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
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,