21 Nov
2001
21 Nov
'01
11:14 a.m.
see inline comments, plz
Dick
----- Original Message -----
From: "Tony Duell" <ard(a)p850ug1.demon.co.uk>
To: <classiccmp(a)classiccmp.org>
Sent: Wednesday, November 21, 2001 5:07 PM
Subject: Re: I'm guessing you have solved this problem...
or
>
> 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,
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.
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. >
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 :-)
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