Dual GPU not working on x399 Taichi

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 happyferret wrote:

Hi MisterJ,

Thanks for the reply. I was planning on installing W10 on this machine eventually, so I used this problem as a good reason to buy a Samsung 960Pro 512Gb to install Windows on. The SSD will be here tomorrow and I will try to see if the same issue arises on Windows too, and will report my findings.

In the meantime, I thought of two other reasons that could explain my problem:

1. I use linux, but boot using BIOS legacy mode. I do not know why this could be an issue, and since both cards were recognized and usable some rare times, I do not think that can be the cause, but maybe ?
2. I do not enough 6-pins power cables to power both 1080 Ti AND the motherboard. I ordered one more cable, which should also arrive tomorrow, and I'll try to see if the problem is resolved when I can power both cards and the motherboard, my guts tell me this problem is power-related, but that's just an intuition.

For the cards, they are two different models, here are the details: (both cards have been swapped, and the problem persists in any configuration):

- PNY GeForce GTX 1080 Ti Graphic Card - 1.58 GHz Boost Clock
- GIGABYTE GeForce GTX 1080 Ti Turbo 11GD, GV-N108TTURBO-11GD

Will be back with more updates, thank you all for your help!

Does your PSU have enough modular connections for a third video power cable? I wonder if the second "CPU" power cable connection is for the SOC VRM stage, or possibly that and the DRAM VRM. The problem may be really one of power distribution rather than power capacity.

It is important to let us know if the third cable allows both video cards to operate correctly. If it doesn't that means the problem might be related to the UEFI/BIOS. 

The manual states the board's six pin graphics 12V power connector must be used when four video cards are installed. That is to provide additional power to the PCIe slots, which otherwise only get power from the +12V connectors on the main 24 pin ATX connector. There are only three +12V pins on the 24 pin ATX connector, which is not much and explains the need for the CPU power connectors. I am skeptical that two video cards will need the board's 12V power cable connected, but we shall see. 

You can install Win 10 temporarily without buying a license, just using an ISO download from MSoft. That will work for at least 30 days.

The legacy boot thing with Linux should not be an issue, since the UEFI is configured by default to operate in legacy mode. Video cards don't require UEFI, but IF you UEFI boot the PC, then the video card must support UEFI/GOP. Your 1080s ought to do that, unless both of your manufactures are way behind in their VBIOS, which would be ridiculous IMO.