Strange behavior from an older motherboard

[grape_tectonics]

I've had 2 rx470's mining in my old 965P-DS4 for quite a while now. Since one of my other motherboards crapped out and is now in warranty, I needed to move its 580's into other rigs. This motherboard is acting strange though.

Basically, it has 5 pcie slots total - 2 of them are 16x and 3 of them are 1x. It has no problems with any video card/riser combo as long as I'm either using the 16x slots exclusively or the 1x slots exclusively, I can't use all of them at the same time. Essentially, as soon as anything is plugged into the 16x slots, the 1x slots stop identifying video cards.

I can plug other stuff in the 1x slots and have them identified, it had no problem running 2 video cards in 16x slots, a wifi card, usb 3.0 card and a sound card in the 1x slots at the same time.

When I switch it up and fill the 1x slots with video cards though, if there is anything in the 16x slots (not just video cards), it won't even boot.

Looking around the BIOS, the only thing associated with PCI-e is its frequency, I can overclock it but I left it on default. Also should mention that the board only supports PCI-e 1.0 and is running the latest bios update.

The type of risers I'm using are the externally powered USB ones, version 006S and are perfectly functional as far as I can tell. I'm testing with a corsair ax1200i so power isn't an issue either.

Anybody else experience something like this?

[grape_tectonics]

Ok, figured it out. There's a really tiny (like literally) footnote in the motherboards manual that says the bottom 16x slot shares its bus with the 1x slots. If there is anything plugged into there then each PCI-e lane it uses shuts off one of the 1x slots.

Armed with this knowledge I could get 4 cards working in that motherboard. Maybe it could do 5 but I'm out of risers and really not in the mood to fuck around with it any further.