29 May
2008
29 May
'08
12:03 p.m.
> 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
tests progress.
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.
-tony