[time-nuts] 60Hz mains clocking in computers

Uno Staver uno at staver.se
Mon Apr 19 21:38:16 UTC 2010

We bought a bunch of PDP-11/23s as part of a communications network 
system. After successful acceptance tests in Boston, MA, the systems 
were commissioned in Sweden with 50Hz AC. To make the RSX-11M O/S 
time-of-day clock run OK, the developers modified some piece of code.

Uno Staver

Bill Hawkins wrote:
> Yes, the whole PDP-11 line used line frequency to update the real-time
> clock.
> DEC had a real-time operating system, very useful for emulation of analog
> process control functions. Of course, an RTOS is more than just the clock.
> We lost that anchor to real time in the interval between the PDP-11 and
> NTP or SNTP when the microprocessors took over. All crystal clocks; time
> of day (social time) set by anybody with a wristwatch.
> Bill Hawkins
> -----Original Message-----
> From: paul swed
> Sent: Saturday, December 12, 2009 10:09 PM
> Talk about dusting off the old brain cells.
> I seem to remember that the PDP 11/23s did indeed allow the use of the 60 hz
> as an interrupt for precision timing if that can actually be said. The data
> general nova 1200 also. Boy thats exposing ones age.
> On Sat, Dec 12, 2009 at 8:29 PM, Colby Gutierrez-Kraybill <
> colby at astro.berkeley.edu> wrote:
>> I'm trying to get to the bottom of whether or not any computing equipment
>> made around the advent of UNIX systems (or any time-slicing system) used
> the
>> mains cycles of 60Hz as phase lock for the internal system clock.  My
> guess
>> is that perhaps they did not as the computing logic is DC based, but, I
> have
>> memories of using an 68000 based UNIX system that I thought had its
> internal
>> clock based off of the 60Hz mains...  Not sure the vendor anymore.
>> Thanks,  Colby
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