Encoder A B Z Phase Explained | Detect Forward & Reverse Rotation in PLC

An encoder is a sensor that converts the rotational motion of a shaft into electrical signals. These signals help a PLC or controller determine speed, direction, and position.

A, B, and Z Phases
Phase A

A square wave signal generated as the encoder shaft rotates.

Represents movement — the faster the shaft turns, the faster the pulses.

Phase B

Another square wave, shifted 90° out of phase with A.

This shift helps determine direction:

If A leads B → Forward rotation

If B leads A → Reverse rotation

🅉 Phase Z (Index Pulse)

Generates one pulse per revolution.

Used to detect the exact zero or reference position of the shaft.

Helpful for resetting counters or homing functions.

How Direction is Detected

When the encoder shaft rotates:

If A pulse rises before B pulse, the system knows the shaft is rotating forward.

If B pulse rises before A pulse, it means reverse rotation.
By continuously monitoring the phase difference, PLCs can track both direction and speed accurately.
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