I'm putting this here in case anyone else comes across something similar and needs any ideas. While I'm not yet 100% sure I've completely fixed it I am about 90% sure, so it should probably be documented.

Some time ago I got a new 40 inch 4K monitor, as previously mentioned. It is amazingly good, and indeed I ended up buying another one so I have a matched pair. Which is very nice indeed

To drive these things I needed a card with two displayport outputs and picked up a second hand AMD Radeon 7870 card, which has the required ports as miniDP, along with a dual-link DVI port and an HDMI one, so it can actually run five monitors. The motherboard I had, a Gigabyte one, also had two more ports, DVI and HDMI, so I could if necessary run 7 of the things, although only two 4K ones at full resolution and 60Hz.

The machine was nothing special, the aforementioned gigabyte motherboard, which I'd originally picked due to the fact it had three graphics card PCI-E slots which were required for the three dual-port cards I was running at the time, and a PCI slot for my 8-port RS232 card. In the machine was an intel i5 processor, 16GB of ram, a 1.5TB samsung HDD, an 840EVO 250GB SSD, and a 1kW psu. It was fast enough for my purposes, reliable, and fairly quiet. It was running windows 7 as well.

The problems arose sometime about two months after I got the first monitor. The machine suddenly began becoming reluctant to start. Normally I hibernate the thing when I stop using it, because I have so many programs in operation that even with the SSD restart times are annoying due to needing to reload everything. It's been working fine like that for a couple of years. Yet, with no obvious reason apparent, it wouldn't start up when you pressed the power button.

It would bring up all the fans, ponder life for a few seconds, beep, have another good think, then go dead right down to the power light going out. Then, after a couple of seconds, it would jump back into life and do the same thing. This could repeat anywhere from a dozen times to indefinitely.

Sometimes hitting the reset button was enough to make it sort its life out and get on with it, but more often it wouldn't do anything useful. Turning it off and back on, again, might sometimes jolt it into action, but again, not always. Turning it off at the wall and waiting for thirty seconds, though, pretty much always made it work.

The problem was made more annoying by the fact that it didn't always display this behaviour, only about eight out of ten times. Sometimes it would work fine. And it never did it unless it had been off for several hours, ie overnight, which made testing the various possibilities very time consuming.

I tried everything I could think of to no avail and ended up living with it for a couple of months. Then, one day, right in the middle of a major project of course, it died completely. No way to get it to come up at all.

The end result was I had to put together a new machine in a hurry. The motherboard was a socket 1155 one, which by now (August this year) was sufficiently obsolete that finding a replacement was difficult. I had to bite the bullet and buy not only a new motherboard (a socket 1150 Gigabyte Z97X-Gaming 5), but a new processor and ram to match. I took the opportunity to increase the ram to 32GB and stick in another SSD, a samsumg 850 evo, to avoid the issues that the 840 evo had, which is another story.

I also replaced the PSU, on the basis I didn't trust it, since the problems smelled of a power fault of some sort. It seems to test OK but even so it was worth a try. I retained the large CPU cooler, the USB3 extension adapter, the case, the graphics card, and the serial card, but everything else got replaced.

Once I'd cloned the old SSD onto the new one, booted in safe mode and uninstalled all the hardware drivers, and rebooted a few times with new drivers being added where necessary, I had the machine back in working order with all my data and programs in place.

Life was good.

The machine is faster, just as stable, even quieter, and everything is working nicely. I got the project finished, which ironically provided exactly the right amount of income to cover the machine needed to do it ( ) and was able to continue working.

Right up to the point that the new machine developed the exact same symptoms...

Press power button, fans come on, beep, fans go off, pause, power cycle, repeat. Over and over again.

Turn it off at the wall, wait thirty seconds, turn it back on, it works.

A month later it died completely.

This time I took it completely to pieces and tested the hell out of everything. By the time I'd finished I could prove it did the same thing with a motherboard and processor only, nothing else at all connected. One of them was clearly bad. Replacing the motherboard was cheapest and also most likely to solve it, so I bought an identical one and put the processor and ram in it. It worked fine.

OK, bad motherboard, coincidental fault. The bad board did the same thing with three different power supplies, the power supply out of the machine worked fine on different hardware, so I put it all back together, leaving the serial card out just in case, and got back to work. Amazon replaced the faulty board, so I now had a spare.

A week later...

You guessed it, exact same initial symptoms.

Clearly something else is wrong.

Replacing the PSU with a known good one, the larger cooler with the stock one, and disconnecting absolutely everything not immediately required didn't help. I even tried taking the UPS I have connected to the machine out just in case there was some esoteric AC issue in play, with no luck.

By this point I'm wondering how the case is eating motherboards. Last night I spent hours poking around on google looking for possible similar problems, which I've done several times to no effect, and finally found something that might explain it. But it's not at all obvious.

It's basically all about displayport cables, I suspect. It turns out that there is a weird gotcha with the damn things, which is that a VESA-spec cable should NOT connect pin 20 from the sink (the monitor) to the source (the PC). Most cables don't. However, some cheap ones do, presumably because they didn't read the spec and just, fairly logically, assumed you connected all the pins through.

The problem is that the sink is supposed to provide 3.3V at 500mA on pin 20, as does the source. I assume this is probably meant to drive an active cable or something similar, but if it's connected straight through, what ends up happening is that the monitor is trying to power the PC's 3.3V rail, which is never going to end well.

I checked with a meter and sure enough, one of the two cables was indeed providing 3.3v on pin 20 at the PC end when it was unplugged from the PC itself.

In my case it seems to be slowly killing something in the power circuitry, quite possibly by reverse-polarising a capacitor or something along those lines. It starts off with this refusal to boot properly, for some reason I'm not entirely sure about, and eventually progresses to a dead motherboard. It's the first actually measurable and testable potential problem that might explain the difficulties I've been having and I'm very hopeful it's the root cause of the issue.

Another indicator that it's doing something is that I noticed when scrabbling around in the dark under the bench that one of the LEDs on the back of the graphics card stays dimly illuminated when that cable is plugged in and the monitor is on, which is proof that the power is in fact going somewhere it shouldn't. Unplugging the cable makes the LED go out.

I replaced the cable with the spare I had which was identical to the other one that doesn't exhibit this issue, double checked there was no voltage present, and turned everything off. This morning when I turned it on, it all works fine.

It will take some time to be reasonably certain I've fixed it, and there's always the possibility that either the graphics card or motherboard are now damaged, but early indications are promising.

Anyway, if you have weird problems with a PC and you've got displayport cables connected to it, it might be worth checking them.

pca

When you turned the computer off 'at the wall' during diagnostics was this also powering down the display that had the through-connected DisplayPort pin-20 cable?

So when the rig was plugged back in both the display and the computer were powered up together?

Interesting that the video card implementation does not protect itself from an out of spec DisplayPort cable. An isolated/separate 3.3 volt feed to the DisplayPort cable would seem prudent. Running an internal system power bus to anywhere outside the computer case always feels risky to me.

This is good info to know Patrick, I'll keep it in mind, thanks!

Interesting. Certainly sounds like an issue but as you say whether it's *the* issue is the question. Also interesting that the same item killed multiple motherboards in similar ways even though they were different (well two were)

I also bought one of those Philips UHD monitors after your review although it seems it's not really a monitor issue. If it takes out my laptop though that's not going to be a good thing (or maybe it will since I can upgrade )

I don't usually turn my laptop off though when it's plugged in. I have had the odd seemingly handshaking type issues when plugging it in hot.

Wow. Incredible sleuthing.

I hear you can become an overnight sensation by reviewing out-of-spec cables...

As I'm now staring at the "CableMatters" mini-DP-to-DP cable between my MacPro and my 40" 4K monitor, and the CableMatters USB-C cable was on the "fail" list.

So, yeah, where do I get a properly compliant mini-DP-to-DP cable? Am I going to fry my MacPro? That would be a barrel of monkeys when I tried to exercise the 3 year AppleCare warranty.

Basically, yes. Initially it was via a UPS so it involved turning the output of that off, which was driving the main machine, one of the monitors, which was the one with the problematic cable, the phone, and an ethernet switch.

Since I've pulled it out everything is plugged directly into a wall outlet. I'll have to put the UPS back in a few days, it's there for a reason

And I agree, there should be a blocking diode or FET or something along those lines at the very least. But, what can you expect from some fly by night company like AMD...

pca

Well, if it helps, the cables that DON'T connect pin 20 came from here while the cable that DOES connect pin 20 came from here.

They're just generic ones, of course, but quite high quality ones. Aside from the wiring being wrong on the second one...

You'd think that any high end expensive one would be right, but life doesn't necessarily work that way.

pca

Apple are usually pretty good at thinking about these things, so I wonder whether they've got protection against this built-in.

If you have a multimeter on hand I suggest doing a continuity check between PIN number 20 on each end of the existing DisplayPort cable.

If there is NO connection between the two ends on pin 20 then the cable is not a risk, at least in terms of this 3.3 volt DisplayPort cable back feeding problem reported by pca.

Displayport pinout

From 2012;
New Theory – displayport cables are not in spec

The broader issue is that cheating on cable specifications seems to be a real thing across all sorts of cable types. Many cable specifcation efforts (standards bodies) apparently allow the manufacturers to self-certify their products. And almost nobody is checking up on them.

I bought a 1000 foot spool of CAT5e outdoor rated cable last year that just does not like running at 1 gigabit speeds. 100mb works just fine, but of course that was not what I wanted. Cable was fully installed (4 runs) before the problem was noticed.

Got it. I'll bring my multimeter to work and see if I can manage to probe those teeny tiny little connector pins.

Ouch. Seems incredibly unlikely, too. Are you 100% sure the end connectors were attached correctly (the right wires going to the correct pins) ?

... unless all 1000 feet are being used in a single run...

What's the brand of that cat 5e cable? Sometimes I wonder if the cat 5 stuff is just cat 3 and they changed what the printer prints.

I am rarely 100% certain of anything

95% in this case. I tested the connections and all had the correct pin sequence. All four runs were in parallel between the same two rooms so I could A-B compare them. I did not have the full cable verification gear with me, so just used a simple LED cable pin sequence tester.

I have since sold off the remainder of the spool as outdoor '100mb' cable.

This was a residence. I strung a temporary regular network cable through the interior and it ran happily at gigabit speed. The outdoor cable was run around the exterior walls to get from a second floor office to a main floor office, with various equipment in each room.

It is possible I was having a senior moment and managed to incorrectly but consistently wire all eight cable ends. There was a time when 100 Mbit wiring was plenty, but now we have actual Internet ISP speeds faster than that.

Edit: The cable was actually marked as CAT6, not CAT5e.

I seem to recall that CAT6 doesn't like going round corners. Is it possible that you stressed it too much?

It is possible. There is a constrained space where all the cables needed to make a 90 degree turn. I was as gentle as I could be given the cable location.

That said, the outdoor rated cable jacket is rather stiff so it mechanically imposes a minimum turn radius of sorts. I do not recall the exact radius those cables ended up at, but it was only a few inches. Cable lengths were about 80 feet each.

Just a quick update on this, the machine has been working perfectly for the last few days with none of the previous symptoms. Bearing in mind how often it was happening, I'm fairly sure it's now fixed.

Many happies for me!

pca