DisplayPort Pin 20 Issue
Computer won’t power on with DisplayPort cable connected?
I came across a weird problem with my PC the other day. Every time I would try to power on my PC, I would press the power button but nothing seemed to happen. After a good amount of troubleshooting, I found that my system would only power on if I unplugged my DisplayPort cable from my graphics card. I tested this issue with 3 different graphics cards; an ATI Radeon 5850, an AMD Radeon R7 360 2GB and an AMD Radeon R9 380X 4GB. The issue occurred on all three cards, with the only difference being that, on the Radeon 5850, the fans would spin up indicating that the card was getting power, but my computer would not go through the standard POST (boot-up) procedure.
My monitors are both Dell P2415Q 4K 24-inch displays and I generally have both monitors connected (one via HDMI, one via Mini-DisplayPort to DisplayPort) using AmazonBasics HDMI cable and a Cable Matters Mini-DisplayPort to DisplayPort cable. Those haven’t changed for the past few months, but just recently I upgraded my motherboard to an ASUS B150-PLUS D3 that has an over-volt or anti-surge protection system in it, where the system will not power on if there is a surge from the power supply or another electrical source, so I was immediately led to believe the issue I was experiencing had something to do with power and not some other problem.
How to fix the problem
After a bit of research, reading through the long and boring IEEE DisplayPort Specification I found that the specification called for a non-powered connector, but most DisplayPort connectors are considered “active” (or powered) connectors. I found that to be a little odd, so I checked with a multimeter and sure enough, Pin 20 on the DisplayPort connector is supplying +3.3V, presumably, from the monitor power, back into my video card. On the Radeon 5850, that doesn’t have any form of protection against that power feed, it causes the fans to spin up to full power, essentially shocking them to life, whereas on the other two cards, they simply don’t power on and end up blocking the POST procedure on the motherboard due to a “device error”.
So, with that in mind, I set out to fix the actual issue. A bit of reading on various forums and a few hours of contemplating the proper way to resolve the issue and I found that, simply covering the 20th pin on the Male-end of the DisplayPort Cable (the one that connects to the video card) with a thin piece of tape, or some other non-conductive material will prevent the electrical surges into your graphics card and all of your problems will be fixed. That assumes, of course, that having the additional +3.3V of electricity being fed into your graphics card has not caused some sort of permanent, irreparable damage along the way. Only time will tell I suppose.
Alternatively, of course, you could always buy a DisplayPort cable with only 19-pins (DisplayPort 1.2 VESA Specification) such as the Rankie R1104 and not have to attempt any manual fixes yourself. I can understand not wanting to pay $9 for another cable though, so if you have the tape laying around, I’d say you give it a shot before spending any additional money.
I hope this saves you a bit of time and hassle if you’re experiencing similar issues with your computer not powering on while a displayport cable is connected to your AMD or nVidia PCIe Video Card.