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From: jhs@MITRE-BEDFORD.ARPA.UUCP
Newsgroups: comp.sys.atari.8bit
Subject: Re: 800xl expansion port
Message-ID: <8701232151.AA01082@mitre-bedford.ARPA>
Date: Fri, 23-Jan-87 16:51:45 EST
Article-I.D.: mitre-be.8701232151.AA01082
Posted: Fri Jan 23 16:51:45 1987
Date-Received: Sat, 24-Jan-87 06:25:31 EST
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The expansion port is basically a direct connection to the 6502 processor bus.
A couple of additional signals are also present, like cassette audio in/out
(?).  There is also a line called "External Select" which has to do with
whether external devices or ones in the Atari console get the bus.  I am not
certain how this works but suspect that it is supposed to be generated in the
external device to let it pre-empt the bus when it feels like it.  That would
seem to give the best flexibility for expansion.

Mapping The Atari, 1985 edition, contains a fairly complete description of the
pinouts in one of the appendices.  Note that the connector is a perfectly
standard 50-pin card-edge connector, but the pin numbering used by Atari and
reproduced in Mapping... is not standard.  So be careful with pin numbering,
especially if you use a standard connector instead of one purchased from
Atari.  The relationship between the two numbering schemes is simple, so there
is no problem using a standard connector and making the translation.

I am not at home now so don't have detailed information readily at hand.

-John Sangster
jhs@mitre-bedford.arpa