tl;dr: I was using non-spec DisplayPort cables and it was causing my computer to reboot during POST. Buy this cable.

Last summer, I treated myself to a new graphics card – the XFX RX 580. I wanted to leave the Nvidia world after hearing that the driver experience under Linux was just so much better with AMD.

I specifically wanted to run DisplayPort cables, because they’re, you know, the future and all. They’re much thinner, easier to run and manage, and they don’t have those annoying screws from the DVI or VGA era.

Soon after installing my new graphics card, I noticed that my computer would reboot just a few seconds after turning it on, almost before the screen even woke up. And this behavior was sporadic… Sometimes it would happen once, and then the computer would work just fine. Other times it would do this reboot cycle twice, very rarely 3 times, and then it would boot just fine. And other times it would boot totally normally.

Google searches for this symptom “computer reboots during post” were not helpful. Everything I read, and everyone I talked to, said that the power supply was to blame. Perhaps it didn’t have enough juice to power the card during boot, a normally very power-intensive time? But the computer would never reset during intense gaming, when the system was under heavy load.

I brought my computer into a different room to hook it up to our wall-mounted TV (over HDMI) and see what the quality was like, and I noticed that the system would boot just fine. I wondered if maybe something was loose in the case, and after moving the system something settled into place? But the moment I took the computer back to my office, it would start acting up again.

I ordered a new PSU, thinking that would solve the issue. After installing it, the system booted once or twice normally, and then the symptom happened again. Reboot right in the middle of POST.

Frustrated doesn’t begin to explain my mood when that happened. I had just wasted $80 on a new PSU I didn’t need. I unplugged the PSU from the wall power, went to go take a quick walk, and when I came back I noticed that the power LED on the graphics card was on.

“That’s funny,” I thought to myself… I know that PSU capacitors can sometimes hold a charge and power tiny things like LEDs for a few seconds, but minutes had gone by.

When I got down on the floor to look inside the case, I saw something moving.

The case fan was spinning.

The freaking case fan was spinning. And I was holding the unplugged power cable in my hand. How is this even possible?!

I unplugged all of the PSU cords from my motherboard, and things shut down. The GPU light turned off, and the fan died.

The next day, I returned to Google, and for the hell of it, searched “computer is on even when unplugged.” I know, it sounds ridiculous, but it lead me to the answer I had been looking for (in addition to a funny meme of The Pope casting holy water for someone with a similar issue).

I scrolled, and scrolled, pages and pages and URL after URL of articles on something called The DisplayPort Pin 20 problem started showing up. It felt so funny to search for months with no solution and then to almost be assaulted by the number of posts on the same issue I was having.

It turns out that cable manufacturers who don’t adhere directly to the official DisplayPort spec end up connecting the 20th pin in the cable on both sides. That pin, carries – you guessed it – power!

This issue was serious. Enough power had been backflowing from my monitors into my GPU to run my case fans when the system was off.

I was stunned. This problem seemed to violate what I know of the computer hardware world, something I’ve been heavily involved with for 30 years. As far as I’m concerned, video cables carry video signals. Maybe a little power for signaling something here or there, but not power like this. I have a working assumption that cables are standardized. DisplayPort is DisplayPort. There shouldn’t be any issues like this in a modern world. Boy, did I receive a rude awakening on this one.

An inexpensive cable is typically just made with poor craftsmanship or out of substandard materials. Inexpensive cables won’t be as durable. Those are things I knew to be true prior to just a few days ago. But now I know, at least for DisplayPort cables, an inexpensive cable now means it’s liable to be made without a full understanding of a nuanced connector pinout and can lead to dangerous current being backfed into expensive and sensitive equipment.

The articles and forums online pointed to high quality cables from Accell. It turns out I had actually been using one in my computer already, as I ordered a long one to reach my third monitor. When I unplugged the other cheap cables, and cursed loudly as I tossed them into the trash, my computer booted perfectly. And that’s why it booted perfectly each and every time over HDMI while testing outputting to the TV… No crappy DisplayPort cables.

I quickly ordered two more cables, and when they arrived, I plugged them in, and booted my computer a few dozen times. Perfect boots, every time.

I never would have thought in a million years that this issue was caused by a goddamn video cable. And now I have this information burned into my mind because of how long it took to uncover the true, underlying cause.