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Network Working Group                            Bill English   SRI/ARCRequest for Comments No. 34                      26 February 1970SOME BRIEF PRELIMINARY NOTES ON THE ARC CLOCKThe ARC clock system provides a time reference that is written into thecore memory of the XDS 940 Computer. There are two types of timeinformation available--absolute and relative.The absolute time is written into two adjacent words of core with thefollowing format:   First word --      Bits 0 thru 7 contain the month code in straight binary with a      range of 1 to 12      Bits 8 thru 15 contain the day code in straight binary with a      range of 1 to 31      Bits 16 thru 23 contain the year code in straight binary with      range of 0 to 99   Second word --      Bits 00 thru 7 contain the hour code written in straight binary      with a range of 0 to 23      Bits 8 thru 15 contain the minute code written in straight binary      with a range of 0 to 60      Bits 16 thru 23 contain the second code written in straight binary      with range ofr 0 to 60These 2 words are written once each second.  It is anticipated that theaccuracy of the initial setting will be on the order of 1 second, asreferred to WWV, and that the oscillator drift rate will not account foran accumlated error of more than 1 second every 250 days.  Theoscillator and clock are provided with standby power in order tomaintain the accuracy of the system.  Because of variable delays in timerequired to obtain access to the 940 core memory, it is anticipated thatthe short-term accuracy will be on the order of 10 to 20 microseconds.The relative time, which is written into one word of core memory, issimply the contents of a 24 bit binary accumulator.  The rate at whichthe accumulator is updated can be chosen to be either once very 100micro seconds or once every millisecond.  In either case the core                                                                [Page 1]

RFC 34              Prelimary Notes on the ARC Clock       February 1970location is written each time the accumulator is updated.  As above theshort-term accuracy will be about 10 to 20 microseconds and the long-term accuracy will be the equivalent of one second every 250 days.       [ This RFC was put into machine readable form for entry ]       [ into the online RFC archives by Katsunori Tanaka 1/98 ]                                                                [Page 2]