IBM XT diagnostic ROM
ard at p850ug1.demon.co.uk
Thu May 29 14:03:42 CDT 2008
> > What you have found is called a Power On Self Test card, or "POST card".
> It was my understanding that only AT-class machines (by age, not
> necessarily by architecture type) wrote to that port. I've never seen a
> PC or XT (or clone thereof) motherboard/BIOS write to that port.
Well, it's a different port address, but it's clear from the source
listings in the IBM PC/XT Techref that it writes out POSTcodes as the
Some (AT and later) clones used other port addresses, BTW.
Soem time ago there was a design fro a POSTcard in elektor magazine. It
was just a GAL for address decoding (equations not given, but very easy
to work out), an 8 bit latch to hold the POSTcodes as they are output,
and another GAL to decode that and drive a couple of 7 segment displays.
There was a DIP switch to select the IBM XT port, IBM AT port, and IIRC 2
or 3 of the odd clone ports.
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