I'd be looking at voltages.

1) What's "missing" between the slot types.
2) What generates the missing ones in the boards that don't work.

https://www.playtool.com/pages/agpcompat/agp.html

myne wrote on 2025-12-29, 05:16:

I'd be looking at voltages. […]
Show full quote

I'd be looking at voltages.

1) What's "missing" between the slot types.
2) What generates the missing ones in the boards that don't work.

https://www.playtool.com/pages/agpcompat/agp.html

Thanks, that’s an interesting link.

I have run into AGP cards that don’t display a picture on the AGP 3.3 board, but I’ve not run into a board that only works in text mode on the AGP 1.5 board.

I've tried a lot of cards in these three boards and most work fine if they fit the slot, including these two newer generation nvidia cards: Geforce FX 5200 ~Mar 2003 & Geforce 6600 ~Aug 2004 which is bonkers to me because that 6600 has an AGP - PCI Express Bridge.

Here are the cards that have had issues:

MSI Geforce 4200 NV25 ~ Feb 2002
-- Via MP3 boots but produces no video. Tried VGA and DVI outputs.
-- Intel 815 is fine
-- Nforce 2 boots, produces text most of the time, but graphics modes rarely produce a signal, and only for short periods. Tried VGA and DVI outputs.

TNT2 Diamond Viper v770 ~ March 1999
-- Via MP3 is fine
-- Intel 815 is fine
-- Nforce 2 won't boot

ATI All In Wonder 7500 ~ Aug 2001
-- Via MP3 - unstable in Windows under load
-- Intel 815 is fine
-- Nforce 2 won't boot

And of course, any PCI card that requires strong v3.3 is going to be problematic on the Via MP3 , since it only has the P* & P8 AT power connectors.

The problems with the NV25 remind me of when I was trying to get PCI cards newer than 1996 to work in an FIC VIP board and a Gateway BAT3 Anigma. Seems to boot OK, but there's no VGA signal. After a lot of fussing, I assumed that it was BIOS issue, not power related, because I had the auxiliary 3.3V power plugged into the Anigma board.

douglar wrote on Yesterday, 04:41:

myne wrote on 2025-12-29, 05:16:

I'd be looking at voltages. […]
Show full quote

I'd be looking at voltages.

1) What's "missing" between the slot types.
2) What generates the missing ones in the boards that don't work.

https://www.playtool.com/pages/agpcompat/agp.html

Thanks, that’s an interesting link.

I have run into AGP cards that don’t display a picture on the AGP 3.3 board, but I’ve not run into a board that only works in text mode on the AGP 1.5 board.

Only thing I can think of is some weird bios settings. IRQ, pallet snoop, memory hole, anything else plausibly related.
Oh, and force agp 2x just in case