Space Shuttle Data Processing System

Digital Processing System - General The Orbiter functions and operations are managed by a computerized data management and processing system called the Data Processing System (DPS) Primary data management is provided by five identical digital computers running in parallel for redundancy Primary data management is provided by five identical digital computers running in parallel for redundancy Secondary data management is furnished by a network of 24 computerized digital system management units called Multiplexers/Demultiplexers (MDMs) Secondary data management is furnished by a network of 24 computerized digital system management units called Multiplexers/Demultiplexers (MDMs) Two dedicated critical event control units supply signal and data management for launch, orbit, deorbit, and landing phases of the STS and Orbiter Two dedicated critical event control units supply signal and data management for launch, orbit, deorbit, and landing phases of the STS and Orbiter Two command and data storage tape units are also provided for multiple-redundant operational information Two command and data storage tape units are also provided for multiple-redundant operational information

DPS Key DPS equipment on the Orbiter 5 General Purpose Computers (GPCs) 24 (20 Orbiter, 4 SRB) Multiplexers/Demultiplexers (MDMs) 2 master events controllers 2 Mass Memory Units (MMUs) 24 Serial digital data buses 3 SSME interface units 4 multifunction CRT display systems (MCDS) 2 data bus isolation amplifiers 1 Master timing unit (MTU)

DPS functional block diagram

General Purpose Computers (GPCs)

DPS - General Purpose Computers IBM was awarded the development contract for the STS data processing and integration and for the KSC launch processing system, and for the JSC mission control center data processing IBM was chosen for its integrated software capability for data management on the STS Vehicle processing Prelaunch and launch Orbital flight Landing Software modifications and changes required for each mission were simplified by developing the digital data systems through the same primary contractor

DPS - General Purpose Computers Off-the-shelf components were emphasized for reliability and rapid integration into the STS An existing model of IBM's AP bit computer was adopted GPC processor was a modified IBM System/4 Pi Initially used as central processor in earlier B-52 digital systems and later in the B-1 bomber Included an assembly language-based software called High-order Assembly Language/Shuttle (HAL/S)

DPS - General Purpose Computers GPC is divided into two major components Central Processor Unit (CPU) CPU provides control logic and computational capabilities Input/Output Processor (IOP) IOP provides control and interface links between the CPU and the hardware that interfaces with the GPCs Both the CPU and the IOP contain software and memory for data storage

GPC Schematic

DPS - General Purpose Computers Basic functions CPU Instruction execution Data storage Data manipulation Input/Output Processor Formats and transmits commands to vehicle systems Receives and validates data from the vehicle systems Maintains interface status with the associated CPU and with the other GPCs Interfaces the DPS 24 data buses with its 24 processors

DPS - General Purpose Computers Weight 30.8 kg (68 lb) Power consumption ~ 560 W Power source Any Main Bus Any Essential Bus Software Four GPCs contain PASS software (GPC 1-4) Output polling used to determine if any of the GPCs or their computations are in error GPC #5 runs BFS software Does not participate in the polling/voting process Used for backup control of the vehicle if the PASS software fails or malfunctions

DPS - General Purpose Computers External mass memory is employed to overcome the inherently small main memory available to the CPU and IOP 128 k of main memory severely restricts the vehicle operations Requires a segmented approach to the GPC's control of the mission operations Mission sequences are loaded into the GPC's main memory from the redundant mass memory storage units This technique allows the entire mission execution but only in sequential bundles

DPS - General Purpose Computers GPC main memory Originally a ferrite core construction like Apollo guidance computers Small, individual wire-wound iron ring for each bit Total of 128 k capacity (half) words was later increased to 256 K by bit manipulation Original limitation was due to the memory's large mass, high power requirement, and GPC heat load

DPS - General Purpose Computers Timer/oscillator GPC's timer/oscillator is used to generate signal timing for pacing the data processing cycles Also used to determine absolute time reference Two time references are maintained for the GPC's operations. GMT - Greenwich Mean Time MET - Mission Elapsed Time A watchdog timer is also used in the GPCs to check on the timing register for the BFS software operations

DPS - GPC GPC panel switches Crew operation of the GPCs is controlled with panel switches on the GPC panel POWER (ON/OFF) OUTPUT (BACKUP / NORMAL / TERMINATE) INITIAL PROGRAM LOAD MODE (RUN / STBY / HALT)

Mass Memory Units

DPS – Mass Memory Units MMUs Two mass memory units (MMUs) are aboard each Orbiter Coaxially mounted, reel-to- reel read/write digital magnetic tape storage device for GPC software and Orbiter systems data Each of the tape systems has a capacity of 34 Mb and stores three copies of the flight software

DPS – Mass Memory Units The principal function of the MMU is to store the basic flight software Secondary function is to store background formats and code for certain CRT displays and the checkpoints that are written periodically to save selected data in case the systems management GPC fails Each mass memory data bus is connected to all five computers But each MMU is connected to only one mass memory data bus All MMU operations are on an on-demand basis only

Data Bus Network

DPS – Data Bus Network Data flow in the Orbiter’s DPS systems is supported 24 data buses categorized in 7 groups 1. Flight-critical (FC) data buses (8) 2. Payload data buses (2) 3. Launch data buses (2) 4. Mass memory data buses (2) 5. Display/keyboard data buses (4) 6. Instrumentation/Pulse Code Modulation Master Unit (PCMMU) data buses (5) 7. Intercomputer communication data buses (5)

DPS – Data Bus Network 1. Flight-critical (FC) data buses (8) FC buses tie the GPCs to: Flight-critical multiplexer/demultiplexers (MDMs) Display driver units Heads-up displays Engine interface units Master events controllers Eight flight critical data buses combined into groups of two (FC strings) for redundancy Each of the four FC strings can be commanded by a different GPC Group 1 FC1, FC2, FC3, FC4 Group 2 FC5, FC6, FC7, FC8

DPS – Data Bus Network 2. Payload data buses (2) These buses tie the GPCs to the payload MDMs and the payload data interleaver (PDI) In some configurations these are mission-dependent flex MDMs or sequence control assemblies Two buses interface 5 GPCs and 2 payload MDMs

DPS – Data Bus Network 3. Launch data buses (2) These buses tie the GPCs to ground support equipment, launch forward MDMs, launch aft MDMs, launch mid MDMs, SRB MDMs, and the manipulator controller interface unit (MCIU) used by the remote manipulator system Two launch data buses are used to interconnect the following: 5 GPCs Ground support equipment Launch processing system 3 Launch MDMs 2 (left & right) SRB MDMs

DPS – Data Bus Network 4. Mass memory data buses (2) The mass memory data buses are used for GPC - Mass Memory Unit transactions Both mass memory data buses are interconnected with each GPC

DPS – Data Bus Network 5. Display/keyboard data buses (4) Used for GPC/display electronics data transactions Each of the four display electronics units are interconnected to each of the GPCs through the Display/keyboard data buses

DPS – Data Bus Network 6. Instrumentation/Pulse Code Modulation Master Unit (PCMMU) data buses (5) Each of the GPCs is interconnected to two of the I/PCMMUs

DPS – Data Bus Network 7. Intercomputer communication data buses (5) Five intercomputer communication (IC) data buses exchange information that includes: 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 tables Operational sequences Master timing unit data Time Internal GPC time System-level display information Uplink data State vectors

DPS – Data Bus Network With the exception of the Instrumentation/PCMMU, each data bus is connected to each GPC, but only one GPC transmits over a bus at any one time

Multiplexers/Demultiplexers (MDMs)

DPS – MDMs MDMs act as system management units that convert and format serial digital GPC commands into separate and parallel (demultiplexed) digital, and analog commands for various vehicle hardware systems MDMs also convert and format the multiple digital, and analog data from vehicle systems into serial (multiplexed) digital data for transmission to the GPCs Each MDM has two redundant multiplexer interface adapters which are connected to a separate data bus on the GPC side MDM's interface with vehicle system hardware is by hard-wired lines

DPS – MDMs Each Orbiter carries 23 multiplexers/demultiplexers onboard Sixteen MDMs are an integral part of the DPS, connected directly to the GPCs and named according to their location in the vehicle and hardware interface Remaining seven MDMs are part of the vehicle instrumentation system Send vehicle instrumentation data to the pulse code modulation master unit

DPS – MDMs 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 SRB launch right MDMs 1 and 2 GSE/LPS launch forward 1 and launch aft 1

Software

DPS – Software Data Processing System software is divided into two functional groups 1. System software 2. Applications software Both are further subdivided into operations functions Primary Orbiter flight software Called the Primary Avionics Software System (PASS) software Used to control the vehicle, vehicle systems, and communications during its mission Second flight software program runs on a separate GPC as a backup for critical operations Called the Backup Flight System (BFS)

DPS – Software System software consists of three program sets 1. Flight Computer Operating System (FCOS) This is the executive software that controls the processors, monitors key system parameters, allocates computer resources, manages program interrupts, and updates computer memory 2. User interface programs These provide instructions for processing flight crew commands or requests 3. System control program These initialize each GPC and arrange for multi-GPC operation during flight-critical phases

DPS – Software Applications software performs the functions required to operate the vehicle and its systems To conserve main memory, the applications soft- ware is divided into three major functions 1. Guidance, navigation, and control (GNC) 2. Systems management (SM) 3. Payload (PL)

DPS – Software Applications software 1. Guidance, navigation, and control (GNC) Specific software required for launch, ascent to orbit, maneuvering in orbit, entry, and landing Only major function where redundant set synchronization occurs Polling circuitry used to evaluate GPC output/calculations in synchronization

DPS – Software Applications software 2. Systems management (SM) This software executes tasks that monitor various orbiter systems, such as life support, thermal control, communications, and payload operations SM is a simplex function The DPS simplex operations use a watchdog timer to evaluate self-consistent results

DPS – Software Applications software 3. Payload (PL) The payload major function runs mass memory utility software Because the PL major function is usually unsupported in flight, none of the GPCs are loaded with PL software Only used in vehicle preprocessing at KSC, and is a simplex major function

DPS – Software Applications software Applications software major functions are divided into mission segments called OPerational Sequences, or OPS Applications software major functions are divided into mission segments called OPerational Sequences, or OPS Each OPS Major Mode is associated with a specific memory configuration loaded into the GPCs from the MMUs Each OPS Major Mode is associated with a specific memory configuration loaded into the GPCs from the MMUs

DPS – Software Applications software All of the software residing in a GPC at any given time consists of system software and one OPS major mode software Except for OPS 1, each memory configuration contains a single OPS major mode Memory configuration OPS 1 which is loaded for countdown and launch contains both OPS 1 (ascent) and OPS 6 (RTLS) major modes Too little time to load in new software for a critical RTLS abort

DPS – Software

DPS – Software OPS-1 Major Modes

DPS – Software Backup Flight System (BFS) Backup Flight System software is an entirely separate DPS software package maintained on the Orbiter's fifth GPC Backup Flight System software is an entirely separate DPS software package maintained on the Orbiter's fifth GPC GPC selection for BFS operation can be changed manually with panel switches GPC selection for BFS operation can be changed manually with panel switches BFS flight software mimics the PASS software for the mission-critical operations (OPS 1, OPS 3 and OPS 6) BFS flight software mimics the PASS software for the mission-critical operations (OPS 1, OPS 3 and OPS 6) Contains notable differences Contains notable differences

DPS – Software Backup Flight System (BFS) BFS software is written by a different vendor Also in the HAL/S language BFS software is simpler than the PASS software since it is used for contingencies in the mission; the critical phases of flight Smaller size of the BFS software allows it to be loaded completely into one GPC for the entire mission and does not require sequential loading Copies of the BFS are contained on both of the MMUs

DPS – Software Backup Flight System (BFS) BFS software is programmed to monitor the state of the GPC and PASS operations so that it would be possible to take control of the vehicle if the PASS software to malfunction Like PASS, the BFS includes both applications software and system software Unlike PASS software, BFS is small enough to run both applications and systems application simultaneously

DPS – Software Backup Flight System (BFS) System functions on the BFS include Time-keeping PASS/BFS interface Multifunction CRT display system operations GPC I/O Communications uplink and downlink

DPS – Software Backup Flight System (BFS) BFS software can be selected with the engage and disengage control switch on the hand conroller BFS can take over control of the vehicle by pressing the engage button on the top of the flight crew's rotational hand controllers BFS also executes system management software (SM) in simplex during ascent and entry since the other four GPCs running PASS software are operating in GNC in redundant set synchronization BFS provides only limited support for on-orbit operations via MM 106 or MM 301

References Shuttle Crew Operations Manual - Digital Processing System (DPS), NASA National Space Transportation System Press Kit, NASA, 1998