Ribbon Transport; Ribbon Drive Block Diagram - Printronix p300 Maintenance Manual

2–7. Ribbon Transport (Figure 2–37 and Figure 2–38)
The ribbon drive assembly contains two DC servomotors that move the ribbon continuously while there is
shuttle action. One motor acts as the driver, drawing the ribbon against opposing torque exerted by the
other motor. When a length of metal tape at either ribbon end is detected, the two motors exchange roles
and ribbon travel is reversed. This system maintains a relatively constant ribbon tension.
The ribbon moves only while the shuttle is in motion. The line side of the AC shuttle drive circuit, which
also powers the hammer bank blower, is switched by a triac switching circuit controlled by the signal NSD.
The switching circuit consists of an opto–isolator device and a zero–crossing detector circuit. The optical
device isolates the control logic from the AC power circuit. The zero–crossing detector prevents an exces-
sive transient by switching the triac only when the AC voltage passes through zero Volts.
The NSD signal enables gates which control switching to the ribbon drive servo.
When an end of ribbon sensor is activated, the corresponding latch turns on the related FET. If, for exam-
ple, the left sensor resets the latch FET, Q1 acts as an open circuit and Q2 conducts, connecting the divider
of amp K2 to the upper end of the resistor Rc. This connects motor M1 as the driving motor, with drive
current flowing through constant voltage amplifier K1, motor M1, and resistor Rc.
When the right end of the ribbon is detected, the latch is set to the opposite state and the motors again re-
verse roles.
When the shuttle motor is not running, that is, NSD is high, both FETs are held off and the motors act in
opposition, as the voltage across Rc provides feedback to resist ribbon movement.
AC LINE
SWITCHING
CIRCUITS
NSD
RIGHT
DETECTOR
DIRECTION
LEFT
CONTROL
DETECTOR
2–56
TRIAC
+28V (P300)
+36V (P600)
RIBBON
Figure 2–37. Ribbon Drive Block Diagram
RIBBON
+5V MOTOR
SERVO
P300/P600 Principles of Operation
TO SHUTTLE MOTOR
0
TO BLOWER
0
M
M