A schematic of the XMM-OM electronics system design is given in
Figure 3.
The overall instrument function is controlled by the Instrument Control Unit (ICU). This and the Digital Processing Unit (DPU) are located together in a separate box called the Digital Electronics Module. The ICU will: monitor and make the instrument safe in breakdown/failure conditions, communicate and receive commands from the spacecraft, and prepare formatted science data from the DPU for telemetry.
The detector electronics and power supply are housed in the rear of the main telescope tube. The mechanisms, heaters, high voltage and detector lookup tables are commanded and read by the ICU via a serial Instrument Control Bus (ICB). In parallel high speed detector data is transmitted to the DPU via a unidirectional data-link (Data Capture).
The system is fully redundant having a duplicate copy of each sub-system independently connected.
The science data is received as a CCD image at the camera head
inside the telescope. Digital video is transmitted to some local
processing electronics where the photon centroiding is performed.
The DPU is responsible for accepting the centroided data from
the detector and removing the effects of spacecraft drift before
accumulating a high resolution windowed image.
ICU
The ICU is a flight computer design with common core functionality for general purpose instrument control and includes the ESA standard spacecraft interface. The design is rad-hard allowing it to be used in almost any space-flight configuration, and is based on the GEC Plessey MIL-STD-1750A processor (MA31750). It incorporates fault tolerance by using an internal watchdog and by being practically immune to radiation induced upsets. Code is initially boot-strapped from ROM into RAM after which the ROM is switched off. Further code is maintained in RAM permanently by the use of 'Keep Alive' power and can be modified or reloaded from the ground. Additional interfaces to the core functionality (for communication with the ICB and DPU) are included to communicate with OM subsystems. The core design is similarly employed by the RGS instrument on-board the XMM spacecraft.