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Orbiter operational instrumentation is used to collect, route and process information from transducers and sensors throughout the orbiter and its payloads. This system also interfaces with the solid rocket boosters, external tank and ground support equipment. Over 2,000 data points are monitored, and the data are routed to OI MDMs. The instrumentation system consists of transducers, signal conditioners, two pulse code modulation master units, encoding equipment, two operational recorders, one payload recorder, master timing equipment and onboard checkout equipment.

The OI system senses, acquires, conditions, digitizes, formats and distributes data for display, telemetry, recording and checkout. It provides for PCM recording, voice recording and master timing for onboard systems.

Dedicated signal conditioners convert digital and analog data signals received from the various sensors to a usable form. Some raw sensor data may need to be conditioned for compatibility with a multiplexing system. Signal conditioning provides the multiplexer with compatible inputs. The DSCs provide input from transducer signals, such as frequency, voltage, current, pressure, temperature (variable resistance and thermocouple), displacement (potentiometer), 28- or 5-volt-dc discrete output signals, analog and digital level changes, polarity changes or an ac signal change to a dc signal. The DSCs send these converted signals to the appropriate MDMs and displays and to the C/W system.

MDMs can operate in two ways. As multiplexers, they take data from several sources, convert the data to serial digital signals (a digitized representation of the applied voltage) and interleave the data into a single data stream. As demultiplexers, the MDMs take interleaved serial digital information; separate and convert it to analog, discrete or serial digital; and send each separate signal to its appropriate destination. The payload MDMs generally act as demultiplexers. They take interleaved commands from the orbiter GPCs, separate them and send each command to its appropriate destination, such as payload ground command interface logic.

The OI MDMs generally act only as multiplexers. Upon request from the pulse code modulation master unit, the MDMs send these interleaved streams to the PCMMU through the OI data buses. When the MDM is addressed by the PCMMU, the MDM selects, digitizes and sends the requested data to the PCMMU in serial digital form. The PCMMU/OI MDM interface is based on demand and response: that is, the OI MDMs do not send data to the PCMMU until the PCMMU makes the request.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
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