Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!tut.cis.ohio-state.edu!att!dptg!sodium!wfl
From: wfl@sodium.ATT.COM (William Linke)
Newsgroups: comp.misc
Subject: Help Hapless Hardware Hacker
Keywords: PC/XT hardware homebrew
Message-ID: <2930@sodium.ATT.COM>
Date: 7 Aug 90 23:29:44 GMT
Organization: AT&T BL Middletown/Lincroft NJ USA
Lines: 30

Greetings:

I've been looking for the proper newsgroup in which to ask my question,
and none of them seem to be completely appropriate.  But you seem to
be a nice, open-minded group, so here goes!

I designed and built a very simple expansion card for the IBM-PC XT bus,
which decodes a single output port and brings the byte written to it out
to a connector as 8 separate data lines.  It doesn't do any interrupt,
DMA or input stuff.  I tried it in my dual-floppy PC clone and it worked,
to my great excitement.  However, soon thereafter I tried to format some
floppys with the board still in the computer, and was unable to do so.
It turns out that when my board is plugged in, any attempt to write to
a floppy disk will destroy everything on the disk, apparently including
the low-level format data; the disk simply can't be read anymore.  With
my board removed, the PC works properly.  The peculiar thing is that
disks can be *read* just fine with the board in; only writes cause
trouble (any kind of writes; even renaming or removing a file).

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.  Has anyone out there seen a similar problem?  I'd appreciate
any thoughts you might have on what could cause this, or how to approach
tracking it down: I'm not sure where to begin.  (Or, ideas about better
places to ask this question!)  If you reply by email to the address
below, I'll summarize here later.  Thanks,

Bill Linke
att!sodium!wfl
(201) 576-3080 (voice)