LG 5k with Dual DisplayPort adapter?

[birdynomnom]

I have the lucky opportunity to get one of those 5k-capable 2xDisplayPort switches for cheap, and would like to use that to switch my LG 5k monitor between two Macs.

As a test, I got two DisplayPort 1.2a cables, and two of these "USB-C to Dual DisplayPort" adapters: https://rocstor.com/product/rocpro-usb-c-to-dual-displayport-multi-monitor-adapter/ (I assumed they must actually be Thunderbolt 3, since they claim to support 2x 4k 60Hz displays).

So I plugged one of the adapters into one of my Macs, the other one into the LG 5k monitor, and connected them together with my two DisplayPort 1.2a cables and... nothing. Nothing at all.

Did I get the wrong stuff? Am I confusing the LG 5k monitor with the adapter and what I want is just not possible? Is there anything else that I fundamentally got wrong?

 

[joevt]

I have the lucky opportunity to get one of those 5k-capable 2xDisplayPort switches for cheap, and would like to use that to switch my LG 5k monitor between two Macs.

What's a 5K-capable 2xDisplayPort switch? Post a link. I assume it has two pairs of inputs for two computers, and one pair of outputs for the display or displays? So you can use the switch to connect two displays (single link SST) or one dual link SST display such as the Dell UP2715K.

As a test, I got two DisplayPort 1.2a cables, and two of these "USB-C to Dual DisplayPort" adapters: https://rocstor.com/product/rocpro-usb-c-to-dual-displayport-multi-monitor-adapter/ (I assumed they must actually be Thunderbolt 3, since they claim to support 2x 4k 60Hz displays).

Those are clearly not Thunderbolt. They are DisplayPort MST hubs. macOS doesn't support MST for multiple displays (or for a single display that requires two DisplayPort inputs, such as the Dell UP2715K or the LG UltraFine 5K via Thunderbolt).

So I plugged one of the adapters into one of my Macs, the other one into the LG 5k monitor, and connected them together with my two DisplayPort 1.2a cables and... nothing. Nothing at all.

An MST hub has one input - in this case the USB-C connector. An MST hub has multiple outputs, the DisplayPort ports.

You used one MST hub on the Mac side which isn't going to work because macOS doesn't support MST for multiple displays. I suppose if it did do something, you would get the same signal on both the left and right half of the display (macOS mirrors the same output to multiple displays connected to MST). Instead, you need a Thunderbolt hub or dock. Connect the hub or dock to the Mac. Connect two DisplayPort cables between the Thunderbolt hub/dock and the switch.

You used a second MST hub on the LG side. That's not going to work because the USB-C of the MST hub is not a output and it is not Thunderbolt. An MST hub cannot take multiple inputs and produce a Thunderbolt output. Instead, you need a Thunderbolt 3 or Thunderbolt 4 add-in card (such as the GC-APLINE RIDGE or the GC-TITAN RIDGE or ASUS ThunderboltEx 4). Connect two DisplayPort cables between the switch and the add-in card. Connecting power to the PCIe card is another matter. Install the add-in card in a PC with a spare PCIe slot, or into a Thunderbolt 3 PCIe expansion chassis, or connect a PCIe riser that is connected to 12V 3A power. Even with all that, it might not work. And you probably won't have access to the USB functions of the display (brightness control, audio, camera, USB ports).

 

[birdynomnom]

Thank you for the detailed explanation, it makes sense.

I am now wondering whether I should actually go through the effort of a Thunderbolt 3 (or 4) add-in card in an external closure (which sounds a bit crazy), along with two Thunderbolt hubs/docks, or whether I should trade my LG 5k monitor for the Dell 5k one since it seems that it will just be "straightforward" Dual DisplayPort, or whether I should just give up entirely and continue manually moving the Thunderbolt cable between my two Macs.

I am a bit surprised that there is no other solution.

 

[joevt]

The add-in card idea might not work.
Dual DisplayPort 5K displays are hard to find. People have converted iMac 5K displays.
https://forums.macrumors.com/threads/diy-5k-monitor-success.2253100/
I wonder if an LG 5K can be converted to not Thunderbolt...

If you have a newer LG UltraFine 5K that accepts Thunderbolt and USB-C input instead of the older LG UltraFine 5K that accepts only Thunderbolt input, then you could connect without Thunderbolt at 4K60 (or 5K39 with a custom timing if you're not using an M1 Mac). Use a Belkin Charge and Sync Cable to combine both USB and DisplayPort into USB-C.

Regarding moving the Thunderbolt cable - maybe consider magnetic connectors?
https://forums.macrumors.com/threads/two-computers-and-one-studio-display.2337419/post-31057228
The same thread discusses using a USB-C switch. That only works with the Apple Studio Display because the Apple Studio Display can use DSC to do 5K60 with a 20Gbps connection. The LG requires a 40Gbps connection to do 5K60.

 

[birdynomnom]

Dual DisplayPort 5K displays are hard to find. People have converted iMac 5K displays.
https://forums.macrumors.com/threads/diy-5k-monitor-success.2253100/
I wonder if an LG 5K can be converted to not Thunderbolt...

May well be possible, but also a bit heavy-handed. I do have an old 27" 5k iMac from 2014 packed away in a box that I was planning to sell, though...

If you have a newer LG UltraFine 5K that accepts Thunderbolt and USB-C input instead of the older LG UltraFine 5K that accepts only Thunderbolt input, then you could connect without Thunderbolt at 4K60 (or 5K39 with a custom timing if you're not using an M1 Mac). Use a Belkin Charge and Sync Cable to combine both USB and DisplayPort into USB-C.

I do have the newer one, but obviously I would prefer keeping 5k!

Regarding moving the Thunderbolt cable - maybe consider magnetic connectors?
https://forums.macrumors.com/threads/two-computers-and-one-studio-display.2337419/post-31057228
The same thread discusses using a USB-C switch. That only works with the Apple Studio Display because the Apple Studio Display can use DSC to do 5K60 with a 20Gbps connection. The LG requires a 40Gbps connection to do 5K60.

Magnetic connectors are a good idea, but I am surprised they work well with those bandwidth? The Thunderbolt cables are already very expensive.

 

[joevt]

I do have the newer one, but obviously I would prefer keeping 5k!

How about 5K at 39 Hertz? Unless you have a M1 Mac...

Magnetic connectors are a good idea, but I am surprised they work well with those bandwidth? The Thunderbolt cables are already very expensive.

The bandwidth is 4 lines at 20 Gbps. 2 per direction. You can stick stuff at the end of a Thunderbolt cable and still get 40 Gbps connection.
https://forums.macrumors.com/thread....2023452/page-104?post=30887187#post-30887187