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.