Troubleshooting a KVM/Mac Pro setup - power cycling and other oddities

Hi all!

I’ve been hunting around for a few days now as to exactly what the issue is and why it’s happening and the only hints I’ve had towards a solution have come from here. I figured I’d take a shot and see if anyone here can shed any more light on the situation.

I have three machines: two 2013 trash can Mac Pros and a 2010 cheesegrater Mac Pro. My primary machine is one of the trash cans, running MacOS Monterey; the other trash can is a work machine running MacOS Catalina, while the cheesegrater Mac Pro is only running Windows 10 (there’s no MacOS installation on it at all) - I use this machine for gaming so it has a GTX 980 in it instead of the old Radeon it shipped with.

I have two monitors: a Thunderbolt display and an LED Cinema Display, both 27".

I bought a cheapie hardware DisplayPort KVM off AliExpress with the idea of sharing the LED Cinema Display between the three machines when needed (I know the solution may boil down to “get a better KVM”, but hear me out first).

The setup is currently this:
Main Trash Can > Mini DisplayPort to DisplayPort Cable > KVM
Work Trash Can > Mini DisplayPort to DisplayPort Cable > KVM
Cheesegrater > DisplayPort to DisplayPort Cable > KVM

KVM > DisplayPort to Mini DisplayPort adaptor > LED Cinema Display

And now we run smack bang into problems.

On the two trash cans, the monitor will power cycle every ten seconds (I timed it; it’s consistent) - the monitor switches off, causing MacOS to re-arrange itself due to a disconnected display. It then turns back on, with MacOS doing its usual thing.

On the Windows machine, it works perfectly. DisplayPort cable directly out of the GTX 980 into the KVM works without any issues.

Now here’s where it gets even weirder.

I figured that there’s some kind of power issue with the trash cans outputting out of Thunderbolt 2/Mini DisplayPort so I bought the Cable Matters DisplayPort booster. If I run this config:

KVM > DisplayPort booster > LED Cinema Display

…and then switch from the cheesegrater booted and running with Windows back to one of the trash cans, the display power cycles a couple of times and then stays on. If the displays go to sleep, however, waking from sleep will resume the ten second power cycle behaviour. Similarly, if I switch from one trash can to the other, it will power cycle. If I remove the booster and try the same action, it doesn’t work (so the booster is certainly doing something).

I’ve tried multiple configurations (based on the cabling and hardware I have available) and cannot get a fully operational experience. My personal trash can has an eGPU attached to it - I tried running an output directly from the RX 580 in the eGPU enclosure into the KVM and it’s the same power cycling behaviour.

So here’s where I stand. Things kind of work, provided I have the cheesegrater booted up anytime I want to use the KVM. I can certainly throw money at the problem for a Level1Techs KVM, but it would be a fairly pricey endeavour without a solid idea of what the actual problem is or whether a new KVM would work.

Does anyone have any idea why this is happening? Or why the setup kind of works sometimes? Would certainly appreciate any light that can be shed on it!

Thanks all. :slight_smile:

Update: Seems that the display will stabilise without having the cheesegrater booted. The Cable Matters DisplayPort booster is definitely doing something as removing or changing its configuration (i.e. changing it to Trash Can > Booster > KVM) will break the setup - it won’t stabilise the display.

Maybe I need a stronger booster coming out of the KVM and into the monitor?

The easiest and cheapest test would be to switch out or run other display port cables. Lots of them aren’t build to spec and it shows in edge cases (KVM use or converters).

Apple has more issues then that when it comes to DP.

I feel like wendell usually has the booster/repeater before the kvm when dealing with mac.

@Dratatoo I’ll try and get my hands on some alternate cables and see if it makes a difference.

@mutation666 I definitely tried it before the KVM but it just wouldn’t stabilise at all. With the current setup of it after the KVM, switching between the two trash cans results in some instability for half a minute or so and then works. An odd one, to be sure!

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