MacBook M3 - Thunderbolt video not working on USB-C to Digital AV adapter

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MacNewb2024 Author

Level 1

4 points

MacBook M3 - Thunderbolt video not working on USB-C to Digital AV adapter

So I'm switching back to a Mac for my primary laptop after many years of using Windoze. Anyway, my desktop setup has two 24" 4K monitors that I previously had hooked up to my Windows laptop via a docking station.

The MacBook M3 really didn't like the docking station so I got an Apple USB-C to Digital AV adaptor and then plugged all my USB-A & C peripherals into the Thunderbolt connected monitor (using it as the docking station). Before I got the adaptor, I just plugged one monitor in via the HDMI port on the laptop and the "main" one via the Apple Thunderbolt connector (which all worked great). I'd just like to get this down to one connection so I can "undock" quickly and take my laptop to work.

That said, when the USB-C to Digital AV adaptor arrived, I plugged it into the MacBook, moved the monitor inputs to the adapter - however, only video from the HDMI monitor worked (the Thunderbolt monitor stayed dark with no input). I plugged the Thunderbolt cable into another USB-C port on the MacBook and both monitors work. Switch it back to the adapter, only the HDMI works.

I guess I'm wondering if there's a compatibility problem with the M3 MacBook (Sonoma 14.6.1) and the Apple adapter. Anyone else run into this scenario? Since everything is new I will likely open up a ticket soon, but just wondering if anyone ran into something along these lines.

Thanks in advance!

Posted on Sep 8, 2024 6:44 AM

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Answer 1

Top-ranking reply

Servant of Cats

Level 8

41,275 points

Sep 8, 2024 8:30 AM in response to MacNewb2024

It's not clear how you are trying to connect these two monitors.

The Apple USB-C Digital AV Multiport Adapter has three ports:

A HDMI port
A USB-A port
A USB-C port where you can plug in a cable from a USB-C power source to provide pass-through power to a Mac notebook

If you were trying to connect two monitors through this adapter, I do not see how that could ever work. HDMI does not support daisy-chaining. USB-A does not carry video. Although USB-C ports can carry video, this one does not. Its only purpose is to accept charging power for the adapter to pass on to a Mac notebook.

In general, you cannot connect two monitors to the same Mac host port, unless

The Mac supports multiple external monitors
The port is a Thunderbolt port
The device connected directly to it is a Thunderbolt device (dock, hub, or dual-monitor adapter) – not a "plain" USB-C one. Macs do not support making a connection to one monitor using USB-C (DisplayPort Alt Mode) and then plugging a second monitor into the first using DisplayPort MST daisy-chaining. They want to see the wide "data highway" that a Thunderbolt connection offers to drive two 4K monitors @ 60 Hz over one docking cable.

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Answer 2

Sep 8, 2024 8:45 AM in response to MacNewb2024

MacNewb2024 wrote:

I'd just like to get this down to one connection so I can "undock" quickly and take my laptop to work.

What I would suggest is a Thunderbolt dock or hub. There are many that let you plug in two "regular" 4K monitors (or USB-C to (whatever) adapters), provided that the Mac in question supports two displays.

You may want to look at offerings from

Other World Computing

SonnetTech

CalDigit

There are other vendors, but these three are long-term suppliers of Thunderbolt gear for Macs.

----------

If your M3 MacBook is one with a base M3 chip, note that it will only be able to drive two external monitors when its lid is closed. Apple's instructions say to close the lid before connecting the second display. I'm not sure how things work if you have two monitors "permanently" connected to a single Thunderbolt dock.

Use dual monitors with your MacBook Air and MacBook Pro with M3 chip - Apple Support

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Answer 3

MacNewb2024 Author

Level 1

4 points

Sep 8, 2024 2:08 PM in response to MacNewb2024

Hello - well, I researched that USB-C to AV adapter a bit more (and read the comments) and I think I understand the issue. Anyway, I had the USB-C adapter plugged into the Thunderbolt port on the M3, I plugged in one monitor into the USB-C port of the adapter (the monitor did have its own power supply) and then the other monitor (again with its own power supply) was plugged into the adapter via HDMI. I was assuming the AV adapter would support dual monitors in this configuration, but apparently that USB-C "in" on the adapter is just a power-input USB connection and won't support a monitor (thanks for that clarification @Servant of Cats).

I went ahead and just plugged both monitors into the M3 via USB-C/Thunderbolt. This lets me us all the ports (A & C) on the monitors and means I only have two cables into the laptop. I will likely research a Thunderbolt hub/dock as @Servant of Cats also recommended above.

Thanks for the feedback - two cables isn't horrible and everything works smoothly, so I will call this a "partial" win. My brain is still bending to thinking in a Mac-mode (coming from many many years of Windows) so I'm sure something else will stump me in the near future.

Thanks again!

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Answer 4

varjak paw

Level 10

182,860 points

Sep 8, 2024 7:40 AM in response to MacNewb2024

Someone else here may have more useful suggestions, but one question: does the Thunderbolt monitor have its own power supply, or does it get powered through Thuderbolt? If the latter, the adapter may not provide enough power to run that monitor. This is probably not the case, but I wanted to mention it as a possibility.

If that's not the case, it'll probably help if you list the monitors you have so people know exactly what you're working with.

Regards.

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MacBook M3 - Thunderbolt video not working on USB-C to Digital AV adapter

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