L1 KVM display issues on Macbook Air

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What I have

I have the usb-c 1.4 Display Port KVM Switch (Single Monitor - Two Computer) which I am trying to use with a desktop PC and a Macbook Air (M1). I have an ultrawide monitor, a keyboard, a mouse, a webcam and a DAC plugged into the KVM.

How I use it

For the Macbook (since it only has two thunderbolt ports), I have the USB-C cable plugged directly into the laptop, and the DP 1.4 cable ( Club3D VESA Certified CAC-2068 DisplayPort to DisplayPort 1.4/Hbr3 Cable DP 1.4 8K 60Hz 2M) is plugged into a Cable Matters multiport adapter (Cable Matters USB C Multiport Adapter (USB C Hub with DisplayPort 8K), 2X USB 2.0, 480 Mbps Ethernet, and 100W Charging) along with the power cable for the Macbook.

The problem

On the Macbook Air, any black colour on the monitor “sparkles” with white pixels, and the screen will periodically go black. The KVM works perfectly on the PC.

Any ideas on what could be wrong and what I could do?

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I have the impression that it is working as intended. Apple uses standardized ports in non-standard implementations for a walled garden experience.

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Sorry, what do you mean by that? I’m totally new to Apple products - this Airbook is the first Apple product I’ve used and I’ve only had it a week

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Apple devices may not be compatible with PC devices by design. As in Apple breaks compatibility because they only want you to use Apple and Apple approved devices.

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Do the adapter and DP cable work as intended if you bypass the KVM?

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I know Wendell had to use DP repeater a lot of the time to make it work right due to

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Can you try it with a moshi bi directional cable or shorter dp cable after the KVm? Symptom sounds like a little bit of cable noise

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Do the adapter and DP cable work as intended if you bypass the KVM?

Yes - please see my testing so far below:

TEST CASE PASS/FAIL
2m DP 1.4 Cable from monitor through adapter with PD into single port – total DisplayPort cable length: 2m PASS
2m DP 1.4 Cable from monitor through adapter without PD into single port – total DisplayPort cable length: 2m PASS
2m DP 1.4 Cable from KVM through adapter with PD into single port – total DisplayPort cable length: 4m FAIL
2m DP 1.4 Cable from KVM through adapter without PD into single port – total DisplayPort cable length: 4m FAIL
1.8m DP 1.4 Cable from KVM through adapter with PD into single port – total DisplayPort cable length: 3.8m FAIL
1.8m DP 1.4 Cable from KVM through adapter without PD into single port – total DisplayPort cable length: 3.8m FAIL
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Hey Wendell (and thanks for popping in!)

Please see above my current test cases. Will order some shorter DP1.4 cables (maybe two 1m cables so total run from monitor > kvm > laptop is 2m).

I’m also experiencing some weird mouse acceleration antics (not wireless - it’s usb plugged into the KVM; specifically a Logitech G403), any ideas on how/whether I can fix that? Basically when moving my mouse across the screen the speed seems to be inconsistent (as in it occurs inconsistently, and when it occurs the speed itself is inconsistent).

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Hey Wendell,

No dice with the shorter DP Cables:

TEST CASE PASS/FAIL
2m DP 1.4 Cable from monitor through adapter with PD into single port – total DisplayPort cable length: 2m PASS
2m DP 1.4 Cable from monitor through adapter without PD into single port – total DisplayPort cable length: 2m PASS
1m DP 1.4 Cable from monitor through adapter with PD into single port – total DisplayPort cable length: 1m PASS
1m DP 1.4 Cable from monitor through adapter without PD into single port – total DisplayPort cable length: 1m PASS
1m DP 1.4 Cable from KVM through adapter with PD into single port – total DisplayPort cable length: 2m FAIL
1m DP 1.4 Cable from KVM through adapter without PD into single port – total DisplayPort cable length: 2m FAIL
2m DP 1.4 Cable from KVM through adapter with PD into single port – total DisplayPort cable length: 4m FAIL
2m DP 1.4 Cable from KVM through adapter without PD into single port – total DisplayPort cable length: 4m FAIL
1.8m DP 1.4 Cable from KVM through adapter with PD into single port – total DisplayPort cable length: 3.8m FAIL
1.8m DP 1.4 Cable from KVM through adapter without PD into single port – total DisplayPort cable length: 3.8m FAIL
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whats the actual usbc to dp adapter? do you have any pics of it? Are they different ones?

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It’s this one here

Though as above the adapter works fine if I go monitor > adapter > laptop, the issue only arises when I go monitor > kvm > adaptor > laptop

Also, any insight into the weird mouse acceleration issues further up in the thread?

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kay, so, this adapter is one of the unfortuante ones that “senses” a monitor is plugged into it by using the ground pins or something. So when you do anything with the kvm, it doesn’t think a monitor is plugged in. These arent dumb adapters. You probably should try the moshi, or another brand, since it is bidirectional.

The accelration thing I’m looking into. You can use it on the usb3 side of the kvm and the kvm will leave it alone. The mouse sometimes seems to be sending an 8 byte mouse packet when its in 20 byte mode, so not sure why that is causing that.

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I can’t use the Moshi cable unfortunately because it doesn’t also provide power delivery. The Macbook Air M1 only has two thunderbolt ports, so if I plug in the USB-C from the KVM and then a DisplayPort-thunderbolt cable, there are no ports left for the charger. So I need to combine the charger and DisplayPort cables - hence this adapter.

Do you think this would work?

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Hmm that’s no good. Does the brick thing with Ethernet not have usb passthrough? Usually when they do multiple functions and charging they also have a passthrough usb port or two

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It only has two USB-A 2.0 ports, which doesn’t help since I have a USB-C coming from the KVM.

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welll a usb3 A to C cable is a thing, which might be worth trying, for diagnostic purposes. I test with a caldigit TS3 dock also, those work great

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You should do a TB dock review/round up and have like a top 3 you recommend

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Before I go spend another $210, would this work?

I would put the DP cable from the kvm straight into that, and then a USB-C → USB-A cable from the KVM into that, plug the adapter into one thunderbolt port on the MacBook and the charger into the other.

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It seems kinda likely?? idk that ive tested that exact 1 tho