Can some inputs on a monitor interfere with other inputs?

June 22, 2017

Okay, so this will be the last time I ask about this issue here as other times have been unsuccessful. I have been stressing over it almost the entire first half of this year and it's giving me a headache, I'm fed up.

So here's the issue:

In February I bought a new SSD and wanted to do a UEFI install of Windows 10. All worked great until I found out I could improve my boot time and gain other features by disabling "CSM" in the BIOS.

After disabling CSM however, it brought around a lot of display issues; the main one being that if I ever restarted the PC, once it loaded back into Windows 10, one or two of my monitors would get no signal, they showed up as "disconnected" in display settings, and only my far left monitor (master monitor that shows POST) would be able to achieve a signal. The only way to get the signal back on the other 2 monitors was to pull the power cable and reconnect around 30 seconds later.

After all of this I decided to do a fresh install of Windows 10 with CSM disabled, thinking that it would probably fix something. Thankfully, after reinstalling Windows 10 the displays were a lot more stable and I didn't experience any drops of signal.

But then, a few weeks later it happened again, the two displays dropped and showed up as "disconnected" in Windows Display Settings. But this time I noticed something; the displays would only drop on restart if I had my ElgatoHD60 connected to the PC via USB, this powers the HDMI that is connected to the PS4 and monitor. I tested it around 4 - 5 times and the displays would only drop on restart if the Elgato was connected to the PC. The PS4 didn't even need to be powered on for this to occur.

So I disconnected the PS4 for a week to see how things went. And then for some reason, a few days later, the displays dropped anyway, even with no PS4 connected!

I then reconnected the PS4 to the display and this is when things get even weirder; the displays would only drop if the PS4 was powered on (HDMI active). Before, the displays would only drop if the PS4 was off.

I called it a day and reinstalled Windows 10 with CSM enabled and the issue has since gone away again. Strange how it works fine with CSM enabled.

Here's my thoughts: All 3 of my monitors are connected via mDP. Disabling CSM makes mDP the first cable the GPU looks for instead of DVI. So could the HDMI be interfering with the mDP cable somewhere alongs the lines? This however doesn't explain how my right monitor gets affected though, as that isn't connected to the HDMI at all, just mDP.

So, does anybody know what the deal is here? I'm not expecting any clear straight answers as this is a very strange issue, but I would like a few theories of what could be going on.

Many thanks, would really appreciate any feedback on this issue.

Other important things to note:

When CSM was disabled, the displays would drop signal more if "fast boot" was enabled. If I disabled "fast boot" the displays would drop very rarely, but they did on occasion.

When enabling/disabling CSM I have also encountered other unrelated boot problems such as sometimes the PC failing to boot up and just giving me a slight "beep" every 1 - 2 minutes.