OVERVIEW OF THE SPACE SHUTTLE'S COMPUTER/DATA PROCESSING SYSTEM =============================================================== Computer/Data Processing The Orbiter data processing subsystem handles data processing,data transfer,data entry,and data display in conjunction with the operations of the Orbiter avionics systems.The data processing system consists of the following. 1. Five general purpose computers for computation and control 2. Two magnetic-tape mass memories for large- volume bulk storage 3. Time-shared serial-digital data buses (essentially party lines)to accomodate the data traffic between the computers and other Orbiter subsystems 4. Nineteen multiplexer-demultiplexer units to convert and format data at various subsystems 5. Three remote engine interface units to command the Orbiter main rocket engines 6. Four multifunction television display systems for the crew to monitor and control the Orbiter and payload systems General-purpose computer.-Each general- purpose computer is a modified IBM AP-101 microprogram-controlled computer.The computer has a 106,496-word(36 bits to the word)memory.The Apollo command module computer had a central memory of 38,912 words (16 bits to the word). As part of the fail-safe design of the avionics system,four of the five computers are arranged as a redundant group during critical flight operations such as launch/ascent or entry/landing.In this mode,the four computers are linked as a voting set,with each one capable of being used as the flight control computer and with each one checking the other three.The crew can select which of the four computers is in control.The fifth computer is used for the backup flight control system,which would control the Shuttle should all four voting computers fail. Each of the four computers in the redundant set synchronizes itself to the other three computers 440 times each second.In this way,the computer set is able to achieve a high degree of reliability. During noncritical flight periods,one computer is used for guidance,navigation,and control tasks and another for systems management.The remaining three can be used for payload mamgement or can be deactivated. During critical phases of the mission,each of the five computers in the system performs approximately 325,000 operations each second using floating-point arithmatic. The crew can ask more than 1000 questions of the system and have the information displayed as alphanumeric symbols, as graphs,or as a combination of the two (including moving and flashing characters or symbols)on any of the four CRTs. Main memory.-In addition to the central memory stored in the computers themselves, 34,000,000 bits of information are also stored in two magnetic-tape devices.Critical programs and data are loaded in both tape machines and are protected from erasure.Normally,one mass memory unit is activated for use and the other is held in reserve for operation if the primary unit fails.However,it is possible to use both units simultaneously on separate data buses or to have both communicate with separate computers. Data bus.-The data bus network consists of digital data signal paths between the computers and the avionics subsystems and secondary channels between the telemetry system and the units that collect instrumentation data.This system is also fail-safe. The data transfer technique uses time-division multiplexing with pulse-code modulation.In this system,data channels are multiplexed together, one after the other,and information is coded on any given channel by a series of binary pulses corresponding to discrete information.Twenty- four data buses are on the Orbiter and an additional 28 buses connect the Orbiter avionics with the Solid Rocket Boosters and the External Tank. Multiplexer-demultiplexer.-The multiplexer- demultiplexer units are used in numerous remote locations of the Orbiter to handle the functions of serial data time multiplexing-demultiplexing associated with the digital data buses and for signal conditioning.They act as translators, putting information on or taking it off the data buses. Master timing unit.-All Orbiter and payload data are time-tagged with coordinated universal time and mission-elapsed time generated by the master timing unit.This device also supplies syncronizing signals to other electronic circuts as required and to the computers. Data Processing Software The software stored in and executed by the Orbiter general-purpose computers is the most sophisticated and complex set of programs ever developed for aerospace use.The programs are written to accommodate almost every aspect of Shuttle operations including vehicle checkout at the manufacturer's plant;flight turnaround activity at the Kennedy Space Center;prelaunch and final countdown;and navigation,guidance,and control during the ascent,orbital,entry,and landing phases and during abort or other contingency mission phases.In-flight programs monitor the status of vehicle subsystems;provide consumables computations;control the opening and closing of payload bay doors;operate the remote manipulator system;perform fault detection and annunciation;provide for payload monitoring, commanding,control,and data acquisition; provide antenna pointing for the various communications systems;and provide backup guidance,navigation,and control for the ascent, orbital,entry,and landing phases and for aborts. These primary computer programs are written so that they can be executed by a single computer or by all computers executing an identical program in the same time frame.This multicomputer mode is used for critical flight phases such as launch, ascent,entry,and aborts. The Orbiter software for a major mission phase must fit into the 106,496-word central memory of each computer.To accomplish all the computing functions referred to(for all phases)would require approximately 400,000 words of computer memory.To fit the software needed into the computer memory space availiable,computer programs have been subdivided into nine memory groups corresponding to the functions executed during specific flight and checkout phases.For example,one memory group accommodates final countdown,ascent,and aborts;another,orbital operations;and yet another,the entry and landing computations.Different memory groups support checkout and turnaround operations and systems management functions.Orbiter computers are loaded with different memory groups from magnetic tapes containing the desired programs. In this way,all the software needed can be stored in mass memory units(tape machines)and loaded into the computers only when actually needed. Software architecture.-The Orbiter computer programs are written in a hierarchy that contains two levels. The first level is the system software group, which consists of three sets of programs:(1)the flight computer operating program(the executive),which controls the processors, monitors key system parameters,allocates computer resources,provides for orderly program interrupts for higher priority activities, and updates computer memory;(2)the user interface program,which provides instructions for processing crew commands or requests;and (3)the system control program which initializes each computer and arranges for multicomputer operation during critical flight periods.The system software group programs also tell the computers how to perform and how to communicate with the other equipment. The second level of memory groups is the applications processing software.This group contains specific software programs for guidance,navigation,and control;systems management;payload operations;and vehicle checkout.The two program groups are combined to form a memory configuration for a specific mission phase.The guidance,navigation,and control programs contain functions required for launching,flying into orbit,maneuvering onorbit, and returning to an Earth landing.The systems management programs handle data management, performance monitoring,and special and display control processing.The payload processing programs contain instructions for control and monitoring of Orbiter payload systems.This set of instructions can be revised depending on the nature of the payload.The vehicle checkout program contains instructions for loading the memories in the main engine computers and for checking the instrumentation system.This program also aids in vehicle subsystem checkout and in ascertaining that the crew displays and controls perform properly.It is also used to update inertial measurement unit state vectors. Programing.-Coding of the Orbiter software programs is accomplished in the same manner as for the general-purpose ground-based computers.In general,the operating system programs are coded in basic assembly language. Applications programs(e.g.,the guidance, navigation,and control software)are written using high-order languages(such as the IBM FORTRAN series).In Orbiter computers,the operating system executive is coded in assembly language.The remaining two operating system programs and all four applications programs are written in HAL/S,a high-order language especially developed for NASA to be used in real-time space applications.It uses a base that is oriented toward the mathematics employed in guidance and navigation algorithms (detailed logical procedures for solving problems). ======================= END ==================================