Well, data sheets are out there, but I don't know about the actual
parts. Unfortunately, the 74AC family has far fewer members than the
74HC and others. I think each step in the evolution loses some types
that aren't expected to be high enough in volume for the most modern
applications. For instance, if you look back through the history of TTL
and its descendants, there are a lot of numbers that have gone extinct,
and new ones born.
I know the 74AC parts are plentiful in the high speed bus type stuff
('374, '244, '541, etc) and glue logic. It seems that many counters
should still be around too. Here's a '390 datasheet I found:
https://pdf1.alldatasheet.com/datasheet-pdf/view/106913/TOSHIBA/TC74AC390F.html
So, it must have existed sometime, but maybe it didn't make the
evolutionary cut. In any case, I think there must be some available
somewhere, even if obsolete. 74AC may be available in other counters
like '160 or '190 families. I haven't looked in a while.
I can't offhand remember all the HC counters, but I don't think any will
be acceptable for 50 MHz. Gotta go with AC or more modern low-level
families. If worse came to worse, you can definitely get some AC74s and
build your own machine, but it won't be as nice as dropping in a single
AC390 and having dividers to spare.
Ed
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