Printronix 108525-001, Maintenance Manual

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2-58 P6040/P6080 Principles of Operation
2-10. Ribbon Drive (Figures 2-27 through 2-29)
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 speed and tension.
The ribbon moves only while the shuttle is in motion. The high 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 deĆ
tector circuit. The optical device isolates the control logic from the ac power circuit. The zero-
crossing detector prevents an excessive 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
example, the left sensor resets the latch FET, Q1 acts as an open circuit and Q2 conducts, connectĆ
ing the divider of amp K2 to the upper end of the resistor Rc. This connects motor M1 as the drivĆ
ing motor, with drive current flowing through constant voltage amplifier K1, motor M1, and resisĆ
tor Rc.
When the right end of the ribbon is detected, the latch is set to the opposite state and the motors
again reverse 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.
Figure 2-27. Ribbon Drive Block Diagram
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