Re: Commodore 610 & 8088 explanation theory

From: Ethan Dicks <>
Date: Mon, 4 Apr 2011 16:19:24 -0400
Message-ID: <>
On Mon, Apr 4, 2011 at 3:18 PM, Michał Pleban <> wrote:
> Hello!
> I have been quiet for some time, but I spent that time productively ;-)
> In this blog post, I have coined a probable theory explaining why the
> 8088 board does not work in Commdore 610:

Well sleuthed!

> Basically, according to the schematics, the chip U20 is supposed to be
> 74S86 quad XOR gate (with Schottky diodes). But on my 610 board, it is a
> plain 7864, which is a slower version. Coincidentally, this very chip is
> used to feed the memory RAS signal to the 8088 board!

Interesting.  I recall a similar issue with a 74S74 vs 74LS74 on a
68000 design I worked on in the 1980s (an embedded intelligent serial
card, so nobody here would have seen one unless they were working on
DEC hardware doing IBM Mainframe serial datacomms).

The important point was that our design engineer made a *big* deal
about not subbing a 74LS74 for that particular part.  It could have
been speed, and it also could have been fan-out draw affecting signal
slew.  Either way, S logic is "beefier" (and hotter) than LS logic and
sometimes, you just can't use LS.

A quick check shows me that (sweeping generalizations) S is 3X faster
than LS, but uses 10X the power.

> I will order a 74S86 and try to make a replacement.

I look forward to hearing about your experimentation.  It sounds like
you have good odds for success.


P.S. - if the 74S86 doesn't help, consider trying a 74F86.  F logic
has a reputation for being noisier, but it's approximately the speed
of S logic at half the power consumption (and then there's AS logic -
50% faster than S and 1/2 the power).

       Message was sent through the cbm-hackers mailing list
Received on 2011-04-04 21:00:07

Archive generated by hypermail 2.2.0.