DisplayPort KVM + LG DualUp flickers, loss of connectivity

perry-mitchell · June 10, 2024, 3:25pm

KVM Model:

Combo USB-C power delivery & Dual-monitor Displayport 1.4 KVM

USB Interface (5-gigabit or 10-gigbit):

5

Problem:

I’m connecting both my Macbook M3 (14") and my PC (dual displayport) to 2 display-port monitors: A Samsung UE28E590 and an LG DualUp. The PC uses DP → DP and DP->USB converter (from L1T) but the Mac has 1 USB-C out and another USB-C->DP cable.

Depending on how I switch the cables around, the DualUp will either not display anything (flickering on and off and disappearing from devices in macOS) or flicker periodically (or not come up at all after sleep).

I’ve systematically tested all the cables. The original DP port cables out of the KVM have always worked (they’re about 3M in length), and the USB-C → DP cable ( CHENLENIC USB C to Displayport 4K Fibre Optic Cable) works fine (no flickering etc.) if plugged directly into either monitor. No matter which way around I rotate the cables (either at the KVM output or Mac output), only the DualUp is affected.

I found that the issues were slightly less pronounced at the 1280x1440 (HiDPI) setting rather than the native 2560x2880 resolution, but I have no idea if that indicates something or not.

Other Notes:

Not sure what to try next… The KVM and cables are obviously quite expensive and I’d hate to have to drop the KVM from my workflow, but at the moment it seems the common denominator in terms of the issues.

EDIT: See diagram in comments.

wendell · June 10, 2024, 3:35pm

its counterintuitive but the cables go in PAIRS not individually.

Think of it this way: What your GPU sees is one long signal corridor with the combined properties of both cables on either side of the kvm. testing cables individually is not diagnostically useful because if you have two cables that are barely good enough the additive losses of both cables are below threshold and then it doesn’t work.

The L1T DP USB converter?
28MQ780 > while this has a USBc input… why use that? Why not use the native DP input on this display? The usbc input on this display is meant for a laptop to charge it kind of a deal.
UE28E590 > This is a displayport monitor, 4k/60hz.

You could try U2 mode if you really need to use the L1T USB+DP > USBc adapter… but it shouldn’t be necessary?

Charles_VT · June 10, 2024, 3:35pm

Have you tried plugging the type A cable into a dedicated wall charger for stable 5v

Level1_Amber · June 10, 2024, 3:44pm

Why are you not using DisplayPort here? Can you give me a better description as to where everything is connected to?

perry-mitchell · June 10, 2024, 6:31pm

Because the dual monitor KVM only has 1 DP port per computer… the other is a USB-C port. Apologies if I wasn’t clear… Below is my entire connection setup:

PC:
  DP -> KVM -> Monitor
  DP -> DP/USB-C converter -> KVM -> monitor

Mac:
  USB-C -> KVM -> monitor
  USB-C (to DP) -> KVM -> monitor

So to be clear here, both monitors work perfectly on the PC setup. The Samsung monitor, on the Mac setup, works perfectly. The LG, on the Mac setup, either glitches out of existence or flickers continuously (minor) and fails to work after sleep. On the mac side switching the cables only affects the LG, not the Samsung (Samsung continues to work well).

Both of the USB-C cables, from the Mac, are between 4.8-5M in length. The USB-C to DP is fibre optic.

perry-mitchell · June 10, 2024, 6:33pm

I’m not sure I follow - The KVM doesn’t need USB power, it has a dedicated plug, which is connected to power.

The DP->USB-C converter can take additional power, but it doesn’t need it (it gets power from the USB-B cable I’m using) and it isn’t on the problematic path (it’s used for the PC, which is working fine).

Level1_Amber · June 10, 2024, 6:46pm

Hmmmmm ok let me see if I can explain why we’re a tad confused. You put in your description, two monitor dp 1.4 kvm, and 10 gigabit as the interface. So, that would lead us to think you’re talking about this KVM:

^ it doesn’t make sense to convert like you are.

Are you meaning you have this KVM?

perry-mitchell · June 10, 2024, 6:48pm

its counterintuitive but the cables go in PAIRS not individually.

Thanks for the detailed description! I didn’t know this. I’ll keep it in mind going forward.

The L1T DP USB converter?

Yes

28MQ780 > while this has a USBc input… why use that?

I think I’ve been unclear, apologies for that - I’m not using the USB-C on the LG. I’m connected, from the Mac to the monitors, via a USB-C cable to the KVM USB-C port and another USB-C-to-Displayport cable to the KVM.

I’ve attached a diagram in case it helps:

You could try U2 mode if you really need to use the L1T USB+DP > USBc adapter… but it shouldn’t be necessary?

I’ve had it on the U2 mode the whole time, but I will try a whole range of different configurations again shortly, to see if I can see some kind of pattern. Currently it just looks like the Mac isn’t happy running the DualUp via the KVM.

Thanks for taking the time to reply! Means a lot.

perry-mitchell · June 10, 2024, 6:50pm

You’re right! Sorry… It is the combo USB-C power variant.

Level1_Amber · June 10, 2024, 6:52pm

Ooooh ok, yeah, that makes all the difference xD FYI though that KVM is 5 gigabit, not 10 gigabit. Now that I understand lemme think on it a moment haha

wendell · June 10, 2024, 6:56pm

okay, so try u2 mode on both the kvm and converter, and make sure the samsung is the one you are using in the usbc row, since its needs are… ahem… more modest.

then the lg dual up usbc to DP and native dp to DP has a much higher chance of working,

Level1_Amber · June 10, 2024, 6:57pm

Can you tell me what kind of DisplayPort cables you have? and what length?

perry-mitchell · June 10, 2024, 7:07pm

Ok, check - U2 mode on both converter and KVM. I plugged in one at a time, and made sure the USB-C only line is the Samsung. Works fine as per usual.

Plugged in the USB-C-DP cable going to the DualUp, and it comes on immediately, then flickers. You can see when it’s on that it’s artifacting a ton at the pixel level.

I recorded a video here: youtube dot com/shorts/cBZL9HT_0AQ

perry-mitchell · June 10, 2024, 7:12pm

Sure, let’s see if I find them all

Mac: USB-C to DP - CHENLENIC USB C to Displayport 4K Fibre Optic Cable @ 10M
Mac: USB-C - Silkland USB C 3.2 Gen 2 all-in-one @ 5M
KVM to monitors (same cables): CSL 4K displayport cable @ 5M

The oldest part of my setup are the CSL DP cables, and are DP 1.2 unlike the others. All cables are from Amazon.

wendell · June 10, 2024, 7:14pm

that’s classic noise, so it’s picking up noise from somewhere. Maybe the fiber cable expects to be at the monitor and not pushing signal another XYZ feet through kvm + another DP cable.

Do you have a shorter cable matters or moshi brand usbc to dp cable you can try? or another short DP cable you can try on the other side of the kvm?

usbc 2 dp > kvm > dp > dual up should work fine no problem.

perry-mitchell · June 10, 2024, 7:17pm

that’s classic noise, so it’s picking up noise from somewhere

Hmm… Unfortunately no other cable, and spent a bit extra on this one to try and get something marginally better than the other junk on Amazon. Only requirement was the 4-5M length I need, but 10 was the min on this particular cable. Not many others suit that requirement.

Couldn’t find any Moshi cables available to ship to Finland, where I’m based. I’ll happily try another cable as it’s the cheapest option here. Any idea on a decent “next-step” cable available in Europe? Or any guidelines I should follow if I need to buy off-brand? I’ll pick one up tonight/tomorrow.

EDIT: I’ve seen plenty of Cable Matters USB-C to DisplayPort dongles, but I suppose the extra connection might also introduce some noise or cause some other issues potentially?

EDIT 2: I might try to move the Macbook far closer to the KVM, I think, and go with something like this @ 1.8M: “Cable Matters Premium Braided USB C DisplayPort Cable 8K 60Hz 1.8 m (USB C to DisplayPort Cable, USB C to DisplayPort 1.4) with 4K @ 120Hz in Grey - Thunderbolt 4 / USB 4 Compatible”.

perry-mitchell · June 11, 2024, 12:23pm

I might try replacing all the cables between the Mac and the monitors. The existing DP cables from the KVM to the screens are thin DP 1.2 cables, 5M, which seem to work but at the moment I don’t trust anything.

I’m going to stick to 2M cables from the Mac to the KVM, in case the distance is an issue somehow.

I’ll start with the Mac->KVM first, to reduce cost… and I was thinking of getting these:

Does this sound logical?

Level1_Amber · June 11, 2024, 2:12pm

If you go for shorter high quality cables, that could potentially solve your issue. We usually like to recommend Club3D they’re pretty good:

perry-mitchell · June 11, 2024, 4:15pm

Good tip! I actually bought, on your recommendation, Club3D cables for the PC->KVM route, which work flawlessly.

I was about to buy Cable Matters but considering I can see the quality of the Club3D ones I’ll switch out for them I think.

Thanks for the help thus far, I’ll report back once I have the cables in-hand.

perry-mitchell · June 24, 2024, 4:38am

@wendell @Level1_Amber Quick update - I got the 2 new cables… a 1m USB-C cable from Club3D (mistakenly only got 1m instead of 2/1.8… oh well ) and a 1.8m USB-C to DisplayPort cable (also from Club3D).

After switching to these, I initially saw severe crackling on the DualUp again… and my heart sank. At this point I’m starting to wonder if it’s something to do with the KVM or cables from it to the monitors… though they work fine for all other purposes.

I did however switch the cable order going OUT of the KVM and this seems to have fixed it for the time being. Both screens are running fine with no static/glitching… So perhaps the DualUp just hates the USB-C input? That’s what it had I believe when it was crackling.

I may end up eventually changing the output cables, as they’re just cheapies, but at the moment I don’t want to sink anymore cash into the setup. So long as it’s working

~~Thanks for the help! I’ll update here again after some longer period of use.~~

EDIT: Nevermind, seems like after a couple of hours use, the DualUp occasionally blinks off and on… usually for a few seconds. No noise or anything else. I might try wiring up the other DisplayPort (Club3D) cables from the KVM → monitors (currently they’re from the PC → KVM, and they’re shorter too) later on, to see if that helps. If it does I’ll invest in new cables OUT from the KVM too.