[COFF] [TUHS] Re: DECtapes under the UNIX room floor

[Aron Insinga via COFF]

Hence the term "jiffy" for a power line frequency clock period:
https://hackersdictionary.com/html/entry/jiffy.html

With the possible exception of a system with only interpreted languages 
you need some sort of clock interrupt in order to implement 
time-sharing.  So there had to be one to allow TSS/8, the various PDP-6 
time-sharing systems, and the PDP-11 time-sharing systems such as 
RSTS(/E) and Unix.  (This is actually a reply to the Unix mailing list, 
but the connection may be too thin.)

In related news,
https://blog.tephra.me/pdp-11-dl-11-repair-uart-2/https://archive.org/details/
[Remove '-2' from the URL to get the previous post which was actually 
about the serial interface.]

At the time, I was told that the [never completed]  "minnow" or "tiny" 
PDP-10 would be the KT20 instead of the KT10 (like the KS10 etc.) 
because there was an existing option named KT10.  IIRC that person 
believed it was perhaps a clock, but I see now that it was the "KT10 
Memory Protection and Relocation" option ('T' for time-sharing??) for 
the KA10.  The clock was actually the DK10.
https://ftp.mirrorservice.org/sites/www.bitsavers.org/pdf/dec/pdp10/KA10/PDP-10_InstallationMan.pdf
https://archive.org/details/bitsavers_decpdp10peealTimeClockMaintenanceManualJul73_2436338/mode/2up

- Aron


On 5/6/26 14:19, Clem Cole via TUHS wrote:
> 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.