A single-ended signal is measured against ground, so any noise on the wire becomes an error. CAN uses a twisted pair: in the recessive state both wires sit at about 2.5 V; in the dominant state CAN_H rises to about 3.5 V and CAN_L falls to about 1.5 V. The receiver only looks at the difference, CAN_H − CAN_L: over 0.9 V is dominant (0), under 0.5 V is recessive (1). Because dominant always overrides recessive, the bus behaves as a wired-AND, which the rest of CAN depends on.
Challenge: Keep the pair twisted and properly terminated, and find the noise level at which even the CAN receiver starts to make errors.
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Termination
Single-ended errors
0 / 20
CAN errors
0 / 20
Peak wire voltage
3.5 V
Resistance H to L (power off)
60 Ω
Termination: a 120 Ω resistor at each end stops edges reflecting back as ringing. On the bench, with the power off, you should measure 60 Ω between CAN_H and CAN_L. Every node taps the same two wires; only the two ends are terminated.
