OK self-update: I did some testing and I believe the card IS used as a passthrough but I think the ebay seller lied - the card is probably NOT a 1MB card after all

I can't find definite proof of this but some of you may be able to tell judging by the chips used. If it's indeed 512kb, I'll need to hunt down a 1MB version ... again.

red_avatar wrote on 2021-02-06, 15:44:

Like the title says, I'm a bit stumped. […]
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Like the title says, I'm a bit stumped.

My on-board chip is a Cirrus Logic one (GD5428) that has a mere 512kb of memory, not enough to run Windows 3.1 in 65k colours which is what I'd like (not for gaming, it's for using an old handscanner I bought).

So I bought a slightly older Cirrus Logic card - the CL-GD5422 - with 1MB of memory. I've tested it in my Pentium 75 before and it works fine but it was too slow compared to the on-board chip (which was a 32bit bus PCI graphics "card") but when I plug it in the IBM (Valuepoint 425SX), it clearly detects it since the BIOS now shows it as the active card and on start up, I see the data in the top left ... but I get a black screen. If I connect it to the old on-board VGA port, I get an image. This is very weird - and clearly the card does not output to the old VGA port either (would that even be possible?) since I still cannot use 65k in Windows (the drivers for the GD5428 and GD5422 seem to be compatible so they work on both).

I know the card is going to be slow but that's not the point of using it - I just want Windows 3.1 to run in true colour mode.

Does anyone have any idea why this would be?

Are there jumpers on the motherboard to disable the onboard VGA ? ISA cards will not passthrough a signal to one another over the ISA bus .

darry wrote on 2021-02-06, 17:21:

Are there jumpers on the motherboard to disable the onboard VGA ? ISA cards will not passthrough a signal to one another over the ISA bus .

There may be - there were a lot of different motherboards used and unlike with my Aptiva and PS/1, I haven't been able to find schematics for the exact same motherboard. Having said that, none of my other IBM motherboards had such a jumper AND the BIOS clearly detects the card, shows it as the active card AND mentions the name upon boot up (top left). This all seems strange behaviour if a jumper would be needed?

EDIT: ignore my post above - I googled similar boards and found that J13 was "Video enable" - changed the jumper and tadaa, it works and detects 1MB. Very odd how it detected the card by name but refused to use it? Thanks for the help.