Altair parts substitutions

From: Richard Erlacher <richard_at_idcomm.com>
Date: Mon May 22 18:40:09 2000

Forgive me for jumping in here, but the '373 is in no way similar to the
8T97. The 373 is a transparent latch with tristate outputs, i.e. a storage
element, whereas the 8T97 is a tristate buffer, in this case two sharing one
enable and the remaining four sharing the other. It's output is heftier
than the otherwise identical 74367. Though it's somewhat of an
oversimplification, the Signetics (that's who first made 'em) 8T series had
inputs pretty much like the LS parts, yet had outputs like the schottky
series parts.

If you're building an Altair work-alike, yet not bent on using the same
parts, I'd certainly recommend you use the 74ACT or even AC series if you
can get them. They have completely symmetrical outputs, i.e. drive upward
as hard as they pull downward, hence have more easily controllable
impedance, hence will make nicer waveforms on the backplane.

What's more, I'd recommend you look into the more up-to-date AC terminations
for the bus, as opposed to the rather poor active DC termination that was
promoted after people got the idea that there was trouble on the bus.
Almost any AC termination together with symmetrical bus drivers as I've
suggested, will improve the behavior of the bus much more than the passive
or active DC terminations that were popular in the '70's. You may find it's
easier to use 8-bit-wide parts for bus drivers/receivers, e.g. 74AC244's
instead of 8T97's. You get two 4-bit buffers per package, but they're
really better than the 8T97's. The fact that they're built in multiples of
4 bits wide makes them handier than the 6-bit 8T97's.

Good luck!

Dick

----- Original Message -----
From: Cini, Richard <RCini_at_congressfinancial.com>
To: 'ClassCompList' <classiccmp_at_classiccmp.org>
Sent: Monday, May 22, 2000 10:05 AM
Subject: Altair parts substitutions


> It's that time of year again, when I think about designing an Altair/IMSAI
> work-alike. Now that I have the ability to generate schematics/layouts, I
> may actually try it.
>
> Anyway, some parts have become scarce, so I would have to use substitutes.
> For example, Jameco no longer carries the 8T97 buffer/driver. As I recall
> the 8T97 is an LS373 without the inverter on the gate *and* is faster and
> has a higher drive current. I've heard that the MC6887 is a sub, but I
can't
> find a datasheet on it.
>
> The 8101 dual-port 256bx4 RAM is no longer available, but Jameco has a
5101
> which looks like a likely sub. I would forego this type for a larger SRAM,
> but I guess that it too would have to be dual-ported. I need to look at
the
> schematics for an S100 memory board.
>
> Someone on-list did a design for his own clone, but I can't remember who
it
> is. I'm sure that it can be done cheaper than Tom Fischer's IMSAI-2
though.
>
>
> Rich
>
Received on Mon May 22 2000 - 18:40:09 BST