Printronix L5000 Series Maintenance Manual page 205

Four hundred pulses are generated during one encoder revolution for phases A and B
(the interval between two neighboring broken lines shown above corresponds to one
pulse) or 1 for phase Z. One encoder revolution is equivalent to 1/2-inch paper
transport. That is, this encoder generates pulses at a frequency twice as high as that of
400-dpi raster scanning (horizontal scanning). Also note that a detent current is
provided to always stop the motor with sensing element Z placed in a slotted
position.
Since the tractor motor needs to be precision controlled for paper loading or parking
operations, all its pulses are fed back to the MPU software, as opposed to the drum
motor and main unit feed motor encoder pulses which are returned to the driver
circuitry.
MPU software
The MPU is interrupted at 281.25 µsec (1/2 of the 400-dpi raster cycle) intervals
under timer control, and control functions (including the transfer elevator motor ON
function) are executed according to the pulse data read upon encoder interruption.
That is, the tractor motor is controlled with double the 400-dpi accuracy.
Principles of Operation 5–45