Larry - GE MAC 5000 Service Manual

Larry

2-32
Equipment Overview: Theory of Operation
Unlike previous keyboard encoder designs, Shemp does not provide
dedicated scan hardware for the shift and / or option keys. These keys are
now located in the scan matrix. Careful placement of keys in the scan
matrix allows simultaneous depression of the shift, option and other keys
without interference.
Larry controls the paper drive motor and digitizes the analog inputs.
The motor control functions are virtually identical to those offered by
the 78310 processor in Max-1 architecture machines, with an expanded
speed control range (down to zero). Since Larry's code is not
field-alterable, every motor control parameter is alterable via BBus.
Hopefully this renders the code immune to minor changes in the
printer drive train.
Motor Speed Control
Larry controls the motor speed by delivering a DAC controlled drive
voltage to the motor windings. The 6-bit DAC is implemented using
discrete, binary-weighted resistors directly driven by Larry's port pins.
The DAC output voltage (approx. 300mV full scale) is compared to a
filtered fraction of the applied DC motor voltage by comparator U59.
If the motor feedback voltage is below the DAC voltage, the comparator
turns on the motor via an H-Bridge driver. One motor terminal (which
one is a function of motor direction) is always grounded. The other is
alternately driven to either 12V or ground. The duty cycle of the drive
signal determines the average applied voltage and therefore the average
motor speed. The feedback voltage signal is the average of both motor
terminals (R274 and R273 driving R309), with a 50:1 ratio,
15Vin = 300mV out, hence 15V full scale). Since one terminal is always
zero (grounded) and the other is driven with a variable duty cycle
between zero and 12V, the feedback signal is positive regardless of motor
direction. C260 filters the switching noise from feedback voltage.
NOTE
The frequency and duty cycle of the motor drive signal are random.
This serves to reduce EMI by spreading any emitted noise across a
wide frequency spectrum. An RC snubber (R272 and C259)
suppresses ringing on the motor lines.
Larry maintains precise motor speed control by comparing the frequency
of the tachometer pulse train emitted by the motor's integral encoder to
an internally generated reference frequency derived from Larry's
resonator. Larry processes motor position information on both edges of
both encoder signals for a total of 64 loop correction cycles per rotation of
the motor shaft. This high angular sampling rate allows Larry to achieve
accurate and smooth speed regulation down to zero speed.
Paper Jam / Pull Detection
Larry monitors the servo error variable to determine whether the servo
loop is closed. If the error variable saturates "on" for more than a
predetermined time it is assumed that the paper drive torque has
become excessive, or the motor has stalled. This condition is reported as
a Paper Jam Error.
MAC™ 5000 resting ECG analysis system
2024917-010
Revision B