On Fri, 23 Feb 2018 14:54:18 -0500, Tony Harminc  wrote:
>
>Timer Units are not TOD clock units. Timer units are approximately
>26.04167 microseconds. They come from the long-gone S/360 Interval
>Timer, which was the fullword at location 80 (x'50'). This was defined
>so that bit position 23 is decremented every 1/300 second, which
>conveniently allowed an implementation that decremented either bits 21
>and 22 every 1/50 second, or bits 21 and 23 every 1/60 second, thus
>being able to run on a 50 or 60 Hz power line. Of course only the
>smallest 360 models actually used the power frequency for timing, but
>the definition lives on.
>
How accurate was that power frequency?  ISTR that a few decades ago my
electric clock would wander over long term, always remaining within
NIST time ±15 seconds, so in long term it was better than crystal accuracy.
I suppose that periodically the power company referred to USNO to make
a correction.  ("It depends.")  Nowadays, it's much better.

Was the granularity actually 1/60 or 1/50 second, or did it actually get finer
granularity by using a PLL with line power as a reference?  I suppose, "It
depends."

-- gil

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