I suppose you could do that, there are some AT power supplies (usually from OEM systems) with the additional 3.3V power connector, and it is in fact 3 +3.3V and 3 GND. On the other hand, I never had a problem with PCI video cards when leaving that connector unplugged, are you sure that is the problem with your card? Intel claims in their manuals that they added it for "full PCI compliance", but it was never clear to me what that meant.

I've tried two cards now, both of which work in other systems. Plus the beep codes--1 long, 2 short--indicate "No video card found," according to the manual.

TheMobRules wrote on 2020-07-12, 05:32:

I suppose you could do that, there are some AT power supplies (usually from OEM systems) with the additional 3.3V power connector, and it is in fact 3 +3.3V and 3 GND. On the other hand, I never had a problem with PCI video cards when leaving that connector unplugged, are you sure that is the problem with your card? Intel claims in their manuals that they added it for "full PCI compliance", but it was never clear to me what that meant.

from the manual:

PCI 3.3 VOLT CAPABILITIES To maintain strict compliance with the PCI specification, the motherboard provides a connector which c […]
Show full quote

PCI 3.3 VOLT CAPABILITIES
To maintain strict compliance with the PCI specification, the motherboard provides a connector which can be used to
route 3.3 volt power to the PCI slots. The connector may be used with a separate 3.3 volt power supply or with a
custom designed voltage converter. Note: The on-board 3.3 volt regulator provides power for the CPU, PCIset and
L2 cache only, not the PCI slots

That last part makes me think the pci slots are getting no power at all, but this probably isn't so. I wish the manual would be more clear.

My Valuepoint 486 motherboard has the same connector, and I had the same question. The factory PSU didn't have the plug for it and so far there hasn't been any issues with the PCI card.

Well I just got finished rewiring the psu (photo attached) and everything works now. I believe older PCI-based video cards required 5V and will work with older boards without additional power requirements. Whereas some, but not all, newer ones, specifically PCI 2.0-compliant cards, require 3.3V. And many will work with both. I did see another post here on vogons about the Rage XL requiring 3.3V, so I guess that's it. Oh, and I should mention, I did try another newer PCI video card, specifically a TNT2 M64 Dell OEM card. That, too, did not work without the additional 3.3V power.

Nice work ! Yeah my Riva TNT PCI is a 3.3 card iirc. Most 3.3v PCI cards have a second groove on the edge connector. I bet an old S3 Trio64 would have worked w/o the extra power cable as they are 5v PCI (unless the chipset is wired to register Video only if the 3.3v is connected whether the card needs it or not)

Horun wrote on 2020-07-12, 20:53:

Nice work ! Yeah my Riva TNT PCI is a 3.3 card iirc. Most 3.3v PCI cards have a second groove on the edge connector. I bet an old S3 Trio64 would have worked w/o the extra power cable as they are 5v PCI (unless the chipset is wired to register Video only if the 3.3v is connected whether the card needs it or not)

Thank you. Yeah I bet you're right about the S3 Trio64. Unfortunately, I don't have any older cards to try right now. But, I still can't figure out why they made it optional. The manual does mention an onboard 3.3v regulator--why doesn't it also supply power to the PCI slots? I know that the 430fx chipset this board has is a repurposed socket-5-era chipset, which probably predates PCI 2.0.

Short answers: Intel also made the same board labeled Intel Advanced/ATX at the same time, which used an ATX PSU (and ATX back panel) which does supply the 3.3v for PCI. I think it was at that time where some companies were building both AT and ATX of same board during the transition from AT to ATX.
In general the 3.3v regulator on AT boards has always been for CPU support not PCI, and Intel did the design work for that so it explains why it needed an external 3.3v supply.... just my opinion.