[time-nuts] 60Hz mains clocking in computers

Hal Murray hmurray at megapathdsl.net
Mon Apr 19 18:32:06 EDT 2010

> 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.

The IBM 360 series bumped a register in low memory by 300 counts per second.

It used an interrupt from the power line.  If you were running on 60 Hz, they 
added 5 each interrupt.  If you were running on 50 Hz, they added 6 each 
interrupt.  I assume that was in the microcode.

I wouldn't call it a phase lock.  The timer was just driven by the interrupts.

I remember a story from the late 70s (I think) of some gear shipped to Europe 
that wasn't keeping good time.  It turned out to be 60 Hz gear that was 
running off a box to make 60 Hz from 50 Hz.  The box wasn't adjusted very 
well so the clock was drifting.  Somebody adjusted the frequency and we went 
back to working on other things.  [I think it was gear that I was familiar 
with, but all the gear I remember from back then derived time from the CPU 
clock.  Old age...]

These are my opinions, not necessarily my employer's.  I hate spam.

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