LG Ultrafine USB-C to RX580 HDMI not working  

The USB-C to HDMI cable probably only works in one direction (USB-C output of computer to HDMI input of display).

What you need is an adapter that converts HDMI to USB-C - such as the Wacom Link Plus. It also transfers a USB 2.0 signal so you can use the USB features of the display (brightness control, USB hub, etc.)

Any reason you don't use the DisplayPort output of the RX580? The Wacom Link Plus can also accept DisplayPort input.

Posted by: @joevt

Any reason you don't use the DisplayPort output of the RX580? The Wacom Link Plus can also accept DisplayPort input.

Reading similar experiences from others I avoided DP ports cause they seem to be problematic. Anyway I bought a DP>USB-C cable that I'll try tomorrow. This is the one I'm using now that doesnt work https://uniaccessories.io/collections/cables/products/usb-c-to-hdmi-cable4k-60hz-uni-usb-type-c-to-hdmi-cable-thunderbolt-3-compatible-for-macbook-pro-2018-2017-macbook-air-ipad-pro-2018-surface-book-2-samsung-s10-and-more-gray-6ft-1-8m

Brand is the same. I was wandering if it could be some kext problem.

I would like to recommend you to have a look at the ongoing discussion here. I believe it is going to give you the necessary approach and angles in order to reach a proper and hassle-free solution. In order to address this issue, for example, I have purchased two different equipments: Huawei VR2 cable and DeLock DisplayPort Adapter for a USB Type-C Monitor. But, I am also thinking to buy Gigabyte GC-TITAN RIDGE Thunderbolt 3 Expansion Card and a discrete GPU with Virtual Link since such equipment is going to give me more space and flexibility in future scenario so that I am not going to be limited by lack of Thunderbolt 3 support for example.

Addendum

This monitor constitutes such a unique case on its own. And that made me several times question my decision afterwards when I have decided to build a desktop PC recently. It poses so many limitations since Thunderbolt 3 is not a common "thing" yet.

@woloban, most USB-C to DP cables only work in one direction also (like USB-C to HDMI cables). Make sure you get a bi-directional USB-C to DP cable like the one from Moshi.

https://hardforum.com/threads/use-usb-c-monitor-without-usb-c.1911817/

Hello. Reviving this post after a year!

I finally bought the Moshi active USB-C>DP cable, as suggested here and in other forums/posts. Plugged it on the TB port of the Ultrafine, but it doesn't work.

Current setup is this: iMac TB1 with Apple adapter to Razer Core with RX580. From DP1 of the RX580 to TB port of the Ultrafine. Tried with rebooting, disconnecting and reconnecting. Monitor is not recognized.

Is it hopeless?

Do I need an Alpine Ridge with a riser? Or a Wacom Link Plus? Or a HP Thunderbolt dock G2?

Thank you

an update: I installed switchResX and it gives me the possibility of switching to "billions of colors" on the Ultrafine external display, while having it still plugged to the TB1 port. I can actually see under mac System report that is nowrecognized as a 30bit framebuffer display. Did gradient target tests and I can see a difference. How is it possible? Can TB1 output 10bit? dGPU is GTX 680MX. 

@woloban, 

I don't think there's such thing as an active USB-C>DP cable. Did you mean bi-directional? Some people have stated that the moshi cable doesn't work in some situations. And in any case, it won't let you use the USB features of the display (audio, brightness control, camera, USB ports). Therefore, for best results, I would use a Wacom Link Plus or Belkin Charge and Sync cable for DisplayPort outputs.
  https://egpu.io/forums/gpu-monitor-peripherals/does-a-usb-c-monitor-support-displayport-input-over-a-usb-c-to-displayport-cable/paged/4/#post-79127

There now exists HDMI 2.0 to DisplayPort 1.2 USB-C adapters such as the Club-3D CAC-1332. 
  https://www.club-3d.com/en/detail/2517/hdmi_to_usb_c_4k60hz_active_adapter_m-f/
These are limited to 4K max width which is good enough for the LG UltraFine 4K displays. 5K width would require an HDMI 2.1 to DisplayPort 1.4 adapter which don't exist yet (but the converter chip to do the job exists)
  https://insights.club-3d.com/thread/hdmi-2-1-to-usb-c-displayport-1-4-alt-mode/

For your 10 bpc test from TB1 port (without eGPU?), what resolution are you using? What timing does SwitchResX say you are using? 10 bit has been around since DisplayPort 1.1. Even if macOS says the frame buffer is 10bpc, it doesn't mean that the output pixel format is matches what it says (same for the resolution). The GPU can scale the resolution (up or down) and transform the pixel format to create the output signal. It could do dithering as well (I'm not sure about this - I'm sure dithering 10bpc to a 8bpc framebuffer can happen in macOS in some apps). Normally I would use the AGDCDiagnose command to see what's happening with the output signal but it doesn't give much info for Intel and NVIDIA GPUs.

@joevt, thank you for your additional input. I returned the Moshi cable (sorry, I was saying active but I meant bidirectional) AND the Wacom link plus. with both I tried any kind of possible setup. Even the Huawei cable + booth order mentioned here https://egpu.io/forums/postid/82424/ to no avail. Nothing really worked and the screen would stay off.

I found a Huawei VR 2 cable the should be arriving soon and that is my last hope. I don’t think I want to resort to a Titan Ridge. It’s probably better to invest in a new workstation at that point.

I will check the output of AGDCDiagnose too.