[time-nuts] 60Hz mains clocking in computers

[Hal Murray]

colby at astro.berkeley.edu said:
> 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 360s bumped a memory location each cycle of the power line.  They 
bumped it by 6 in 50 HZ countries and by 5 in 60 HZ countries.  So the units 
were 300ths of a second.  I think that was used for the system time-of-day 
clock.  (I assume it was a few lines of microcode.)


> 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.

I'm not sure what "DC based" means.  For something like this, you would need 
an IO device that generated an interrupt.  It's a pretty simple IO device, 
but as far as the CPU is concerned, the power line is part of the outside 
world.  Most of the hardware for this sort of IO device would probably be 
filtering out noise.



-- 
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