Does a USB-C monitor support DisplayPort input over a USB-C to DisplayPort cable?
I have the new generation of the LG Ultrafine 4k for Mac, which eschews DisplayPort and HDMI inputs for just two (2) Thunderbolt 3 inputs. LG states that the monitor can receive input over USB-C, non-Thunderbolt 3 devices such as the iPad Pro and MacBook (not pro), which leads me to believe that the monitor accepts a DisplayPort signal over USB-C.
All of my internet research has led me to many threads on using a USB-C to DisplayPort cable with the USB-C end as the source, but is this protocol bi-directional? Will I be able to use the DisplayPort output on an eGPU to connect to my monitor using a USB-C to DisplayPort cable?
I am looking to add an eGPU to help with gaming and video editing on my 2018 MBP 15". I don't like the BlackMagic solutions solely because they are non-upgradeable. At the time I purchased this monitor I wasn't considering an eGPU at all, so the limited choice of inputs was a non-issue for me. Since the monitor is somewhat of a new product, more people may find themselves in a similar situation in the future. Hoping one of you guys can help me understand how a display signal is passed over a DisplayPort cable vs USB-C.
@seandre The only bi-directional USB-C to DisplayPort cable I'm aware of is Moshi 5 ft (1.5 m) cable. DisplayPort output to USB-C input in particular can be tricky. It needs to be supported by the monitor. So far Moshi has listed the following models as compatible on its product page.
For bi-directional support, please check with the monitor manufacturer to ensure the monitor accepts a DisplayPort signal over USB-C (UFP_D pin assignment E). While not all USB-C monitors support DisplayPort signals, there is now a growing number of certified displays that do support this. For example:
@itsage Thanks for your reply and your continued support of the eGPU community.
I've started a support ticket with LG to enquire if this new monitor supports DisplayPort over USB-C and will reply to this thread when I hear back.
That's a great plan! I hope it works because many people really need an easy way to power USB-C monitors.
For anyone doing their research - this story ends quite unfortunately for an eGPU-based solution. After many hours of research and contacting LG directly (not as helpful as I had hoped), I have learned a couple things:
USB-C with DisplayPort Alt Mode is the protocol used for any display connection passing over a usb cable. DisplayPort Alt Mode requires that power be passed over the cable from the output (source) to the input (monitor) in order to work. Because of the power requirement, a DP-to-USB-C cable will not work with USB-C monitors.
All the solutions I have found involve the use of an add-in card which takes a DP-in signal and assigns it to the proper USB channels, then outputs a powered signal over USB-C. Add in cards require a power connection and a PCI-e slot. You could conceivably use a large eGPU enclosure with PCI-e bifurcation support in order to accommodate these requirements, but even the best case scenarios involve an inelegant external wiring of a DP cable from GPU->Add-in card.
Apologies to the experts who may be reading this for any misinformation, but as far as I know, this is the reality of trying to get USB-C monitor inputs to work. You will be limited to using devices which support DisplayPort Alt Mode over USB-C natively, if you want to avoid using an add-in card.
Some USB-C displays can work with the Moshi cable (the new LG UltraFine 5K). Some do not (old LG UltraFine 4K) but they can work with a cable that includes USB such as the Huawei VR2 cable. There's a new adapter that is specifically meant to convert DisplayPort to USB-C DisplayPort alt mode. It doesn't have a USB input but it might have a USB output required by some displays (uses a Cypress chipset) https://www.delock.com/produkte/G_63928/technische_details.html
This and other solutions (including the Cypress chipsets) are discussed at #148
That is fascinating! What is it specifically about the new LG UltraFine 5k that makes the Moshi cable compatible, as opposed to the new LG UltraFine 4k? Both the LG UltraFine 5k and 4k received product refreshes this year.
Thank you for providing those useful links. I have some reading to do.
I don't know. These LG displays made for Macs don't have buttons or menus. Maybe the LG UltraFine 4K was made to turn on only when there was a USB 2.0 signal? I don't know or remember if the new LG UltraFine 4K is the same. Maybe it's improved like the new LG UltraFine 5K? Well, the new LG UltraFine 5K is newer than the new LG UltraFine 4K so maybe not? It's starting to get confusing. Maybe should start using model numbers instead of names...
Hah! Yes. I think these monitors are beautiful, but the lack of documentation and settings management is frustrating.
I have ordered the Moshi cable and will report back with my findings. Cheers!
I have two of the Moshi cables (from Apple) and a Huawei VR2 cable (from eBay). I don't have a display for them. I was using them to test the HP Thunderbolt Dock G2's Titan Ridge USB-C DisplayPort alt mode input support. The HP Thunderbolt Dock G2 seems to accept only 2 lane DisplayPort alt mode with USB 3.x so the cables didn't work. I think HP could have made the dock smarter and allow it to accept 4 lane DisplayPort input with USB 2.0. But then if you connect it to a USB-C port that has DisplayPort alt mode and USB 3.x - how would you make it choose 4 lane DisplayPort input to maximize the display capabilities instead of the USB capabilities?