There are 23 multiplexers/demultiplexers aboard the orbiter; 16 are part of the DPS, connected directly to the GPCs and named according to their location in the vehicle and hardware interface. The remaining seven MDMs are part of the vehicle instrumentation system and send vehicle instrumentation data to the pulse code modulation master unit.

The data processing system MDMs consist of flight-critical forward MDMs 1 through 4, flight-critical aft MDMs 1 through 4, payload MDMs 1 and 2, SRB launch left MDMs 1 and 2 and launch right MDMs 1 and 2, and GSE/LPS launch forward 1 and launch aft 1.

Of the seven operational instrumentation MDMs, four are located forward (OF1, OF2, OF3 and OF4) and three are located aft (OA1, OA2 and OA3).

The system software in each redundant set of GPCs activates a GN&C; executive program and issues commands to the bus and MDM to provide a set of input data. Each MDM receives the command from the GPC assigned to it, acquires the requested data from the GN&C; hardware wired to it and sends the data to the GPCs.

The DPS MDMs convert and format serial digital GPC commands into separate parallel discrete, digital and analog commands for various vehicle system hardware. This operation is called demultiplexing. The MDMs also multiplex, or convert, and format the discrete, digital and analog data from vehicle systems into serial digital data for transmission to the GPCs. Each MDM has two redundant multiplexer interface adapters that function the same as the GPC MIAs and are connected to two data buses. The MDM's other functional interface is its connection to the appropriate vehicle system hardware by hard-wired lines.

When the sets of GN&C; hardware data arrive at the GPCs through the MDMs and data buses, the information is generally not in the proper format, units or form for use by flight control, guidance or navigation. A subsystem operating program for each type of hardware processes the data to make it usable by GN&C; software. These programs contain the software necessary for hardware operation, activation, self-testing and moding. The level of redundancy varies from two to four, depending on the particular unit. The software that processes data from redundant GN&C; hardware is called redundancy management. It performs two functions: (1) selecting, from redundant sets of hardware data, one set of data for use by flight control, guidance and navigation and (2) detecting out-of-tolerance data, identifying the faulty unit and announcing the failure to the flight crew and to the data collection software.

In the case of four redundant hardware units, the redundancy management software uses three and holds the fourth in reserve. It utilizes a middle value select until one of the three is bad and then uses the fourth. If one of the remaining three is lost, the software downmodes to two and uses the average of two. If one of the remaining two is lost, the software downmodes to one and passes only the data it receives.

The three main engine interface units between the GPCs and the three main engine controllers accept GPC main engine commands, reformat them and transfer them to each main engine controller. In return, the EIUs accept data from the main engine controller, reformat it and transfer it to GPCs and operational instrumentation. Main engine functions, such as ignition, gimbaling, throttling and shutdown, are controlled by each main engine controller internally through inputs from the guidance equations that are computed in the orbiter GPCs.

Each flight-critical data bus is connected to a flight forward and flight aft MDM. Each MDM has two MIAs, or ports, and each port has a channel through which the GPCs can communicate with an MDM; however, the FC data buses can interface with only one MIA port at a time. Port moding is the software method used to control the MIA port that is used in an MDM. Initially, these MDMs operate with MIA port 1; if a failure occurs in MIA port 1, the flight crew can select MIA port 2. Since port moding involves a pair of buses, both MDMs must be ported at the same time. The control of all other units connected to the affected data buses is unaffected by port moding.

Payload data bus 1 is normally connected to the primary MIA port of payload MDM 1 and payload data bus 2 is connected to the primary MIA port of payload MDM 2. Payload data bus 1 can be connected to the secondary MIA port of payload MDM 2 and payload data bus 2 can be connected to the secondary MIA port of payload MDM 1 by flight crew selection.

The two launch data buses are also connected to dual MDM MIA ports. The flight crew cannot switch these ports; however, if an input/output error is detected on LF1 or LA1 during ascent, an automatic switchover occurs.

The only hardware controls for the MDMs are the MDM FC and MDM PL power switches on panel O6. These are on/off switches that provide or remove power for the four aft and four forward flight-critical MDMs and PL1, PL2 and PL3 MDMs. The PL3 MDM switch is unwired and is not used. There are no flight crew controls for the SRB MDMs.

Each MDM is redundantly powered by two main buses. The power switches control bus power for activation of a remote power controller for main power bus to an MDM. The main buses power separate power supplies in the MDM. Loss of either the main bus or MDM power supply does not cause a loss of function because each power supply powers both channels in the MDM. Turning power off to an MDM resets all the commands to subsystems.

The SRB MDMs receive power through SRB buses A and B; they are tied to the orbiter main buses and controlled by the master events controller circuitry. The launch forward and aft MDMs receive their power through the preflight test buses.

The FF1, PL1 and LF1 MDMs are located in the forward avionics bays and are cooled by water coolant loop cold plates. LA1 and the FA MDMs are in the aft avionics bays and are cooled by Freon coolant loop cold plates. MDMs LL1, LL2, LR1 and LR2 located in the SRBs are cooled by passive cold plates.

Modules and cards in an MDM depend on the hardware components accessed by that type of MDM. An FF MDM and an FA MDM are not interchangeable. However, one FF MDM may be interchanged with another or one payload MDM with another.

Each MDM is 13 by 10 by 7 inches and weighs 36.7 pounds. MDMs use less than 80 watts of power.

The MDM contractor is Honeywell Inc., Sperry Space Systems Division, Phoenix, Ariz.