If one isn't averse to the inherent anachronism of putting a PAL or other
modern component in an old PET or KIM, it certainly would make more sense to
replace ALL the RAM with a single 64Kx8 device, and disable the 32-pin SRAM
with a single gate, either in a PAL or simply a 74S133, which is a 13-input
NAND gate. The latter would be a negative logic NOR, outputting a HIGH
level each time one of its inputs, which, in this case, would be a select
from another memory-mapped device. That way you'd always have RAM wherever
you don't have something else, and, since everybody's got these 64KB SRAMs
lying in the closet, masquerading as CACHE memory on an obsolete '486 board
(that's where I get mine!) they're plenty fast enough (15 ns).
OTOH, if you prefer to maintain technological compatibility with the state
of the art at the time of the machine's design, the SSI solution is more
appropriate. Either one should work.
I'm not sure whether I am more inclined to one approach or the other, but I
can see why one might prefer one or the other.
In reality, I still haven't gotten to a solid position on what to call the
computer with boards from one manufacturer, yet housed in sheet metal
bearing another's name. Take for example the IMSAI box with North-Star
boards inside. Is it a CRAY if it has that printed on the cabinet, even
though the computer inside is an 1802 board?
Is it not an IMSAI an IMSAI if all its boards are from IMSAI? Oh, well . .
.
Dick
----- Original Message -----
From: Pete Turnbull <pete_at_dunnington.u-net.com>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, March 22, 2000 6:04 PM
Subject: Re: Are you sure the 6550 is dead? (was: Mos Technology RAM wanted)
> On Mar 22, 23:56, Tony Duell wrote:
>
> > You _honestly_ think that a 22V10 (or whatever) is simpler, cheaper, and
> > easier to use than a couple of 74LS20s? You know, a 22V10 has 24 pins,
> > and the 2 74LS20s have 28. That's only 4 more pins to solder. And
there's
> > no requirement to program a chip if you use the '20s.
>
> No, but I can make more choices with the 22V10 -- I want to be able to
> switch ROM sets (between the original on the motherboard and possibly two
> on the expansion) and RAM sizes and that would need a little more effort
> with the TTL, because I want to *replace* the 74154, not add to it.
>
>
> --
>
> Pete Peter Turnbull
> Dept. of Computer Science
> University of York
Received on Wed Mar 22 2000 - 20:06:53 GMT