Double GTX 780 in ASUS P7P55D-E PRO not working

3v3stuff · May 5, 2018

I'm trying to run two GTX 780s with ASUS P7P55D-E PRO motherboard, and I'm stuck.
MB manual says that single video card should run at x16, and double should take x8 + x8.

When I connect both video cards directly to MB only first video card is detected, and it runs x16. Second 780 is powered (at least logo is lighted and fan is rotating), but not detected by the system.
When I connect first video card directly to MB, and using PCI-E x1 to x16 riser for the second one - both video cards are detected, but first one is running at x8, and the second one is running at x1 (obviously).

As far as I can tell, ASUS just don't bothered to provide power lines to second part of x16 socket, so my second video card when connected to second PCI-E x16 slot get power only via x8 lines and additional power cables.
So the question is - is there anything I missing, will it be enough for SLI setup to have first videocard on x8 and second on x1 PCI-E lines, or is it somehow possible to make video card work in second slot?

PSU is obliously not an issue, as I can run both video cards via riser.
UPD.: BIOS is updated to last version.

HW Specs:
Video - 2x GTX 780 Zotac
MB - ASUS P7P55D-E PRO
CPU - Xeon X3460 (OC to 3.6GHz)
RAM - 16Gb DDR3 (4x4Gb)

Lutfij · May 5, 2018

How did you rule out that the PSU is not the issue? How old is it? Which BIOS for your motherboard are you on? You could try and reflash the BIOS if your hardware(PSU) isn't the issue.

3v3stuff · May 5, 2018

Lutfij :

I've run exact same everything (GPUs, HDDs, RAM, CPU and PSU) on another MB, which unfortunately does not support SLI and has no second PCI-E x16 slot, so I used second video card via x1 to x16 riser. I suppose it's solid evidence that PSU is trustable.
I also stress-tested this system, so I'm sure it's not PSU problem. After all, it works perfectly (without SLI) using PCI-E riser, which provides basically 1 PCI-E x1 extension with additional power supply for non-connected pins in x16 socket.
And yes, I've tried to work with different BIOS versions, currently I'm on the latest one, 1703, so I assume it cannot be issue too. Maybe I'm missing some BIOS settings for this exact scenario, but I've checked it multiple times.
So my most obvious assumption is that all GPUs I've tested (780s and 1060) are required to be connected to all power and ground pins of PCI-E x16 socket (even with external power lines), no mater what data pins they use. And as I told earlier, on this MB pins connected only up to PCI-E x8, so no pins in half of the x16 socket.

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Double GTX 780 in ASUS P7P55D-E PRO not working

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