down by resistors R501 and R502. The current drawn by this network
is returned to HV RETURN so it bypasses the current sensing
network. This requires that the voltage is sensed as the difference
between the HV and ILOW outputs. R505, C504, and diodes D507 -
508 protects the CPU board in case of power failure. AC capacitor
C504 reverses the polarity of the power supply without changing the
metering circuits.
R504 is the current shunt for IHIGH output, while R503 provides a
similar function for ILOW output. When high current is drawn, D501-
502 conducts, limiting heat dissipated in R503. C502-503, D503-506,
and R506-507 also provide protection to the CPU board components.
5.3
Low Voltage Power Supply (Schematic 625703)
The power supply board provides power to run the microprocessor
analog circuits and to control high voltage primary power.
Input line voltage is fused by F 401-402 to protect against power
failure. The fan is wired across one primary of T 401 to provide Vac
operation. For fire protection, the secondary of T 401 contains internal,
temperature sensitive fuses. The output of T401 is rectified, filtered,
and regulated to provide the +5V, ± 15V regulated, and + 12V
umegulated outputs.
A control signal from the CPU board turns Q402 on and off to enable
the primary of the high voltage transformer. U401-402 and Q401
control the HV enable lamp on the front panel.
5.4
CPU Board (Schematic 615741)
The CPU board consists of a digital section ( 100 series components)
and an analog section (300 series components). It is the main control
point for all activities in the DIGITEL.
The main CPU clock is controlled by a 2.4576 mHz crystal. This
signal is divided down to 614.4 kHz at Ul0l for the timing of the
digital logic. U 102 decodes the CPU address and control signals to
enable memory and I/O devices as required. Ul 03 contains the main
DIGITEL software EPROM. The 128-byte RAM (Ul04) increases the
CPU RAM to provide a total of256 bytes. U105 and U107 are
universal, parallel, I/O devices. U105 provides control of analog
conditioning circuits on pins 2 through 9. Pins 10 through 13 read rear
panel status switches. Pin 39 provides a watchdog output signal. If
5-2
Theory of Operation