STS-131: Payload arrives as engineers work PCMMU issue on Discovery

by Chris Gebhardt

As work continues at Launch Pad 39A in terms of STS-131 payload arrival and transfer to the Payload Changeout Room, engineers are continuing to work an issue with Discovery’s PCMMU (Pulse Code Modulator Master Unit), which first experienced a “momentary failure” on March 5, that kicked off a round of engineering evaluations and troubleshooting efforts.

STS-131: Processing Latest:

Engineers are continuing to process Discovery for an April 5 launch, pending what will be an Agency FRR (Flight Readiness Review) decision on Flight Rationale next week, relating to the leak on Discovery’s Right Reaction Control System (RRCS).

Tests on two regulators downstream of the helium isolation valve have shown positive results on their performance, although a weekend test will involve a full test of the entire system via test equipment that wasn’t available during the initial problem. This test will provide full data for the FRR to debate.

“IPR 33 Update: Over the weekend additional data will be gathered on the Right RCS fuel system helium pressure regulators using a GSE (Ground Support Equipment) test panel,” noted Friday processing from the NASA Test Director (NTD), on L2.

“The RSS was moved to the park position yesterday morning to support payload installation activities. The payload arrived at Pad A at 0251 EDT this morning. Installation of the payload into the PCR (Payload Changeout Room) is expected to start today.”

STS-131 Specific Articles:

Meanwhile, LOX and LH2 dew point and conditioning has been completed, EMU installation and continues today with functional checkout, and Orbiter aft compartment closeout for flight has already begun.

Pulse Code Modulation Master Unit Background:

The PCMMU – which operates through Telemetry Format Loads (TFLs) programming – is a devise that “routes the orbiter data to the OPS recorders and the communications systems for downlink to the MCC (Mission Control Center),” notes a Data Processing System (DPS) Familiarization Workbook (L2).

Overall, there are three types of telemetry data downlinked via the PCMMUs (of which there are two): GPC (General Purpose Computer) downlist, Orbiter systems data from the OI (Operational Increment) software/system, and “mission specific payload data.”

In fact, most post-insertion, on-orbit operations, and deorbit preparations are conducted by loading TFLs into the PCMMUs.

For GPC downlist, “data is provided by the operating GPCs and is stored in special memory areas in the PCMMU called ‘toggle buffers,'” notes the DPS overview.

In all, there are five “toggle buffers” with two, independent sections. This enables the PCMMU to read one side of the “buffer” while the other side continues to receive information from the GPCs.

When the PCMMU is done reading information from one side of the “toggle buffer,” the functions of the two sides are “swapped, or toggled (thus, the name).”

This process of constantly switching – or toggling – between the two sides “allows for a more rapid rate of downlist from the GPCs.”

However, the GPCs also have the ability to “poll” the PCMMUs for data that has been accumulated from the OI and payload systems. “This is the usual method for the SM GPC and the BFS GPC to obtain data to monitor the orbiter’s mechanical, electrical power, life support systems, and the payloads,” notes the DPS overview.

To obtain this data, the PCMMU cyclically polls the OI MDM (Operational Increment Multiplexer/Demultiplexer) and Payload Data Interleaver (PDI) for information (data) on the orbiter’s systems and payload, respectively. That information is then stored within the PCMMU’s memory.

PCMMU Failure and Forward Path:

According to a Space Shuttle Program (SSP) Flight Readiness Review (FRR) summary (available on L2), “On March 5th, the four GPCs running in the redundant set all logged an I/O (input/output) error. Reviewed the data and saw that the PCMMU and several OI MDMs had logged data validity errors.”

This issue was first mentioned via official NASA documentation on March 8, however, two days before the SSP FRR conveined.

According to a NASA Test Director (NTD) morning status update from March 8, “New IPR-131V0029 to ISL: PCMMU 1 BITE Bit 10 (input data valid) had a momentary failure. At this time the only planned troubleshooting will be to keep the PCMMU select switch in 1 and watch for the error to repeat.”

Following the March 5 failure, a second failure occurred the following day with a third failure occurring on March 9.

“The GPCs responded nominally so the focus is now on the instrumentation buses,” notes the SSP FRR summary.

During the first two failures, “no vehicle commanding” was being performed and all “affected LRUs (Line Replacement Units) have been associated with same data bus.”

Furthermore, there are no Flight Control (FC) MDMs on this data bus; therefore, no FC communication problems can occur from this particular failure.

The SPP FRR summary goes on to note that the problem appears to a “real issue on (the) data bus rather than a particular box having problems with BITE circuitry.”

“Possible causes include any of the 8 MIAs (Multiplex Interface Adapters) as source of noise causing communication errors (7 OI MDMs and the PCMMU), noise due to damaged wires/connectors, or PCMMU internal failure.”

The forward plan, as outlined by the SSP FRR, is to install breakout boxes on the OI data buses and “go back to PCMMU-1 to collect further data for troubleshooting.”

The week’s morning NTD reports indicate that work on this front began after Launch Pad 39A was reopened following the hazardous portions of the RRCS (Right Reaction Control System) leak troubleshooting operations.

“IPR 131-0029 Update: PCMMU #1 troubleshooting was performed yesterday with no definitive answers,” stated Monday’s NTD report.

“Engineering continues to monitor and analyze data to better understand the problem. Following troubleshooting for IPR-0033, crossover cables tying PCMMU 1 to OI Data Bus 2 and PCMMU 2 to OI Data Bus 1 will be installed for the next phase of troubleshooting.”

These crossover cables were installed on Wednesday and, according to Thursday’s report, “Monitoring will continue through Friday afternoon. At this time no bit errors have occurred.”

Should the R&R (Removal and Replacement) of a PCMMU or OI MDMs become necessary, spare parts are available for use.

“IPR 131-0029 Update: The switch to PCMMU 2 as active was completed last night. Monitoring indicated that no bit errors have occurred to date,” added Friday information. “Engineering will be evaluating additional troubleshooting options today.”

L2 members : Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4500 gbs in size

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