Incremental Encoders output a pulse string according to the rotational displacement of an axis. The number of rotations can be detected by counting the number of pulses.
A Direction Discrimination Unit accepts phase difference signal from the Encoder to detect the direction of rotation. Either voltage or open-collector outputs can be connected.
Absolute Encoders output the rotational angle using an absolute code. The rotational position can be detected by reading the code. This eliminates the need to return to the origin at startup.