The UVCS instrument maintains electronics subassemblies within the TSU and the REU as shown in Figure 16 
. The TSU-mounted
electronics include subassemblies for the differential impedance transducers (not shown),
Sun sensors, image processors, and the XDL detectors. The Sun sensor
electronics utilize difference and summing amplifiers to translate
relative photodiode signals into a fine-resolution solar-center
pointing sensor. The image processor
utilizes first-in-first-out buffers, detector column and row
mask RAMS and a one-microsecond cycle-time state machine for proper
data integration. The XDL detector subsystems include detector head
subassemblies with integral high-voltage power supplies and amplifier
modules. The amplifiers contain high speed and pulse amplitude amplifiers
for the delay line signals and the XDL time-to-digital converter electronics contain constant fraction discriminators, time-to-amplitude
converters and analog-to-digital converters to decode the detected
signals and define event addresses. The XDL monitor electronics provide
command and housekeeping activities to operate and monitor the delay line
detector.
The REU subassemblies including the central processing unit (CPU) electronics,
low-voltage power supplies (LVPS), power switching electronics, mechanism controller
circuitry, housekeeping, thermal control and interface electronics are shown in Figure 16 .
There are two physically-redundant CPU boards (card CPU A
and CPU B) within the REU, each containing an experiment controller
(EXCON) 80C86-type microprocessor with 5 MHz clock speed and
associated 4K by 16-bits of fusible-link read-only memory (PROM),
128K by 16-bits of electrically-erasable programmable memory (EEPROM)
and 32K by 16-bits of RAM. The PROM contains
``bootstrap'' code to allow instrument restart and initialization.
The EEPROM is used to upload and modify observing sequences and to modify
system configuration parameters during flight. The EEPROM also allows
revised flight software uploads. The RAM is primarily used for temporary
image storage.
The low-voltage power supplies provide regulated +5V, 
V and
V and switchable V,
physically-redundant and independently integrated on two REU printed circuit board (PCB) assemblies.
A power-switching pulse code modulation (PCM) assembly provides the necessary level of power
establishment and control. Four types (3 closed-loop and one open-loop
control) of mechanism controllers are integrated within three PCB
assemblies. Critical position controllers for the mirror, occulter and
grating mechanisms employ proportional-integral (PI) control architecture.
Housekeeping electronics are maintained on one PCB assembly and provide
multiplexed, differential-input position and monitor information through
a CS5016 analog-to-digital converter for transmission on the
133 bit-per-second SOHO telemetry system. The instrument grounding and
power distribution network utilizes a single-point grounding structure
with appropriate line isolation. Open-loop thermal control in the form
of pulse-width-modulated heater power control exists to maintain
appropriate instrument temperature distributions. The single-card
interface electronics provide the appropriate command and telemetry
decoding and timing electronics to adequately function via SOHO-initiated
commanding and telemetry.
The REU employs surface mount technology of passive components for PCB assembly economization.
