I've said this before, and I'll say it again The safest way to power cards using risers #1, using a server power supply and #2 using 6 pin powered riser cards. I use a server power supply like this t… (View Post)
You have to plug in some input into one of the graphics cards. Or use a headless HDMI, like this https://www.amazon.com/CompuLab-4K-Display-Emulator-fit-Headless/dp/B00JKFTYA8 (View Post)