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RFC 34 UNKNOWN HARDWARE · TIMING · NETWORK

Some Brief Preliminary Notes on the Augmentation Research Center Clock

Technical description of the Augmentation Research Center's network clock system

On rfc-editor.org ↗
PUBLISHED
February 1970
AUTHORS
W.K. English
STREAM
Legacy
ABSOLUTE TIME UPDATE RATE
Once per second
RELATIVE COUNTER UPDATE RATES
Every 100 microseconds or 1 millisecond
LONG-TERM ACCURACY
About 1 second per 250 days
SHORT-TERM ACCURACY
10 to 20 microseconds
COMPUTER
XDS 940
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 ]

AI ELI5

On this page
PROBLEMNetwork hosts needed accurate time references to support measurement and synchronization, but clock implementations were not yet standardized.
MECHANISMThe Augmentation Research Center built a clock system that stored both absolute time (date and clock values in core memory) and relative time (a running counter) updated at fixed intervals.
TIMING INFRASTRUCTUREThe design demonstrated practical clock implementation for network nodes and informed future timing infrastructure.

§1Clock System Overview

The ARC (Augmentation Research Center) clock system at SRI provided a time reference stored in the core memory of an XDS 940 computer. The system offered two types of time information: absolute time (the actual date and time of day) and relative time (a running counter for interval measurement).

The absolute time was written as two adjacent core memory words, refreshed once each second. The relative time was a 24-bit binary accumulator that could be updated either every 100 microseconds or every 1 millisecond, depending on the application's needs.

The clock stores two things: what time it is right now, and a counter that keeps track of how much time has passed.

§2Absolute and Relative Time

The absolute time uses two core memory words with a binary format. The first word holds the month (1 to 12), day (1 to 31), and year (0 to 99) as three 8-bit binary fields. The second word holds the hour (0 to 23), minute (0 to 60), and second (0 to 60) as three more 8-bit binary fields.

The relative time uses one core memory word containing a 24-bit counter. This counter increments either every 100 microseconds or every 1 millisecond, allowing applications to measure short intervals without reading the full date and time.

The system provided long-term accuracy of about 1 second per 250 days through a crystal-controlled oscillator with standby power. Short-term accuracy was about 10 to 20 microseconds due to variable memory access delays.

One part stores what time and date it is, another part counts ticks to measure how fast things happen.

§3Practical Implementation

The design reflected the needs of early network hosts. The absolute time's accuracy on initial setting was about 1 second compared to radio time standard WWV. The oscillator had a drift rate of less than 1 second every 250 days. The system was backed by standby power to maintain accuracy across power disruptions.

The RFC notes that variable delays in accessing the 940 core memory meant that short-term absolute time reading was limited to 10 to 20 microseconds precision. This was acceptable for network measurement purposes, where the relative time counter could handle finer intervals while the absolute time provided synchronization anchors.

The clock stays accurate for months, and even brief measurements are fast enough for network work.