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From: ardai@bass.bu.edu (Michael Ardai)
Newsgroups: comp.misc
Subject: Re: Help Hapless Hardware Hacker
Keywords: PC/XT hardware homebrew
Message-ID: <62399@bu.edu.bu.edu>
Date: 11 Aug 90 01:12:39 GMT
References: <2930@sodium.ATT.COM>
Sender: news@bu.edu.bu.edu
Organization: Boston University
Lines: 31

In article <2930@sodium.ATT.COM> wfl@sodium.ATT.COM (William Linke) writes:
-I designed and built a very simple expansion card for the IBM-PC XT bus,

[... It is an 8 bit output port, and it frotzes his floppy drive ]

-The board decodes 12 bits of I/O address.  The I/O port address used
-was 0x250, but I also tried changing it to a few other values to no
-avail.

All 12 bits???  There are only 10 bits used for I/O addressing.

0x250 is listed as 'reserved' in my old copy of the XT tech ref.
A better choice would be 0x300 - 0x31f (Prototype card), 0x380 -
0x38F (SDLC and 2nd Bysynch) or 0x3A0 - 0x3AF (1st bysynch).

Your address decoder should decode A0 - A9, AEN*, and IOR*/IOW*.
IOR* can be used to control the direction of an LS245 if you want 
to make a bidir port.  For example, the prototype card is selected by
((not A5) and (not A6) and (not A7) and (not A8) and A9 and (not AEN*))
with A0 - A4 available for on-board registers.  One nice device to use
for address decoding is the LS138 3-8 decoder with 2 active HI and one
active LO enables.

-Bill Linke

/mike


\|/ Michael L. Ardai     ardai@bu-pub.bu.edu   
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