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Recovery from Transient Failures of the Apollo Guidance Computer

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Recovery from Transient Failures of the Apollo Guidance Computer ,.,. Control and Flight Dynamics Conference, Pasadena, California, August 12-14, 1968. E-2307 RECOVERYFROMTRANSIENTFAILURESOF THEAPOLLOGUIDANCECOMPUTER by Edward M. Copps, Jr. AUGUST 1968 R T T w(o) T“i Yr CAMBRIDGE 39, MASSACHUSETTS -. GUIDANCE AND NAVIGATIQN 6s to be presented at the AIAA Guidance, Control and Flight Dynamics Conference, Pasadena, California, August 12-14, 1968. E-2307 RECOVERYFROMTRANSIENTFAILURESOF THE APOLLO GUIDANCE COMPUTER bY Edward M. Copps, Jr. AUGUST 1968 pf CAMBRIDGE 39, MASSACHUSETTS 0 K ACKNOWLEDGEMENT This report was preparedunder DSR Project 55- 23370 sponsored by the Manned Spacecraft Center of the National Aeronautics and Space Administration through Contract NAS g-4065 with the Instrumentation Laboratory, Massachusetts Institute of Technology, Cambridge, Mass. The publication of this report does not constitute approval by the National Aeronautics and Space Administration of the findings or the conclusions contained therein. It is published only for the exchange and stimulation of ideas. RECOVERY FROM TRANSIENT FAILURES OF THE APOLLO GUIDANCE COMPUTER Edward M. Copps, Jr. Massachusetts Institute of Technology Cambridge, Massachusetts ABSTRACT might have been done instead and implications, large In the Apollo Guidance Computer, nearly one and small, of various design decisions. hundred thousand word transfers ccur each second. A random error rate of one in 10 11 actions would be considered good in today’s technology, but at that Thename restart refers to a built-in recovery rate an error might occur within several hundred procedure which responds to hardware detected hours. Added to the random error rate are externally induced errors, including power and computer malfunctions. Basically, the hardware signal interface transients, program overloads, and transfers control to a specific location in the AGC operator errors. The incidence of induced errors has so far been very much greater than random memory. The software then restarts the programs errors (if indeed there have been any at all) in AGC in progress by reference to stored data which points experience. to the last restart point passed by each program The AGC is a control computer, and has been before the malfunction was detected. designed to detect and to recover from random or induced transient failures. The techniques used are the subject of this paper. II. The Apollo Guidance Computer The AGC is a control computer. It receives The paper outlines the character of the computer and its system software to a depth suf- incremental angular and velocity information from ficient for the discussion. The built-inmalfunction an Inertial Measurement Unit, from a telescope, a alarm logic is discussed, along with the software- based computer Self Check. These systems, upon sextant, and in the case of the Lunar Module it detection of a failure, force an involuntary re- ordering of the signal interface, and an involuntary receives range, range rate and angular data from a transfer to the restart program logic. From this Rendezvous Radar and a Landing Radar. There is a point, the software, without recourse to suspect Display and Keyboard (DSKY) which is illustrated in information in the central processor, reconstructs the output interface conditions, and reconstitutes the Figure 1. There is an uptelemetry receiving system, control processes in progress at the time of failure. Thereare a number of groundrules which limit the and a downtelemetry system. recovery capability, based on presumptions about the nature of the failure, These are presented. The software associated with a restart is described, with a typical program flow derived from an Apollo Mission Program. The amount of memory assigned to restart protection is stated. Several interesting sidelights are briefly discussed such as manual break-in to prevent restart looping, the adoption of the restart technique for scheduling the termination of active programs, and the use of the failure recovery technique to remove a temporary computer overload. I. Introduction The Apollo Guidance Computer (AGC) is part of the Guidance, Navigation and Control System of the Apollo Spacecraft. There aretwo systems, one in the Command Module and one in the Lunar Module. This paper is about the restart feature of the hardware and software. The intent of the paper is to explain what restart protection is, why it is done, what things Figure 1 Display and Keyboard Assembly 1 There are a number of discrete signals sent and intricate part of the AGC software, since display received, indicating, for example, such events as information, possibly requiring an astronaut res- , liftoff, accelerometer failure, a-rd vehicle stage ponse, may be “buried” under display information separation. generated by an auxiliary routine, which in turn may have been interrupted by a priority display, or by The computer directly controls the numerical astronaut keyboard activity. There is aKey Release displays, the rocket engine gimbals, the attitude jet light which informs the astronaut about buried commands, angular commands to the radar, optics, information. and inertial measurement unit, attitude error displays, engine throttle and engine on/off com- mands, among other things. III. AGC Memory The AGC memory is composed of three The computer performs many functions, almost significantly different components. The first all of which are variations on four purposes: consists of 36,864 words of non-alterable program 1. to maintain and improve knowledge of storage, called fixed memory. The contents of the position and velocity, memory locations are fixed at manufacture time. 2. to compute changes in positions ani velocity The question of content is one of physical threading by thrust vector control to carry out various or bypassing of wires through a sequence of tape mission phases, wound magnetic cores. Restart protection presumes 3. to control vehicle attitude, and, that fixed memory is intact. 4. to compute and display a wide variety of information necessary for astronaut control. There are 2040 words of program-alterable memory, called erasable memory. These consist of All of these functions are under the control of a conventional destructive read out ferrite core the astronaut, through the DSKY. There are about 40 matrix. The question of content is one of the polarity separate programs available to him. These of magnetic fluxset in theindividual core. Restart programs are function oriented, and are strung protection presumes that erasable memory is intact, together by theastronaut according to his needs by although a small portion is redundantly stored and a succession of keyboard actions. In addition to the compared during a restart. programs, there are approximately 30 routines Table 1 which can be run simultaneously with the programs 1?.EGISTER to yield information as desired. In general, only one NAME REMARKS TYPE ~-__ program can occupy the machine at one time. There O”O0” A is an exception to this which will be discussed in 0000 1 L detail later because it is relevant to the restart On”02 Q program. 00003 FB Erasable Rank Rrgistrr Flip-Flop The progress through a program or a routine is O”O”4 FB Fixed Bank Register Registera characterized in most cases by a sequence of 00005 z Next Address Regint~r displays. There are many decision points, at which 00006 BE Roth Rank Regl~ters the relevant data is displayed and a flashing an- nouncement indicates that an astronaut response is There are seven Central registers, designed for required. The usual astronaut response is to high speed operation. The names of these registers Proceed with the computational path and values are in Table 1. These registers are composed of flip displayed. flops, formed by interconnecting logic NOR gates, with content dependent on continuity of electrical Provision is made to put programs “to sleep” signal from the time thememoryis set, to the time until a desired response or other necessary external that it is read. Restart protection presumes that the event occurs. The display system is a special and content of these registers may have been destroyed. 2 There are sixteen input/output channels, flight. This Task reads the accelerometer velocity similar to the central registers, employing logic flip increment registers, makes a Job request to the flops. Executive which will complete a rather lengthy series of computations, makes a request to the IV. The AGC Software Operating System Waitlist to reschedule itself for 2 seconds from now There is one central processor. There is an and then terminates the interrupt mode. When this Executive program which schedules Jobs, according happens, the Executive resumes where it left off, to program assigned priority. Each Job is required unless another interrupt has occurred in the to periodically interrogate the NEW JOB register meantime. There is an instruction, INHINT, that which indicates the presence of Jobs of higher delays interrupts until execution of another priority. If there is a Job of higher priority awaiting instruction, RELINT. execution, the current Job is replaced. At the completion of a Job, Jobs of successively lower A restart may be considered to be a special case priority are executed until finally the Job of lowest of interrupt. However this and only this event will priority called DUMMY JOB is continuously interrupt a program already operating in interrupt executed. A computer Self Test program, designed mode, or a program operating with interrupt to test the reading of fixed memory and the reading inhibited. Also, restart does not return control to and writing of erasable memory can be executed the interrupted program. during this time. When a Job request is made, a fixed length set of erasable core is assigned to that Job and VI. Need For Restart is kept intact until the Job is terminated. Once a Job In the AGC, nearly one hundred thousand word is active, it is not possible to recognize it among the transfers are performed each second. A random other Jobs in various stages of completion in the error rate of one in 10 12 actions would be considered Executive.
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