Altair parts substitutions

From: Richard Erlacher <richard_at_idcomm.com>
Date: Tue May 23 21:53:19 2000

All this verbage about the i8205 has forced me to dig out an old
(pre-Columbian) data book. IIRC Intel really didn't make any of its TTL
parts, as their fab was dedicated to PMOS, NMOS, and later, CMOS devices.
Therefore I'd suspect that the i8205 (and also the i3205) were strictly TTL,
or, more correctly, schottky TTL. The i8205, by the way, sinks only 10 mA
as opposed to the schottky equivalent, the 'S138, which sinks somewhere
between 16 and 24 mA, depending on whose databook of that day you consult.
This was probably more specsmanship than technology, however.

I do, however, believe that applying a negative voltage lower than -0.6
volts would cause damage. The 1978 datasheet says very little about that.
I'm sure there's a better reference, but I'm too lazy to dig for it in the
basement. I can see no reason why one would wish to apply a negative
voltage to one of these babies, however, since the only parts from the '70's
that needed that negative voltage were the PMOS and a few NMOS memories,
which, admittedly could fail in some mode that might short the Vbb supply to
the select logic, though that's highly unlikely. Parts normally aren't
designed with unusual failure modes of other components taken into
consideration. In any event I'm unable to find any spec indicating the
device will tolerate any signal more negative than a silicon diode forward
voltage below its own GND.

Dick

----- Original Message -----
From: Dwight Elvey <elvey_at_hal.com>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, May 23, 2000 6:06 PM
Subject: Re[2]: Altair parts substitutions


> ard_at_p850ug1.demon.co.uk (Tony Duell) wrote:
> >
> > Now that's interesting. When I got my (second-hand) Intellec MCS8i, it
> > didn't work. One of the faults was soon traced to an 8205 chip on (IIRC)
> > one of the memory boards.
> >
> > At the time I'd never heard of this device (and didn't have a suitable
> > Intel databook), but from the schematics in the manual it appeared to
> > have the same pinout as an 74LS138. So I stuck one of those in and of
> > course it worked fine (and still works AFAIK).
> >
> > I wondered at the time what the difference was between an 8205 and a
> > 74LS138. Now I know...
> >
> > -tony
>
> Hi Tony
> As I recall, the difference was that the Intel parts could
> stand a much lower negative voltage ( -10V? ) than the general purpose
> TTL. This made is compatible with the 8080's. Also, as
> I recall, the 8205 were also Schottky's.
> Dwight
>
Received on Tue May 23 2000 - 21:53:19 BST

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