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 [email protected] -----Original Message----- From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of Paul Gilmartin Sent: Friday, February 23, 2018 12:13 PM To: [email protected] 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 depends." -- gil ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
