Long before I came to work for one, I heard that power companies are pretty 
rigorous in maintaining a constant sine wave, whether 50 or 60 Hz. The power 
might go out (!), but it won't wander in frequency. 

J.O.Skip Robinson
Southern California Edison Company
Electric Dragon Team Paddler 
SHARE MVS Program Co-Manager
323-715-0595 Mobile
626-543-6132 Office ⇐=== NEW

-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf 
Of Paul Gilmartin
Sent: Friday, February 23, 2018 12:13 PM
Subject: (External):Timer Unis (was: ... time change ...)

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 

-- gil

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