Operator`s manual

The NMU relies on the inherent stepper motor operation for position control. It is assumed
that the motor faithfully steps once each time it is commanded to do so. This method of
control differs from most printers which employ closed loop servo motors and encoder
devices. The NMU keeps track of carriage position by counting the number and polarity of
steps given to the motor.
For reliable position control, it is necessary to ramp the stepper motor from one speed to
another. This ramping action is under MPU control, and is accomplished by frequency
modulating current excitation to the motor. With this technique, step commands are
applied to the motor which take full advantage of the available torque at every speed
required by the printer.
During the print cycle, dot formation is accomplished by synchronizing print-wire
excitation to the motor steps as the printhead moves along the print line.
The open-loop, encoderless design of the carriage system is not a problem during a normal
print cycle since the printhead is in motion, and sufficient torque is available to resist any
external disturbance. However, the system is vulnerable at rest due to the lack of feedback.
This could happen, for instance, if the operator manually moved the printhead during a
routine ribbon change. The MPU would not be aware of the incorrect carriage position, and
the machine would not operate properly when required to print the next line. This problem
is avoided by returning the carriage to the left margin when the machine is idle. Since the
photosensor can monitor this position, the MPU will detect any motion of the carriage
away from the sensor. A restore cycle will be automatically commanded if the carriage is
not positioned at the left margin when the next print line is initiated.
Since the carriage is returned to the physical left margin when the printer is idle, the logical
and physical position of the printhead may not correspond. The MPU maintains the logical
position at all times so that the next character will be printed in its proper physical location
on the paper.
1.12: UNIVERSAL POWER SUPPLY
The power supply consists of a 50/60 Hz transformer-rectifier- filter which produces a
nominal output voltage of +55 volts D.C. The +55 volt unregulated potential is used
directly by the paper drive motor, carriage motor and printhead power amplifiers. The +55
volt output is also used by a switching regulator to produce regulated voltages of +5, +12
and -12 volts. The regulated voltages are obtained from a high efficiency power converter
of the “fly back” variety. This type of regulator uses a single switch transistor and is able
to regulate multiple outputs with only one control loop. Benefits include cool, reliable
operation with a wide tolerance to line voltage fluctuations.
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