Issue with 3 HDMI Monitors on M1 MacBook Pro late 2021

I am trying to drive 3 external displays all via HDMI using the UD-ULTC4K dock but I can only seem to get 2 out of the 3 displays to work using the dock, specifically the displays connected to the top 2 HDMI ports. When I connect any display to the bottom HDMI port, it is not recognized by the Displaylink app / MacOS. This is the port that is labeled “Alternate Mode”. Is there something that I need to do to enable this mode with a late 2021 MacBook Pro using the M1 Pro chip?


Thank you for contacting Plugable support! Sorry to hear about this issue. I’d be more than happy to assist you.

We also received your direct email to us about this issue and looked at the diagnostics you sent to us. Looking at the diagnostics the dock is not connecting at the full speed to your computer. I suspect you may not be using the provided USB-C to USB-C cable the dock shipped with as such.

Please ensure to connect the dock using the provided C cable as this should correct this issue (as not all C cables are alike). For example, if you used the white Apple C cable your Mac shipped with it’s not able to support the dock as it is for “charging only” and doesn’t support USB-C Alternate Mode video or high speed USB data.

Please don’t hesitate to let us know of other questions.

Thanks again for contacting Plugable support and best wishes!

Joshua Henry
Senior Engineer | Product Owner
Plugable Technologies

Thanks so much Josh for spotting this. I was in fact using a different USB-C cable and swapping it out to the one that came with the dock worked! The reason I had used the other cable is that it is a bit longer and allows me more options to place the MacBook pro on my desk where I have more free space.

You’re most welcome!

If a longer cable is needed it is possible to go up to 2m in length (double the original). For example, we’ve tested our 2m 20Gbps Thunderbolt 3 cable ( with various USB-C docks and have found that the high quality Thunderbolt cable can properly maintain signal integrity and fully support the various dock functions. (We don’t recommend long regular USB-C cables instead of Thunderbolt as we have encountered some issues with the signal not being stable from some USB-C cables.)

Best wishes,

Got it, thank you very much for helping here Josh.

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