The propagation delays cited for the three popular bipolar technologies in
which the '138 was offered seem to vary pretty widely. My experience has
been that these spec's reflect worst-case conditions: conditions more
frequently encountered then than now, since it was likely a designer in '76
or so would be using two levels of these decoders on a memory board, one to
select a bunch of memories, and one to select a bunch of '138's that
selected the memories. Back when small (1K-bit) memories were the most
common sort, there were lots of chips expecting to see that select strobe,
and the worst case load made the prop-delay long.
When there's only a single 2764 to drive, the difference between the three
available technologies is much smaller. A fairly typical memory
configuration might have been a bank of 72 2102's (they liked parity back
then) and about eight 1702's. One 138 would drive each of the rows of RAMs,
while another drove each of the 8 1702's. That's lots of capacitance, not
to mention a fair DC load. I'd say the difference between STTL and standard
TTL would have shown up there in the form of a difference of maybe 15-25 ns.
Due to the size of memory devices back then, the decoding tree would have
been pretty long, therefore combining the delay through several decoders.
This would probably have been a depth of two in the case of the memory
layout I suggested. 2102's back then had a typical access time of 600 ns
and 1702's were somewhere between 750 and 1000 nanoseconds. I don't think
one could claim that 50 ns is negligible with a 2 MHz 8080 under those
conditions.
Dick
----- Original Message -----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, May 23, 2000 7:19 PM
Subject: Re: Re[2]: Altair parts substitutions
>
> > TTL. This made is compatible with the 8080's. Also, as
>
> _Apart_ from the clock lines (and power supplies :-)), I thought all pins
> on the 8080 were at standard TTL levels. Certainly the address bus was,
> which is where you'd be most likely to use a 3-8 decoder.
>
> > I recall, the 8205 were also Schottky's.
>
> Possibly, But whether it's a 74138, 74S138 or 74LS138 makes little
> difference on the average 8080 system memory board...
>
> > Dwight
> >
> >
>
> -tony
>
Received on Tue May 23 2000 - 22:07:49 BST