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RFC 34 UNKNOWN HARDWARE · TIMING · NETWORK

Some Brief Preliminary Notes on the Augmentation Research Center Clock

How the ARC clock stored absolute time and elapsed-time counts.

On rfc-editor.org ↗
PUBLISHED
February 1970
AUTHORS
W.K. English
STREAM
Legacy
ABSOLUTE TIME UPDATE
Once each second
RELATIVE ACCUMULATOR
24 bits
RELATIVE UPDATE RATES
Every 100 microseconds or every millisecond
LONG-TERM DRIFT
No more than 1 second every 250 days
SHORT-TERM ACCURACY
10 to 20 microseconds
DOI
10.17487/RFC0034

RFC text

Original text · RFC Editor View on rfc-editor.org ↗ 
Network Working Group                            Bill English   SRI/ARC
Request for Comments No. 34                      26 February 1970


SOME BRIEF PRELIMINARY NOTES ON THE ARC CLOCK

The ARC clock system provides a time reference that is written into the
core memory of the XDS 940 Computer. There are two types of time
information available--absolute and relative.

The absolute time is written into two adjacent words of core with the
following 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 60

These 2 words are written once each second.  It is anticipated that the
accuracy of the initial setting will be on the order of 1 second, as
referred to WWV, and that the oscillator drift rate will not account for
an accumlated error of more than 1 second every 250 days.  The
oscillator and clock are provided with standby power in order to
maintain the accuracy of the system.  Because of variable delays in time
required to obtain access to the 940 core memory, it is anticipated that
the 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, is
simply the contents of a 24 bit binary accumulator.  The rate at which
the accumulator is updated can be chosen to be either once very 100
micro seconds or once every millisecond.  In either case the core

location is written each time the accumulator is updated.  As above the
short-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 ]

ISO Plain Language

On this page
PROBLEMThe Augmentation Research Center needed a clock reference that could provide date and time as well as support interval measurements.
MECHANISMThe XDS 940 clock stores absolute time in two core words and relative time in a 24-bit accumulator updated at a chosen interval.
CLOCK DESIGNThe note describes the clock's data layout, update rates and stated accuracy; it is a preliminary technical description.

§1Absolute Time in Two Words

The ARC clock writes absolute time into two adjacent words of XDS 940 core memory. The first word stores month, day and year in binary fields; the second stores hour, minute and second in binary fields.

The two words are written once each second. The note gives ranges for each date and time field.

Two memory words hold the calendar date and clock time. The first stores the date, and the second stores the hour, minute and second.

§1Relative Time in an Accumulator

Relative time is stored in one core word as the contents of a 24-bit binary accumulator. The update rate can be chosen as once every 100 microseconds or once every millisecond.

The core location is written each time the accumulator is updated.

A separate counter measures elapsed time instead of storing a date and time. The clock can update this counter at either of the two rates stated in the note.

§1Accuracy and Standby Power

The note anticipates that the initial setting will be accurate to about 1 second relative to WWV. It says oscillator drift should not cause more than 1 second of accumulated error every 250 days.

The oscillator and clock have standby power to maintain accuracy. Variable delays in accessing core memory limit short-term accuracy to about 10 to 20 microseconds.

The clock is expected to keep close time over long periods and has standby power. Reading memory takes a variable amount of time, which limits short-term accuracy.