Printronix p300 Maintenance Manual page 75

2–6. Paper Feed State
–continued
m. Paper Out Switch Logic
When the paper out switch is closed, NFLT goes low and the troubleshooting "paper out" LED on the
power supply PCBA lights. NFLT begins to count TFLC pulses, each representing twelve dot rows.
As the paper out switch is mounted approximately two inches (12 line spaces at six lines per inch) below
the print station, when the counter has counted 12 TFLC pulses, RDY goes low, inhibiting further input to
the printer, and the CHECK indicator lights.
Closing the platen or loading paper in the printer should clear the paper out fault. Note that this fault, as
well as a paper motion fault, is recognized only when the printer enters the buffer load state.
n. Paper Motion Detection Logic (Figure 2–35)
This logic detects the condition wherein the paper tractors are driven but there is no paper motion. Paper
movement is detected using an LED emitter/phototransistor sensor which illuminates the paper in the line
of sprocket holes. When paper is in front of the sensor, reflected light causes the sensor output to follow the
LED emitter. When the sensor sees a sprocket hole, the sensor output is flat.
The sensor is connected to a tone encoder circuit on the power supply PCBA. This circuit uses a phase–
locked loop generating a frequency which lights the LED in the sensor. When paper reflects the LED emit-
ted light to the sensor, its output is a signal at the oscillator frequency causing the output of the quadrature
detector to be true.
If, however, there is a sprocket hole in front of the sensor, and emitted light is therefore not reflected to the
phototransistor, the sensor output is low and the loss of phase lock causes the signal to become false. These
outputs are directed to the B logic as Paper Motion Detect (PMD) level.
Paper motion error detector circuits check the sensor output every eight character lines in six lines per inch
operation. If the sensor output has not changed from high to low during this interval, the paper motion
flip–flop is reset, READY is negated, inhibiting further data transfer to the printer, and an error message
appears on the display. Because there are at least two sprocket holes in every eight–line interval on the
paper, the logic has two chances to detect a fault during that interval.
Circuits on the logic B PCBA check the state of the PMD signal by comparing with signal TC3 which is
stage three in the top of form counter. If the state of PMD has not changed within the eight–line interval,
that is, no sprocket hole has passed the sensor, RDY is negated and the CHECK indicator lights. The fault
is cleared by correcting the paper jam and then pressing the CHECK switch on the console. When certain
colored forms are used, the lower reflectivity of the paper surface may cause an occasional false paper
motion fault. This paper motion fault may be defeated by removing jumper W6 on the logic B PCBA.
P300/P600 Principles of Operation 2–53