[TUHS] DECtapes under the UNIX room floor

[Phil Budne via TUHS]

John Cowan wrote:
> See the '"Electronics" and "Computing" sections of
> https://en.wikipedia.org/wiki/Jiffy_(time).

The "Computing" section is pretty light on history.

DEC had a long history of 60Hz "line time" or "mains" clocks.

Wall clocks that use synchronous motors (demonstrated by Tesla in
1893, and common since the first part of the 20th century) depend on
long-term accuracy of line frequency.

PDP-7 UNIX depends on the PDP-4/7 feature of location 7 being
decremented at 60Hz and generating an interrupt on overflow.

The PDP-6 has a "power line frequency" clock in the built-in APR
(arithmetic processor) device, and the KA10 (PDP-10, the PDP-6
follow-on) has readable bit in the APR device indicating that the
system was on 50Hz power.

I know less about PDP-5/8 and PDP-11s, but line time clocks were
present, or available as options (DK8-EA on the PDP-8/E, DL11-W, KW11
on PDP-11s, the 11/03 has a switch on the power supply for LTC).