Source Code

This source code is very close to the Apollo Guidance Computer software for the Apollo 13 Lunar module. This revision of the Luminary 131 program is from December of 1969, whereas there was a later revision in January of 1970, and still another revision in February, which is the code that flew on the mission. As far as this transcription is concerned, it was originally from a copy made in 1991 of a printout from the collection of AGC developer Don Eyles for collector David Craig. It was subsequently scanned by Gary Neff, reprocessed for online presentation at the now-discontinued History of Recent Science and Technology (HRST) website of MIT's Dibner Institute (the full-quality scans being discarded in the process), and finally transcribed by Ron Burkey for the Virtual AGC Project. Although a high-quality replacement scan for a completely illegible page was later provided by Gary Neff, the reduced legibility of the reprocessed HRST posting nevertheless caused many errors to be introduced into the transcription. Accordingly, a second scan of the same physical printout was made in 2017 for the Virtual AGC Project's collection at the Internet Archive, and used to correct the transcription errors. All of the scanned materials mentioned, as well as other Luminary 131 related material, are available online. Don Eyles apprently made additional hand-written notes in printout between 1991 and 2017, so the two scans are not identical in that respect. The page-headings in the printout read, in part:
	GAP:  ASSEMBLE REVISION 131 OF AGC PROGRAM LUMINARY BY NASA 2021112-091
	17:53 DEC. 19, 1969
Note that the date is the date the printout was made, not the date on which the program revision was released, although these happen to be very close together.

063858,000002:                                                                                                  ## Copyright:   Public domain.
063859,000003:                                                                                                  ## Filename:    SPS_BACK-UP_RCS_CONTROL.agc
063860,000004:                                                                                                  ## Purpose:     A section of Luminary 1C, revision 131.
063861,000005:                                                                                                  ##              It is part of the source code for the Lunar Module's (LM)
063862,000006:                                                                                                  ##              Apollo Guidance Computer (AGC) for Apollo 13.
063863,000007:                                                                                                  ##              This file is intended to be a faithful transcription, except
063864,000008:                                                                                                  ##              that the code format has been changed to conform to the
063865,000009:                                                                                                  ##              requirements of the yaYUL assembler rather than the
063866,000010:                                                                                                  ##              original YUL assembler.
063867,000011:                                                                                                  ## Reference:   pp. 1499-1502 
063868,000012:                                                                                                  ## Contact:     Ron Burkey <info@sandroid.org>.
063869,000013:                                                                                                  ## Website:     www.ibiblio.org/apollo/index.html
063870,000014:                                                                                                  ## Mod history: 06/08/03 RSB.   Began transcribing.
063871,000015:                                                                                                  ##              05/14/05 RSB    Corrected website reference above.
063872,000016:                                                                                                  ##              2010-10-25 JL   Indentation fixes.
063873,000017:                                                                                                  ##              2017-01-06 RSB  Page numbers now agree with those on the
063874,000018:                                                                                                  ##                              original hardcopy, as opposed to the PDF page
063875,000019:                                                                                                  ##                              numbers in 1701.pdf.
063876,000020:                                                                                                  ##              2017-02-28 RSB  Proofed comment text using octopus/ProoferComments.
063877,000021:                                                                                                  ##              2017-03-14 RSB  Comment-text fixes noted in proofing Luminary 116.
063878,000022: 

Page 1499

063880,000024:                                                                                                  #  PROGRAM NAME:         SPSRCS
063881,000025:                                                                                                  #  AUTHOR:               EDGAR M. OSHIKA (AC ELECTRONICS)
063882,000026:                                                                                                  #  MODIFIED:             TO RETURN TO ALL AXES VIA Q BY P. S. WEISSMAN, OCT 7, 1968
063883,000027:                                                                                                  #  MODIFIED TO IMPROVE BENDING STABILITY BY G. KALAN, FEB. 14, 1969
063884,000028: 
063885,000029:                                                                                                  #  FUNCTIONAL DESCRIPTION:
063886,000030:                                                                                                  #        THIS PROGRAM CONTROLS THE FIRING OF ALL RCS JETS IN THE DOCKED CONFIGURATION ACCORDING TO THE FOLLOWING PHASE
063887,000031:                                                                                                  #        PLANE LOGIC.
063888,000032: 
063889,000033:                                                                                                  #        1. JET SENSE TEST (SPSRCS)
063890,000034:                                                                                                  #                IF JETS ARE FIRING NEGATIVELY, SET OLDSENSE NEGATIVE AND CONTINUE
063891,000035:                                                                                                  #                IF JETS ARE FIRING POSITIVELY, SET OLDSENSE POSITIVE AND CONTINUE
063892,000036:                                                                                                  #                IF JETS ARE NOT FIRING, SET OLDSENSE TO ZERO AND GO TO OUTER RATE LIMIT TEST
063893,000037: 
063894,000038:                                                                                                  #        2. RATE DEAD BAND TEST
063895,000039:                                                                                                  #                IF JETS ARE FIRING NEGATIVELY AND RATE IS GREATER THAN TARGET RATE, LEAVE
063896,000040:                                                                                                  #                JETS ON AND GO TO INHIBITION LOGIC.  OTHERWISE, CONTINUE.
063897,000041:                                                                                                  #                IF JETS ARE FIRING POSITIVELY AND RATE IS LESS THAN TARGET RATE, LEAVE
063898,000042:                                                                                                  #                JETS ON AND GO TO INHIBITION LOGIC.  OTHERWISE, CONTINUE.
063899,000043: 
063900,000044:                                                                                                  #        3. OUTER RATE LIMIT TEST (SPSSTART)
063901,000045:                                                                                                  #                IF MAGNITUDE OF EDOT IS GREATER THAN 1.73 DEG/SEC SET JET FIRING TIME
063902,000046:                                                                                                  #                TO REDUCE RATE AND GO TO INHIBITION LOGIC.  OTHERWISE, CONTINUE.
063903,000047: 
063904,000048:                                                                                                  #        4. COAST ZONE TEST
063905,000049:                                                                                                  #                IF STATE (E,EDOT) IS BELOW LINE E + 4 X EDOT > -1.4 DEG AND EDOT IS LESS THAN 1.30 DEG/SEC SET JET TIME POSI-
063906,000050:                                                                                                  #                TIVE AND CONTINUE.  OTHERWISE, SET JET FIRING TIME TO ZERO AND CONTINUE.
063907,000051:                                                                                                  #                IF STATE IS ABOVE LINE E + 4 X EDOT > +1.4 DEG AND EDOT IS GREATER THAN -1.30 DEG/SEC, SET JET TIME NEGATIVE
063908,000052:                                                                                                  #                AND CONTINUE.  OTHERWISE, SET JET FIRING TIME TO ZERO AND CONTINUE.
063909,000053: 
063910,000054:                                                                                                  #        5. INHIBITION LOGIC
063911,000055:                                                                                                  #                IF OLDSENSE IS NON-ZERO:
063912,000056:                                                                                                  #                        A) RETURN IF JET TIME HAS SAME SIGN AS OLDSENSE
063913,000057:                                                                                                  #                        B) SET INHIBITION COUNTER* AND RETURN IF JET TIME IS ZERO
063914,000058:                                                                                                  #                        C) SET INHIBITION COUNTER,* SET JET TIME TO ZERO AND RETURN IF SIGN
063915,000059:                                                                                                  #                           OF JET TIME IS OPPOSITE TO THAT OF OLDSENSE
063916,000060:                                                                                                  #                IF OLDSENSE IS ZERO:
063917,000061:                                                                                                  #                        A) RETURN IF INHIBITION COUNTER IS NOT POSITIVE
063918,000062:                                                                                                  #                        B) SET JET TIME TO ZERO AND RETURN IF INHIBITION COUNTER IS POSITIVE
063919,000063:                                                                                                  #                *NOTE: INHIBITION COUNTERS CAN BE SET TO 4 OR 10 FOR THE P,AND UV AXES,
063920,000064:                                                                                                  #                RESPECTIVELY, IN SPSRCS.  THEY ARE DECREMENTED BY ONE AT THE BEGINNING OF

Page 1500

063922,000066:                                                                                                  #                EACH DAP PASS.
063923,000067: 
063924,000068:                                                                                                  #        THE MINIMUM PULSE WIDTH OF THIS CONTROLLER IS DETERMINED BY THE REPETITION RATE AT WHICH THIS ROUTINE IS CALLED
063925,000069:                                                                                                  #        AND IS NOMINALLY 100 MS FOR ALL AXES IN DRIFTING FLIGHT.  DURING POWERED FLIGHT THE MINIMUM IS 100 MS FOR THE
063926,000070:                                                                                                  #        P AXIS AND 200 MS FOR THE CONTROL OF THE U AND V AXES.
063927,000071: 
063928,000072:                                                                                                  #  CALLING SEQUENCE:
063929,000073:                                                                                                  #                INHINT
063930,000074:                                                                                                  #                TC      IBNKCALL
063931,000075:                                                                                                  #                CADR    SPSRCS
063932,000076: 
063933,000077:                                                                                                  #  EXIT:
063934,000078:                                                                                                  #                TC      Q
063935,000079: 
063936,000080:                                                                                                  #  ALARM/ABORT MODE:     NONE
063937,000081: 
063938,000082:                                                                                                  #  SUBROUTINES CALLED:   NONE
063939,000083: 
063940,000084:                                                                                                  #  INPUT:                E, EDOT
063941,000085:                                                                                                  #                        TJP, TJV, TJU           TJ MUST NOT BE NEGATIVE ZERO
063942,000086: 
063943,000087:                                                                                                  #  OUTPUT:               TJP, TJV, TJU
063944,000088: 
063945,000089: 21,3655                                           BANK     21                                    
063946,000090: 21,2000                                           SETLOC   DAPS4                                 
063947,000091: 21,2000                                           BANK                                           
063948,000092: 
063949,000093: 21,3655                                           COUNT*   $$/DAPBU                              
063950,000094: 
063951,000095: 21,3655  E6,1525                                  EBANK=   TJU                                   
063952,000096: 21,3655           00632        RATELIM2           OCT      00632                                 #  1.125 DEG/SEC
063953,000097: 21,3656           34736        POSTHRST           CA       HALF                                  
063954,000098: 
063955,000099: 21,3657           51505                           NDX      AXISCTR                               
063956,000100: 21,3660           55525                           TS       TJU                                   
063957,000101: 21,3661           11744                           CCS      OLDSENSE                              
063958,000102: 21,3662           13702                           TCF      POSCHECK                              #  JETS FIRING POSITIVELY
063959,000103: 21,3663           13705                           TCF      CTRCHECK                              #  JETS OFF.  CHECK INHIBITION CTR
063960,000104: 21,3664           51505        NEGCHECK           INDEX    AXISCTR                               #  JETS FIRING NEGATIVELY
063961,000105: 21,3665           41525                           CS       TJU                                   
063962,000106: 21,3666           10000                           CCS      A                                     
063963,000107: 21,3667           00002                           TC       Q                                     #  RETURN
063964,000108: 21,3670           13672                           TCF      +2                                    
063965,000109: 21,3671           13672                           TCF      +1                                    #  JETS COMMANDED OFF.  SET CTR AND RETURN
063966,000110: 21,3672           51505        SETCTR             INDEX    AXISCTR                               #  JET FIRING REVERSAL COMMANDED.  SET CTR,
063967,000111: 21,3673           33714                           CA       UTIME                                 #  SET JET TIME TO ZERO,AND RETURN

Page 1501

063969,000113: 21,3674           51505                           INDEX    AXISCTR                               
063970,000114: 21,3675           55767                           TS       UJETCTR                               
063971,000115: 21,3676           34755        ZAPTJ              CA       ZERO                                  
063972,000116: 21,3677           51505                           INDEX    AXISCTR                               
063973,000117: 21,3700           55525                           TS       TJU                                   
063974,000118: 21,3701           00002                           TC       Q                                     
063975,000119: 21,3702           51505        POSCHECK           INDEX    AXISCTR                               
063976,000120: 21,3703           31525                           CA       TJU                                   
063977,000121: 21,3704           13666                           TCF      NEGCHECK   +2                         
063978,000122: 21,3705           51505        CTRCHECK           INDEX    AXISCTR                               #  CHECK JET INHIBITION COUNTER
063979,000123: 21,3706           11767                           CCS      UJETCTR                               
063980,000124: 21,3707           13711                           TCF      +2                                    
063981,000125: 21,3710           00002                           TC       Q                                     #  CTR IS NOT POSITIVE.  RETURN
063982,000126: 21,3711           13676                           TCF      ZAPTJ                                 #  CTR IS POSITIVE.  INHIBIT FIRINGS
063983,000127: 21,3712           00002                           TC       Q                                     #  CTR IS NOT POSITIVE.  RETURN
063984,000128: 21,3713           00004                           OCT      00004                                 
063985,000129: 21,3714           00012        UTIME              OCT      00012                                 
063986,000130: 21,3715           00012                           OCT      00012                                 
063987,000131: 21,3716  E6,1744               OLDSENSE           EQUALS   DAPTREG1                              
063988,000132: 21,3716           44753        NEGFIRE            CS       ONE                                   #  JETS FIRING NEGATIVELY
063989,000133: 21,3717           55744                           TS       OLDSENSE                              
063990,000134: 21,3720           31427                           CA       EDOT                                  
063991,000135: 21,3721           13725                           TCF      +4                                    
063992,000136: 21,3722           34753        PLUSFIRE           CA       ONE                                   
063993,000137: 21,3723           55744                           TS       OLDSENSE                              
063994,000138: 21,3724           41427                           CS       EDOT                                  #  RATE DEAD BAND TEST
063995,000139: 21,3725           22000                           LXCH     A                                     
063996,000140: 21,3726           40111                           CS       DAPBOOLS                              #  IF DRIFTBIT = 1, USE ZERO TARGET RATE
063997,000141: 21,3727           74744                           MASK     DRIFTBIT                              #  IF DRIFTBIT = 0, USE 0.10 RATE TARGET
063998,000142: 21,3730           10000                           CCS      A                                     
063999,000143: 21,3731           34767                           CA       RATEDB1                               
064000,000144: 21,3732           60001                           AD       L                                     
064001,000145: 21,3733           00006                           EXTEND                                         
064002,000146: 21,3734           63744                           BZMF     SPSSTART                              
064003,000147: 21,3735           13661                           TCF      POSTHRST   +3                         
064004,000148: 
064005,000149: 21,3736           51505        SPSRCS             INDEX    AXISCTR                               #  JET SENSE TEST
064006,000150: 21,3737           11525                           CCS      TJU                                   
064007,000151: 21,3740           13722                           TCF      PLUSFIRE                              #  JETS FIRING POSITIVELY
064008,000152: 21,3741           13743                           TCF      +2                                    
064009,000153: 21,3742           13716                           TCF      NEGFIRE                               #  JETS FIRING NEGATIVELY
064010,000154: 21,3743           55744                           TS       OLDSENSE                              #  JETS OFF
064011,000155: 21,3744           31427        SPSSTART           CA       EDOT                                  #  OUTER RATE LIMIT TEST
064012,000156: 21,3745           00006                           EXTEND                                         
064013,000157: 21,3746           74766                           MP       RATELIM1                              
064014,000158: 21,3747           10000                           CCS      A                                     
064015,000159: 21,3750           13767                           TCF      NEGTHRST                              #  OUTER RATE LIMIT EXCEEDED
064016,000160: 21,3751           13753                           TCF      +2                                    
064017,000161: 21,3752           13656                           TCF      POSTHRST                              #  OUTER RATE LIMIT EXCEEDED
064018,000162: 21,3753           31427                           CA       EDOT                                  #  COAST ZONE TEST

Page 1502

064020,000164: 21,3754           61751                           AD       E                                     
064021,000165: 21,3755           00006                           EXTEND                                         
064022,000166: 21,3756           71411                           MP       DKDB                                  #  PAD LOADED DEADBAND.  FRESHSTART: 1.4 DEG
064023,000167: 21,3757           00006                           EXTEND                                         
064024,000168: 21,3760           13775                           BZF      TJZERO                                
064025,000169: 
064026,000170: 21,3761           00006                           EXTEND                                         
064027,000171: 21,3762           63771                           BZMF     +7                                    
064028,000172: 21,3763           31427                           CA       EDOT                                  
064029,000173: 21,3764           63655                           AD       RATELIM2                              
064030,000174: 21,3765           00006                           EXTEND                                         
064031,000175: 21,3766           63775                           BZMF     TJZERO                                
064032,000176: 21,3767           44736        NEGTHRST           CS       HALF                                  
064033,000177: 21,3770           13657                           TCF      POSTHRST   +1                         
064034,000178: 21,3771           43655                 +7        CS       RATELIM2                              
064035,000179: 21,3772           61427                           AD       EDOT                                  
064036,000180: 21,3773           00006                           EXTEND                                         
064037,000181: 21,3774           63656                           BZMF     POSTHRST                              
064038,000182: 21,3775           34755        TJZERO             CA       ZERO                                  
064039,000183: 21,3776           13657                           TCF      POSTHRST   +1                         
064040,000184: 
064041,000185: 21,3777     4766               RATELIM1           =        CALLCODE                              #  = 00032, CORRESPONDING TO 1.73 DEG/SEC
064042,000186: 21,3777     4767               RATEDB1            =        TBUILDFX                              #  = 00045, CORRESPONDS TO 0.101 DEG/SEC

End of include-file SPS_BACK-UP_RCS_CONTROL.agc.  Parent file is MAIN.agc