[TUHS] DECtapes under the UNIX room floor

[Clem Cole via TUHS]

On Wed, May 6, 2026 at 4:12 AM Thalia Archibald via TUHS <tuhs at tuhs.org>
wrote:

>
>
> Was there a precedent for the choice of sixtieths of a second?
>
Yes, DEC used line frequency from very early on in their processors.  For
the original PDP-11s (/20, 40, 45, 70), the KW11-L  (not be confused with
the KW-11P) was a single high card
http://www.bitsavers.org/pdf/dec/pdp11/1140/EK-KW11L_TM-002_KW11-L_Line_Time_Clock_Manual_Jul74.pdf
 [there is a picture of in on gunkies:
https://gunkies.org/wiki/KW11-L_Line_Time_Clock ], the functionality was
added to the DL11-W, so when it was used as the console KL11 it also acted
as a KW11-L



The purpose is to produce interrupts at a rate of 50 or 60 Hz, driven from
the AC power provided to the CPU's power supply [which is why UNIX is
configured appropriately].

If I understand the history correctly, the PDP-5 (1963): is the first DEC
machine to offer a Type 137 Real Time Clock, which could be configured to
trigger interrupts at the power line frequency (60Hz or 50Hz). This design
directly influenced the later PDP-8. PDP-8 (1965): Supported the DK8-EA
(and later the DK8-L and DK8-P) real-time clocks. The DK8-L Line Frequency
Clock was functionally identical to the KW11-L, providing a flag and
interrupt every 16.6ms or 20ms.   PDP-6 (1964): As a large-scale system, it
had a central clock (the Type 701) that provided several fixed frequencies,
including a line frequency signal for system timekeeping.   PDP-7 (1965):
Offered the Type 175 or Type 144 real-time clocks.   Like the PDP-5, these
could be set to line frequency for basic task scheduling.