ENCAD CADJET - Quadrature Signal Generation

2-12 Theory of Operation
CadJet 3D Service Manual
Figure 2-8. Quadrature Signal Generation.
The direction that the Carriage Assembly is moving is known based
upon the state of one detector’s output and the direction of the transition
of the other detector’s output.
A hardware counter in the gate array increments as the Carriage
Assembly moves left and decrements as the Carriage Assembly moves
right. The hardware counter is only eight bits wide, so it cannot store a
value large enough to represent an absolute Carriage Assembly position.
Instead, it is read during the servo interrupt and its value compared
with that from the previous interrupt. This difference is used to update
the absolute position value in the software.