At 1:44 AM +0000 12/13/09, time-nuts-requ...@febo.com wrote:
Date: Sat, 12 Dec 2009 17:29:17 -0800
From: Colby Gutierrez-Kraybill <co...@astro.berkeley.edu>
Subject: [time-nuts] 60Hz mains clocking in computers
To: time-nuts@febo.com
Message-ID: <3058527a-cc99-4174-be75-21dd92334...@astro.berkeley.edu>
Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
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. 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.
In the 1980s and 1990s, before networks capable of carrying NTP time
to the millions became common, the computer local clock was very
often derived from the local AC power mains, and the frequency was
steered to match atomic time once per day. The POSIX standards
reflect this common approach by the tolerance on CLOCK_REALTIME, 20
milliseconds, this being one cycle of 50 Hz power.
The CPU logic clock was not generally phase-locked to the AC power
lines, instead being generated by a cheap crystal having a very large
tempco. The exception to this was that video generators were (and
still are) often locked to the AC line so that hum bars would not
drift across the screen.
Joe Gwinn
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