How can two pulse signals reveal which way a shaft is turning?

An incremental rotary encoder produces repeating electrical signals as a shaft turns. In a common design, two channels called A and B are displaced by a quarter of a repeating cycle. Their pulses therefore change at different points in the movement.

The sequence of combined states reveals direction. One selected direction can produce 00 → 10 → 11 → 01 → 00, while reversing motion produces the reverse order. A single repeating channel can count changes but cannot distinguish those two directions by that count alone.

A controller can increment or decrement a counter as valid transitions arrive. If all four edges are counted, one illustrated A–B cycle contributes four counts. The number of cycles around the shaft and the chosen decoding method determine how counts relate to angle.

This records movement relative to a starting reference. It does not by itself reveal an absolute angle after power is lost; a reference mark or a different encoder design can help establish that. Noise, missed edges and mechanical coupling also matter, so the counter is a measurement requiring interpretation rather than a perfect memory of motion.