Dual 4K60 with Thunderbolt Dock

[coffee_tranquility]

Hi All,

A while back I purchased a Satechi Thunderbolt 4 dock a while back for my 2021 MBP 16 M1 Max. I use a dual 4K display setup, one with just HDMI and the other with HDMI and DP with the DP being used by my desktop PC.

Using the Satechi with USB-C to HDMI cables I was stuck with 4K 60 on one display and 4K 30 on the other. I could swap the display settings to 60 Hz on either display but not on both simultaneously. Running the cables direct and not through a dock, no issues and 4K 60 on both.

I ended up returning the Satechi and opting to go with a similar OWC Thunderbolt 4 dock which stated specifically that it could do 4K 60 on dual monitors. Strangely enough though, same issue with the OWC dock. Is this a cabling issue where I'll have to use a USB-C to DP and switch out the port used by my PC or is it just not possible without using a Thunderbolt display as the second monitor?

Appreciate any help.

 

[joevt]

Were any of the USB-C to HDMI cables specified as DisplayPort 1.4 or HDMI 2.1? You'll want them both to be DisplayPort 1.2/HDMI 2.0.

 

[coffee_tranquility]

Hi Joevt, I checked on the cabling and no spec given. Normally I buy certified cables from Monoprice for home equipment and other vendors for work jobs. Figured I would give the new Anker cables a try but no HDMI versioning specs on these aside from the supported spec. I had ordered a USB-C to DP 1.4 cable from Cable Matters a bit back when I ran into the issue and hooked it up today and it's working as expected. Still not sure if its an issue with utilizing two USB-C to HDMI's since they both worked direct without the dock or just an issue when two of the same are used but it's working, so I'm happy .

 

[joevt]

Maybe you can post some URLs for the cables.

The issue is - if a DisplayPort to HDMI adapter uses HBR3 link rate (for DisplayPort 1.4 to HDMI 2.1 conversion) then that leaves only HBR link rate for the other display connected to the same Thunderbolt port.
If it's a DisplayPort 1.2 to HDMI 2.0 converter, then it will only use HBR2 link rate and two of those can work from the same Thunderbolt port.
To find out what link rate is being used, on an Intel Mac I would use the AGDCDiagnose command but I don't think that works on M1 Macs. I'm making a utility to query the info directly from the DisplayPort connection but I don't have an M1 Mac to test it with (first I need to find out if I can actually do DisplayPort transactions on a M1 Mac - I'm hoping it will be similar to the methods for doing DDC/CI except DCPDPServiceProxy is used instead of DCPAVServiceProxy ).

You can force HBR2 link rate if you first connect a display that only requires HBR2 link rate. That will leave enough bandwidth for another HBR2 connection so even if you connect a HBR3 display, it will connect as HBR2. Then you can disconnect the first display and attach a second HBR3 display which should connect as HBR2 (as long as macOS doesn't redo the link for the first HBR3 display). This is how you would connect two HBR2 displays to a Thunderbolt 1 or Thunderbolt 2 Mac - except in that case the first display connected should be HBR to force the next two displays to connect as HBR instead of HBR2.

 

[coffee_tranquility]

Thanks for the detailed response. I found it very informative and enjoyed it.

Tested out AGDCDiagnose but like you said, would not run, just returned "Node Count: 0 - bailing" each time.

If it helps for reference, here is the setup I'm currently using in detail:

OWC Thunderbolt 4 Dock - https://www.owcdigital.com/products/thunderbolt-dock.

Anker USB-C to HDMI - 6 ft - https://us.anker.com/products/a8730

UNI USB-C to HDMI 2.0 - 6 ft - https://uniaccessories.com/products/usb-c-to-hdmi-cable-3ft-6ft-10ft

Cable Matters USB-C to DisplayPort 1.4 - 6 ft - https://www.cablematters.com/pc-710-124-usb-c-to-displayport-cable-8k-ready.aspx

LG 4K 43" 43UM7300PUA LCD TV - https://www.lg.com/us/tvs/lg-43UM7300PUA-4k-uhd-tv (Main Display)

Acer VG280K (Nitro VG0) 27 Inch Monitor - https://www.acer.com/ac/en/US/content/model/UM.PV0AA.001 (Secondary Display)

Got the Uni cable delivered earlier today and tried combining it with one of the Anker and same situation. Will have to keep experimenting with my setup and some different configurations to see what works best but if you do come across a method for checking the link rates on the Apple silicon, definitely let me know.

 

[joevt]

Will have to keep experimenting with my setup and some different configurations to see what works best but if you do come across a method for checking the link rates on the Apple silicon, definitely let me know.

Right now I'm trying to figure out how to construct a DisplayPort Sideband Message to communicate with DisplayPort sinks (devices) that are connected to a DisplayPort branch (MST hub). Some DisplayPort 1.4 to HDMI 2.1 adapters are implemented as an MST hub so they might have some interesting info beyond the branch device (info that AGDCDiagnose doesn't show).