Incremental Encoder (Quadrature)

An incremental encoder generates a pulse each time the shaft rotates by a fixed angle. The number of pulses per revolution (PPR) — also called resolution or line count — determines how finely position can be measured. A 1000 PPR encoder generates 1000 complete A-B cycles per shaft revolution.

Quadrature encoders have two output channels, A and B, shifted 90 degrees apart in phase. By examining which channel leads the other, the PLC can determine direction of rotation: if A leads B, the shaft is rotating forward; if B leads A, it is rotating backward. With quadrature decoding (counting both edges of A and B), the effective resolution is multiplied by four.

The Z channel (index pulse) produces one pulse per revolution at a fixed reference position. This allows the controller to re-home the axis after power-up without returning to a physical limit switch.

PLC high-speed counter (HSC) modules process encoder signals directly in hardware, decoding the quadrature and accumulating the count at speeds up to several MHz — far faster than the PLC scan cycle could handle in software.

Common output types are HTL (push-pull, 10-30 V, suitable for long cable runs) and TTL (differential RS-422, fast, noise-immune). Always match the encoder output type to the HSC module's input specification.