Commodore 610 & 8088 explanation theory

From: Michał Pleban <>
Date: Mon, 04 Apr 2011 21:18:16 +0200
Message-ID: <>

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:

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!

Having a slower chip provide the signal to the board may very well cause
memory control signals to arrive too late under certain conditions,
which is a perfect explanation for memory corruption problem experienced
under the 8088 board.

There is also U13 which is supposed to be 7486, but is 74LS86 on my
board, which can be a culprit here, though my bet is on U20. You may
check your 610's too to see what chips you have in these places.

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

Michal Pleban

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Received on 2011-04-04 20:00:13

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