quick way to check termination. Without proper termination, none, only one, or a wrong value, reflections superimpose on the signal and distort the bit edges, causing bit errors, failed arbitration, and ACK errors that escalate into error frames and retransmissions; the bus may work at low speed or short length and then fail mysteriously as you go faster or longer. Putting termination in the middle of the bus, or scattering many resistors, is also wrong, it must be at the two ends. Termination problems are one of the most common CAN bring-up failures, which is why measuring ~60 Ω across an unpowered bus is a standard first check.
Communication Protocols · Interview question
Why does CAN need exactly two 120 Ω termination resistors, and what happens without them?
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From the lesson
CAN Bus
The robust multi-master bus behind vehicles: differential dominant/recessive bits, non-destructive bitwise arbitration by message ID, and built-in error detection with fault confinement.