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
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
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.