see inline comments, plz
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, November 21, 2001 5:07 PM
Subject: Re: I'm guessing you have solved this problem...
> >
> > Just how fast do these devices have to be, and how deep are they? I've got
some
>
> 10's of ns at worst. And typically 256*4 bits or so.
>
So those would be 82S129's or the like? There are some such devices that still
show up in popular surplus supplies.
>
> > 35ns 8kx8 CMOS EPROMs, in skinny DIP packages, and I'd imagine that other
>
> I suspect those would be fine for many of the PROMs in old minicomputers...
>
> > configurations are out there as well. These are contemporaries of the 25ns
> > 22V10 UV-erasable PALs from CYPRESS. That would date them about 1990.
> >
> > Small bipolar PROMs were often used like PALs, which were not yet popular,
or
>
> In the case of the 11/45 (1972), I don't think PALs were even available
> at the time it was made.
>
> > were more expensive than the PROMs. I've not seen EPROMs that small,
however,
> > but it's likely a big EPROM would work if it's fast enough. In a case where
the
>
> Of course it would. EPROMs never need a refresh (for obvious reasons), so
> it's safe to tie unused address inputs low and to program the required
> pattern into the first few words of the EPROM. I don't see how that could
> ever not work.
>
> > PROM is a logic element rather than a program store, a PAL could well be
> > substituted.
>
> Well, if you want to take a dump of the PROM, work out the logic
> equations from it, then fit them into a PAL, and get it working, good
> luck :-)
>
The PROM is listed as a truth-table, right? The only ABEL version I used simply
took a truth table as input. That wouldn't take much processing. >
> Yes, some PROMs were used as logic functions rather than (say) microcode
> store. But it's not that easy to replace them with PALs (it's possible in
> some cases to fit the logic into a small-ish PAL). It's a lot easier to
> use a PROM/EPROM device.
>
If one doesn't have a suitable bit of software, that Quine-McLuskey (?) method
that I learned about in college some 30+ years back (and promptly forgot) can be
automated easily enough to produce reduced equations. Reducing the prom listing
to a set of equations by isolating each bit in the output word and OR'ing the
locations at which those bits are true is one reasonably way to do the job. I
know of no PAL generation software that doesn't automatically reduce the
equations for you.
PROMs are programmable-OR-fixed-AND devices, while PALs are programmable
AND-fixed-OR devices. The process of generating them is, therefore different,
for a given logic function, but, expanded and re-reduced, the result should be
the same. Of course a registered PROM is needed to produced registered outputs.
I'd say a 16L8 would do for most PROM-based applications. A 16L8 is capable of
generating any logic function of 16 inputs. A 256x4 or 32x8 PROM has less logic
than that in it.
>
> -tony
>
>
Received on Wed Nov 21 2001 - 19:14:17 GMT