Port 80 POST card in XT \ VOGONS

Port 80 POST card in XT

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First post, by Predator99

Posted on 2019-11-21, 19:57

Predator99 Offline

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I hope somebody can help me in explaining this observation with a China-POST-card.

When sending a byte to port 80 with i.e.
mov al,66
out 80,al
The number "66" is displayed on the card when doing this in a 286+ PC. Thats what it should do.

But when trying the same on a 8088 XT nothing is displayed on the card. Why? For me there is no reason why this shouldnt work. I have tried this in many XTs and 286+. It doesnt work in any XT.

Its clear for me that nothing is displayed on the card during the startup of (most) XTs as its not implemented in BIOS, but this is another topic.

Somebody an idea?

Reply 1 of 5, by derSammler

Posted on 2019-11-21, 20:14

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Afaik, port 80h on XTs is used by something else (DMA registers, I think).

Reply 2 of 5, by Predator99

Posted on 2019-11-21, 20:33

Predator99 Offline

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Thats indeed a good idea!

http://stanislavs.org/helppc/ports.html
080 Manufacturer systems checkpoint port (used during POST)
080-090 DMA Page Registers
081 High order 4 bits of DMA channel 2 address
082 High order 4 bits of DMA channel 3 address
083 High order 4 bits of DMA channel 1 address

But I assume for compatibility reasons this should be the same on a 286?
And even its for DMA the output should be forwarded to the port80-card as well?

Reply 3 of 5, by spark2k06

Posted on 2019-11-22, 07:09

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It could also be related to the clock frequency of the ISA BUS, which will be slower in XT. The Chinese boards I think they have a frequency detection option, being possible to configure it with the tactile switch that they have integrated into the board.

It is possible that they are designed for a fast bus and not working well at low frequencies like those of the XT.

Perhaps it would be interesting to get a simpler design, similar to this:

Build your own ISA POST analysis card?

If the problem were finally the port, the discrete logic of an alleged simple board could be replaced by SPLDs and set it to a free port.

https://www.tindie.com/stores/spark2k06/
https://hackaday.io/spark2k06