There are two mass memory
units aboard the orbiter. Each is a coaxially mounted, reel-to-reel
digital magnetic tape storage device for GPC software and orbiter
systems data that can be written to or read from. The MMU tape
is 602 feet long and 0.5 of an inch wide and has nine tracks (eight
data tracks and one control track). These tracks are divided into
files and subfiles for finding particular locations.
Computing functions for all mission phases requires approximately
400,000 words of computer memory. The orbiter GPCs are loaded
with different memory groups from the MMUs containing the desired
program. In this way, software can be stored in MMUs and loaded
into the GPCs when actually needed.
To fit the required software into the available GPC memory space,
programs are subdivided into nine memory groups corre sponding
to functions executed during specific flight and checkout phases.
Thus, in addition to the central memory in the GPCs themselves,
34 million bytes of information can be stored in each of the two
mass memory units. Critical programs and data are loaded in both
MMUs and protected from erasure.
The principal function of the MMU, besides storing the basic
flight software, is to store background formats for certain CRT
displays and the checkpoints that are written periodically to
save system data in case the systems management GPC fails.
MMU operations are controlled by logic and the read and write
electronics that activate the proper tape heads (read or write/erase)
and validate the data.
Each MMU interfaces with its mass memory data bus through multiplexer
interface adapters, which function like the GPCs. Each mass memory
data bus is connected to all five computers; however, each MMU
is connected to only one mass memory data bus. All MMU operations
are on an on-demand basis only.
The mass memory units are an advanced form of data storage that
fills the gap between slow-access drives of high storage capacity
and discs or drums with fast access but relatively low storage
The power switches are located on panel O14 for MMU 1 and panel
O15 for MMU 2. The MMU 1 switch positioned to on allows control
bus power to activate an RPC, which allows MNA power to MMU 1.
The MMU 2 switch positioned to on operates in a similar manner
with MNB power. A mass memory unit uses 20 watts of power in standby
and 50 watts when the tape is moving.
MMU 1 is located in crew compartment middeck avionics bay 1,
and MMU 2 is in avionics bay 2. Each unit is cooled by water coolant
loop cold plates. Each MMU is 7.6 inches high, 11.6 inches wide
and 15 inches long and weighs 22 pounds.