Flight-critical data buses tie the GPCs to flight-critical MDMs,
display driver units, head-up displays, main engine interface
units and master events controllers. Intercomputer communication
data buses are for GPC-to-GPC transactions. Mass memory data buses
conduct GPC/mass memory unit transactions. Display keyboard data
buses are for GPC/display electronic unit transactions. Instrumentation/pulse
code modulation master unit data buses are for GPC/PCMMU transactions.
Launch/boost data buses tie the GPCs to ground support equipment,
launch forward and launch aft MDMs, solid rocket booster MDMs
and the remote manipulator system manipulator control interface
unit. Payload data buses tie the GPCs to payload MDMs and the
payload data interleaver.
Although all data buses except the instrumentation/PCMMU buses
are connected to all five GPCs, only one GPC at a time controls
(transmits commands over) each bus. However, several GPCs may
listen (receive data) from the same bus simultaneously. The flight
crew can select the GPC that controls a given bus.
Each data bus, with the exception of the intercomputer communication
data buses, is bidirectional; that is, traffic can flow in either
direction. The intercomputer communication data bus traffic flows
in only one direction.
There are five intercomputer communication data buses. The following
information is exchanged over the IC buses for proper data processing
system operation: input/output errors, fault messages, GPC status
matrix data, display electronics unit major function switch settings,
GPC/CRT keyboard entries, resident GPC memory configuration, memory
configuration table, operational sequences, master timing unit,
internal GPC time, system-level display information, uplink data
and state vector.
All GPCs processing primary avionics software exchange status
information over the IC data buses. During critical mission phases
(launch, ascent and entry), usually GPCs 1, 2, 3 and 4 are assigned
to perform GN&C; tasks, operating as a cooperative redundant set,
with GPC 5 as the backup flight system. One of the PASS GPCs acts
as a commander of a given data bus in the flight control scheme
and initiates all data bus transactions.
Cross-strapping the four intercomputer communication buses to
the four PASS GPCs allows each GPC access to the status of data
received or transmitted by the other GPCs so that identical results
among the four PASS GPCs can be verified. The four PASS GPCs are
loaded with the same software programs. Each IC bus is assigned
to one of the four PASS GPCs in the command mode, and the remaining
GPCs operate in the listening mode for the bus. Each GPC can receive
data with the other three GPCs, pass data to the others, request
data from the others and perform any other tasks required to operate
the redundant set. In addition, GPC 5 requires certain information
to perform its function as the backup flight system because it
must listen to the transactions on the IC data bus.
Flight-critical buses tie the GPCs to flight-critical MDMs, display
driver units, head-up displays, main engine interface units and
master events controllers. These buses are directed into groups
of four compatible with the grouping of four PASS GPCs. Four of
these buses-FC1, 2, 3 and 4-connect the GPCs with the four flight-critical
forward MDMs, the four aft flight-critical MDMs, the three DDUs
and the two HUDs. The other four flight-critical buses-FC5, 6,
7 and 8-connect the GPCs to four forward MDMs, the four aft MDMs,
the two mission events controllers and the three main engine interface
units. The specific manner in which these units interface is referred
to as a string. A string is composed of two flight-critical data
buses-one from the first group (FC1, 2, 3 or 4) and one from the
second group (FC5, 6, 7 or 8).
The GPC in the command mode issues data requests and commands
to the applicable vehicle systems over its assigned flight-critical
(dedicated) bus. The remaining three buses in each group are assigned
to the remaining GPCs in the listening mode. A GPC operating in
the listening mode can only receive data. Thus, if GPC 1 operates
in the command mode on FC1 and FC5, it listens on the three remaining
buses. For example, GPC 1 is assigned as the commander of string
1, which includes flight-critical data bus 1 and flight-critical
forward MDM 1. GPC 1's transmitter is enabled on FC1, and the
three remaining non-commander PASS GPCs need to receive the same
redundant information at the same time and verified as consistent
identical information; thus, their receivers must also be enabled
on FC1 to listen in on the data bus.
In this example, when all GPCs require a time update from the
master timing unit, GPC 1 is the only GPC that actually issues
the command to the MTU because it is in command of the intercomputer
communication data bus connected to the MDM that interfaces with
accumulator 1 of the MTU. All five GPCs receive this time update
because they all are listening to the response data transmitted
over their own dedicated IC bus.
Each flight-critical bus in a group of four is commanded by a
different GPC. Multiple units of each GN&C; hardware item are wired
to a different MDM and flight-critical bus.
In this example, string 1 consists of FC data buses 1 and 5;
MDMs flight forward 1 and flight aft 1 and their hard-wired hardware,
controls and displays; the three EIUs; the two MECs; the three
DDUs; HUD 1; and their associated displays. Thus, four strings
are defined in this manner.
During launch, ascent and entry, when there are four PASS GN&C;
GPCs, each of the four strings is assigned to a different GPC
to maximize redundancy. All flight-critical units are redundant,
and the redundant units are on different strings. The string concept
provides failure protection during dynamic phases by allowing
exclusive command of a specific group of vehicle hardware by one
GPC, which can be transferred to another GPC in case of failure.
Additional redundancy is provided because each FF and FA MDM is
connected to the GPCs by two flight-critical data buses; thus,
all or part of one string can be lost and all functions will still
be retained through the other string.
The four display electronics unit keyboard data buses, one for
each DEU, are connected to each of the five GPCs. The computer
in command of a particular keyboard data bus is a function of
the current major func switch setting of the associated CRT, current
memory configuration, GPC/CRT keyboard entries and the position
of the backup flight control CRT switches.
Two payload data buses interface the five GPCs with the two payload
MDMs (also called payload forward MDMs), which interface with
orbiter systems and payloads. A payload data interleaver is connected
to payload data bus 1. Each payload MDM is connected to two payload
data buses. Up to five safety-critical payload status parameters
may be hard-wired; then these parameters and others can be recorded
as part of the vehicle's systems management, which is transmitted
and received over two payload buses. To accommodate the various
forms of payload data, the payload data interleaver integrates
payload data for transmission to ground telemetry.
The five instrumentation/pulse code modulation master unit data
buses are unique in that each GPC commands its own individual
data bus to two PCMMUs. All the other data buses go to every GPC.
Flight controllers in the Mission Control Center monitor the
status of the vehicle's onboard systems through data transmissions
from the vehicle to the ground. These transmissions, called downlink,
include GPC-collected data, payload data, instrumentation data
and onboard voice. The GPC-collected data, called downlist, includes
a set of parameters chosen before flight for each mission phase.
The system software in each GPC assimilates the specified GN&C;,
systems management, payload or DPS data according to the premission-defined
format for inclusion in the downlist. Each GPC is physically capable
of transmitting its downlist to the current active PCMMU over
its dedicated instrumentation/PCMMU data bus. Only one PCMMU is
powered at a time. It interleaves the downlist data from the different
GPCs with the instrumentation and payload data according to the
telemetry format load programmed in the PCMMU. The resulting composite
data set, called the operational downlink, is transmitted to the
network signal processor. Only one NSP is powered at a time. In
the NSP, the operational downlink is combined with onboard recorded
voice for transmission to the ground. The S-band system transmits
the data to the space flight tracking and data network remote
site ground stations, which send it to the MCC. Or the downlink
is routed through the orbiter's Ku-band system to the Tracking
and Data Relay Satellite system.
Uplink is the method by which ground commands originating in
the MCC are formatted, generated and transmitted to the orbiter
for validation, processing and eventual execution by onboard software.
This capability allows the ground to control software processing,
change modes in orbiter hardware and store or change software
in GPC memory and mass memory.
From MCC consoles, operators issue commands and request uplink.
The command requests are formatted into a command load for transmission
to the orbiter either by the STDN sites and S-band or by the Ku-band
system. The S-band or Ku-band transponder receivers aboard the
orbiter send the commands to the active network signal processor.
The NSP validates the commands until they are requested by the
GPCs through an FF MDM. The GPCs also validate the commands before
executing them. Those GPCs not listening directly to the flight-critical
data buses receive uplink commands over the intercomputer communication
The PCMMU also contains a programmable read-only memory for accessing
subsystem data, a random-access memory in which to store data
and a memory in which GPC data is stored for incorporation into
To prevent the uplink of spurious commands from somewhere other
than the MCC, the flight crew can control when the GPCs accept
uplink commands and when uplink is blocked. The GPC block position
of the uplink NSP switch on panel C3 inhibits uplink commands
during ascent and entry when the orbiter is not over a ground
station or in TDRS coverage. The flight crew selects this switch
position when the capsule communicator at the MCC requests loss-of-signal
configuration. The flight crew selects the enable position of
the switch during ascent or entry when the capsule communicator
requests acquisition-of-signal configuration.
Two launch data buses, also referred to as launch/boost data
buses, are used primarily for ground checkout and launch phase
activities. They connect the five GPCs with ground support equipment/launch
processing system, the launch forward (LF1) and launch aft (LA1)
MDMs aboard the orbiter, and the two left and right SRB MDMs (LL1,
LL2, LR1 and LR2). The GSE/LPS interface is disconnected at lift-off
by the T-0 umbilical. The solid rocket booster interfaces are
disconnected at SRB separation. Launch data bus 1 is used on orbit
for interface with the remote manipulator controller by the systems