Printronix L5000 Series Maintenance Manual page 204

Tractor motor
Since this motor moves the fan-fold paper forward and backward to determine the
print position, it is necessary to control not only its rotating speed but also its rotation
direction and amount. Therefore, the encoder for this motor has three sensing
elements (consisting of a light emitter and receptor) as shown below. The pulse trains
(phases A and B) detected by sensing elements A and B determine the rotating speed
and direction. The pulse trains (phase Z) detected by sensing element Z determine the
rotation amount.
(within motor)
(Forward tractor rotation)
5–44
Principles of Operation