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Does a USB-C monitor support DisplayPort input over a USB-C to DisplayPort cable...
 

Does a USB-C monitor support DisplayPort input over a USB-C to DisplayPort cable?  

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Seandre
(@seandre)
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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. 

TIA!

2018 MBP 15" | i7-8850H @ 2.6GHz | RP 560x | 16GB RAM | 500GB SSD | Mojave 10.14.5 | Win10
eGPU: TBD


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itsage
(@itsage)
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@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:
BENQ EX3501R
BENQ EW3270U
BENQ SW271
ACER PE320QK
ACER XR342CKP

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Seandre
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@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.  

2018 MBP 15" | i7-8850H @ 2.6GHz | RP 560x | 16GB RAM | 500GB SSD | Mojave 10.14.5 | Win10
eGPU: TBD


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itsage
(@itsage)
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That's a great plan! I hope it works because many people really need an easy way to power USB-C monitors.

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Seandre
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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.  

2018 MBP 15" | i7-8850H @ 2.6GHz | RP 560x | 16GB RAM | 500GB SSD | Mojave 10.14.5 | Win10
eGPU: TBD


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joevt
(@joevt)
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@seandre

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

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Seandre
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@joevt

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.  

2018 MBP 15" | i7-8850H @ 2.6GHz | RP 560x | 16GB RAM | 500GB SSD | Mojave 10.14.5 | Win10
eGPU: TBD


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joevt
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@seandre

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...

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Seandre
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@joevt

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!

 

 

2018 MBP 15" | i7-8850H @ 2.6GHz | RP 560x | 16GB RAM | 500GB SSD | Mojave 10.14.5 | Win10
eGPU: TBD


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joevt
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@seandre

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?

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Seandre
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@joevt

So I tested and validated that the Moshi cable sold online through Apple will work* with the new LG UltraFine 23.7 inch display (24MD4KL-B.AUSA).

I tested this using a fresh Windows 10 install on a native machine (not bootcamp) with the latest updates and drivers from Nvidia.  I conducted separate tests using the USB-C output of two Nvidia 2080Ti cards- one being a Founders Edition,  the other card being an EVGA FTW3 Gaming card.  Both cards were had the latest Nvidia drivers.  

The caveat is that I cannot for the life of me figure out how to get the cards and monitor paired consistently.  Through numerous combinations of connecting/disconnecting the cable on both the output and input side,  it sometimes pairs and sometimes doesn't.  I've found the most consistent way to get a connection is to boot up the machine without the monitor connected,  then plug it in some time after the POST.  

Has anyone discovered a way to force this handshake to occur?  

In addition,  I validated that using a USB-C to USB-C cable acted in an identical manner - sometimes the handshake was successful,  but it often wasn't,  without clear reproduction steps.  

I will add that to anyone who attempts to use the monitor with either cable will get the full 4k, 60Hz,  but the color space is limited to srgb.  Attempting to load the .icc profile for the monitor from my MacBook Pro on the windows machine led to unpredicted results.  

 

 

2018 MBP 15" | i7-8850H @ 2.6GHz | RP 560x | 16GB RAM | 500GB SSD | Mojave 10.14.5 | Win10
eGPU: TBD


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joevt
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Posted by: @seandre

I tested this using a fresh Windows 10 install on a native machine (not bootcamp) with the latest updates and drivers from Nvidia.  I conducted separate tests using the USB-C output of two Nvidia 2080Ti cards- one being a Founders Edition,  the other card being an EVGA FTW3 Gaming card.  Both cards were had the latest Nvidia drivers.  

To be clear, you did two tests:
1) the Moshi cable connected to a DisplayPort port of the 2080Ti
2) a USB-C cable connected to the USB-C port of the 2080Ti

Interesting that they have the same behavior. Without the second test, I would have thought that maybe the display works better with a USB connection.

Did you find that the USB-C cable allows using the USB functions of the display?

Like the new LG UltraFine 5K display, this new 4K display works without a USB connection, unlike the old 4K display.

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Seandre
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Joined: 4 weeks ago
 

@joevt

Thats correct!  The behavior surprised me as well.  

The USB-C connection (using the monitor-supplied USB-C to USB-C cable,  NOT the Moshi cable) allowed for the USB-C functions of the monitor (I have an audio interface connected directly to the monitor which worked when using this configuration). Although,  I had to use a DDC utility to control the monitor brightness as brightness controls were not showing up on Windows 10. 

To your last statement,  yes!  I can confirm that this new monitor works without a USB connection using a Moshi cable connected from the DisplayPort output of 2080Ti cards.    

Regarding my issues of ensuring a connection is made,  have you come across this behavior before with other USB-C devices?  What prompts the monitor or card to "check" for a connection?  It's puzzling that I have to disconnect/reconnect multiple times to get an image to appear on the monitor...

2018 MBP 15" | i7-8850H @ 2.6GHz | RP 560x | 16GB RAM | 500GB SSD | Mojave 10.14.5 | Win10
eGPU: TBD


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joevt
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Posted by: @seandre

Regarding my issues of ensuring a connection is made,  have you come across this behavior before with other USB-C devices?  What prompts the monitor or card to "check" for a connection?  It's puzzling that I have to disconnect/reconnect multiple times to get an image to appear on the monitor...

This is strange. Usually no reconnects are required. On my Mac Pro 2008 with GC-TITAN RIDGE cards (not normally supported on anything that doesn't have a Thunderbolt header), I may need to reconnect a device at most once after startup. The first card seems to not require this.

You only tested on one PC with Nvidia graphics? Try Intel graphics? Try a different computer such as a Mac? If you have a Thunderbolt 2 Mac, you can add a USB-C with DisplayPort alt mode port by connecting a Thunderbolt 3 device that has two Thunderbolt 3 ports (but then the LG will connect as Thunderbolt instead of USB-C) or a Thunderbolt 3 dock that has a USB-C with DisplayPort alt mode port such as the HP Thunderbolt Dock G2.

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