Altair parts substitutions

From: Richard Erlacher <richard_at_idcomm.com>
Date: Mon May 22 20:06:51 2000

Please see embedded remarks below.

Dick

----- Original Message -----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Monday, May 22, 2000 6:07 PM
Subject: Re: Altair parts substitutions


> >
> > 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
>
> True...
>
> > element, whereas the 8T97 is a tristate buffer, in this case two sharing
one
>
> However, it is _possible_ to use a '373 as a 3-state buffer. You tie the
> enable to whichever state (high?) makes it transparent (rather than
> holding the last value), apply the inputs to the D pins, put Q to the bus
> and control it with the OE/ line.
>
Yes, but given that there are parts available that are more suitable, as
he's building a new device this time, I'd say it is probably preferable that
a tristate buffer be used instead of a transparent latch. I've seen this
sort of thing, but the advantage in using a '244, for example, is that it,
when used as a receiver, has schmidt-trigger inputs, which yields better
noise immunity, and when used as a driver, sources and sinks, in the CMOS
case, 24mA, while the LS has widely varying parameters, depending on the
maker, but you can rely on the output low (sink) current being at least
twice the output high (source) current.
>
> I don't recomend this (the output drive capability is not that good for
> one thing), but I did use it one in an emergency when I had to make an
> input port for a microcomputer (a TRS-80 model 1 IIRC) in a hurry and I
> didn't have any 3-state buffers in stock.
>
> -tony
>
with CMOS bus buffers you needn't worry so much about the source and sink
current. The CMOS doesn't dc-load the bus, though it does put a bit of
AC-load in the form of capacitance on it. The great thing about AC
terminations is that they put a capcitance in the transient state in
parallel with the load capacitance. I'll let you work out the details, but
it does improve things, as anyone who's hung a 'scope on and watched a few
million transitions will tell you. They're not always textbook perfect, but
VERY close to it!

Dick
Received on Mon May 22 2000 - 20:06:51 BST

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