Code Modulation Master Unit
The PCMMU receives
the data requested from the OI MDMs, downlists data from the GPCs
under control of flight software and payload telemetry from the
payload data interleaver and Spacelab computers, interleaves the
data, formats data according to programmed instructions stored within
the PCMMU, and sends the interleaved data to the network signal
processor to be mixed with the analog air-to-ground voice data from
the audio central control unit for transmission through the S-band
PM downlink or Ku-band system return links. Telemetry from the PCMMU
also is sent through the NSP to the operational recorders for storage
and future payloads to be downlinked on the S-band FM or Ku-band
system. OI and payload data collected by the PCMMU are sent to the
onboard GPCs for display and monitoring purposes upon request. All
data received by the PCMMU is stored in memory and periodically
The PCMMU has
two formatter memories: programmable read only and random access.
The read-only memory is programmed only before launch; the random-access
memory is reprogrammed several times during flight. The PCMMU uses
the format memories to downlink data from the computers and OI MDMs
into PCM telemetry data streams.
Only one of
the redundant PCMMUs and NSPs operates at a time. The one used is
controlled by the crew through the flight deck display and control
panel. The primary port of an MDM operates with PCMMU 1 and the
secondary port operates with PCMMU 2.
receive a synchronization clock signal from the master timing unit.
If this signal is not present, the PCMMU provides its own timing
and continues to send synchronization signals to the payload data
interleaver and network signal processor.
data interleaver is programmed onboard from mass memory through
the GPCs to select specific data from each payload PCM signal and
to store the data within its buffer memory locations.