Printronix P4280, Maintenance Manual

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E–13 Principles of Operation
MDIC ASIC. The CCB writes a word to the 8032, which writes a word to the
MDIC. The MDIC generates a clock signal based on this word.
The clock signal from the MDIC is the reference input to a phase detector.
The other input to the phase detector is a logical combination of the pulses
from three Hall–effect sensors in the brushless three–phase dc shuttle motor.
The Hall–effect sensors are mounted to the motor stator 120 electrical
degrees phase–shifted from each other. The phase detector is both a phase
and frequency detector. It operates as a frequency detector when out of lock,
and as a phase detector when the loop is phase–locked. During shuttle start,
the frequency error from the phase frequency detector drives the servo
system towards phase lock. At approximately 85% of final speed, the servo
integrator resets to prevent overshoot. When the correct speed is reached, the
system is in phase lock with the MDIC clock signal.
The shuttle is protected from overspeed, over current, missing hall–effect
sensors, and out–of–sync hall–effect sensors.
The Paper Feed System
Dot row advance and slew tables are stored in the CCB. The paper feed drive
circuit takes commands directly from the CCB to control the two–phase dc
paper feed stepper motor. A CCB paper feed command is a digital word
containing a value proportional to the desired current level in the paper feed
motor, enabling the motor to be quarter stepped in P4280 mode. Two PWM
current mode amplifiers, protected against overloads and short circuits, drive
the paper feed motor. The paper feed motor is energized whenever printer
power is on, unless the platen is open, in order to maintain tension and
position of the paper. The paper feed motor is disabled in a platen open fault
condition.
The Reverse Paper Feed System
To reverse paper motion, the printer must open the platen, move the paper
backwards vertically, close the platen, and remove the slack in the paper. A
platen open or close command is generated on the CCB and communicated
to the RSP 8032 processor. The RSP generates control and step clock signals
for the platen driver circuitry. The platen driver circuitry is connected to a
stepper motor that drives the platen through a toothed belt. The platen motor
is only energized during the open and close cycles. The platen driver is
protected from over current.