Are you sure the 6550 is dead? (was: Mos Technology RAM wanted)

From: Richard Erlacher <edick_at_idcomm.com>
Date: Wed Mar 22 20:19:57 2000

Well, I did it again . . . please replace "... negative logic NOR ..." with
"...negative logic OR..." (have I spelled that stuff right ??) The point
is that it drives the active low enable on the memory device high, disabling
it, whenever one of its inputs is driven low. Therefore, given that there
are fewer than 14 competing enables, which certainly is not guaranteed to be
the case, though quite likely, the SRAM will occupy all the otherwise
unoccupied memory.

Dick

----- Original Message -----
From: Richard Erlacher <edick_at_idcomm.com>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, March 22, 2000 7:06 PM
Subject: Re: Are you sure the 6550 is dead? (was: Mos Technology RAM wanted)


> 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 - 20:19:57 GMT