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 updated.

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.

The PCMMUs 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.

The payload 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.