> Subject: RE: PDP11/23+ in a BA411 enclosure ?
> Date: Wed, 22 Dec 2004 13:00:31 -0700
> Thread-Topic: PDP11/23+ in a BA411 enclosure ?
> Thread-Index: AcToXvQtewvRv8yURQOBKkEle3PYzAAACczQ
> From: "McCollum, Peter"
> To: "John Wilson" ,
>
> >(There used to be a Motorola chip that got 60 Hz from a 3.579 MHz color

> burst xtal, but they aren't available any more so I slapped this thing
> together.)
>
> Perhaps you mean the National MM5369? (I think that's the right
> number...) I have one of those - I used it in college to make the time
> base for a home-made frequency counter.
>
> Note that overall, the 60 Hz AC power line is more 'accurate' than
> nearly any signal source you can build yourself. It may drift away from
> exactly 60 Hz at times, but averaged over time it is extremely accurate.
> (I think the term would be it is 'accurate', but not 'precise'). So,
> stealing a bit of 60 Hz out of your power supply is simpler and more
> accurate than building a crystal-based circuit.
>
> Meanwhile, another way to do 'divide-by-n' without using lots of weird
> flip-flop configurations is to use one-shots. This works for dividing
> relatively low freqs, where the input freq is known.


There is a rather neat way to take a 5MHz or 10MHz output from a
frequency standard and multiply it by 3/10, using a ring counter.
After that the division to 60Hz is fairly straightforward. I used
to do that on a plug-in card to make Qbus 11 systems keep good
time while on board ships with a wandering mains frequency.

carl

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