The title may be a little bit of clickbait, but this very well could be the world's smallest V2 SLI setup. If it worked...
No longer clickbait! This is the World's smallest V2 SLI setup!

Let's start with the hardware. This is a (pcisa-c400r-rs-r20) half-sized single-board computer running a Celeron Mobile 400 Mhz. It's using a compatible (IP-4S2-RS-R40) backplane with 2 PCI slots + 1 ISA slot. The edge connector is PISA standard and carries both ISA and PCI pinouts. The SBC has a CF card slot so I can reduce the footprint by excluding the hard drive. Using the built-in AGP graphics and the 2 x Voodoo 2's, this makes for a tiny but fully capable machine.

So let's get to the issue. While I can get one Voodoo 2 to work in either slot, I can't get both to work in SLI. Both show up in the device manager correctly and both register in the Voodoo display properties window. If I disable SLI then games like Quake seem to run fine on one card while both are still installed. I've tried a few different versions of the 3dfx drivers but the result is always the same. These Voodoo 2's are definitely working as I've tested both on my socket 7 system.

This is what it looks like when SLI is on...

As I said, one card in either slot works fine. I tried something else by installing an ATI PCI graphics card + Voodoo 2 on the backplane. Interestingly, the ATI card works fine in either slot, but when adding the Voodoo 2, it only works if the ATI card is in slot 1. (Slot 1 is the furthest from the SBC). And, the Voodoo 2 shows up in the device manager in either case but only works in Quake when the ATI is in the first position.

I'm kinda stumped and my attention is now turning to the backplane for compatibility reasons.

This is a fully passive backplane using the SBC Northbridge. I'm a little lacking in information here, but the PISA spec as well as the backplane itself lists a hardware PCI address for each slot. It's printed on the PCB but goes like this.

1DEVICE|IDSLE|REQ#|GNT#|INT#
2PCI1|AD19|0|0|A,B,C,D
3PCI2|AD20|1|1|D,A,B,C

The PDF for the PISA spec is here. https://www.kontron.com/download/download?fil … f&product=87221

On section 4.4 Interrupt-Routing on page 16, it clarifies the hardware PCI routing. This doesn't seem to be adjustable and is a property of the physical wiring of the PCI bus on the backplane.

Is it possible this SBC is not capable of handling traffic between both PCI slots at the same time? How could I test this out? Is there some kind of software I can use to check this?

It's strange because a single Voodoo 2 works fine in either slot as well as using an additional graphics card plus one Voodoo 2. However, only if the ATI is in the first slot, otherwise I get a black screen on the Voodoo but the system is still working.

Since either slot can use one Voodoo 2 I have to assume the physical wiring is capable of accessing them. And with two installed, everything appears normal like the device manager and system info on the display window. Once the game starts, one card seems to only render 1/2 of the screen and is offset horribly. I'm not sure it's a resource conflict as the Voodoo 2's don't use an IRQ... I've played with some of the BIOS settings but got the same results which neither advanced nor degraded things. Both Voodoo 2's also show in the BIOS POST screen as "multimedia devices". This is a good sign, but something else is causing an issue when the cards are rendering something.

Any and all advice is welcome. Whether this problem is solved may determine the fate of another project I'm working on so any help would be greatly appreciated.