Control System Hardware
of GN&C; hardware is hard-wired to one of eight flight-critical multiplexers/demultiplexers,
which are connected to each of the five GPCs by data buses. Each
GPC is assigned to command on one or more data buses; assignments
can be changed.
A sensor, controller
or flight control effector cannot be assigned or rerouted to another
MDM during flight, but the MDM it is wired to can be assigned to
a different GPC. Each multiple unit of each type of GN&C; hardware
is hard-wired to a different MDM. For example, there are four accelerometer
assemblies on board the orbiter. AA 1 is wired to forward flight
MDM 1 and is part of string 1, AA 2 is wired to FF MDM 2 on string
2, AA 3 is wired to FF MDM 3 on string 3, and AA 4 is wired to FF
MDM 4 on string 4.