Read the differential pair

RS-422 and RS-485 send a bit as the difference between A and B, so noise that hits both wires cancels. A receiver decides the bit from that difference, and a difference inside about ±200 mV is too small to call. Idle, a mark, drives A high and B low.

The three traces are A, B, and A minus B. Decide whether this pair is healthy, swapped, missing its termination, or open. New capture moves the probe to another pair.