Latitude 5480 not detecting external display

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@Miwako  A USB-C to DisplayPort cable should be fine, although using the WD15 is a bad idea because if you connect through a WD15, you'll only get 4K 30 Hz rather than 4K 60 Hz.  A USB-C to DisplayPort cable when plugged directly into the USB-C port would allow 4K 60 Hz because in that setup, all four high speed lanes in the USB-C interface are being dedicated to video.  The WD15 sets up the USB-C link to carry video and USB 3.x, and carrying USB 3.x cuts available video bandwidth in half, which is why you're limited to 4K 30 Hz.

Hi jphughan

First connection I tried was via usb-c cable that came with the display (usb-c over DP port to usb-c port). I tried HDMI to usb-c as well. Unfortunately, display remained black and laptop showed a message "no display detected". Good to know that WD15 won't help me.

@Miwako  Ok, a couple of things are confusing me here.  First, which exact LG UltraFine 4K display model do you have?  LG has more than one display that they label as "UltraFine 4K".  It would help to know exactly what display you're working with.  And second, you've said "USB-C over DP" twice now, and that doesn't make any sense, so I don't know what you mean there.  You can run DisplayPort over USB-C if you are using a regular USB-C cable or something like a USB-C to DP cable, but there is no such thing as "USB-C over DP".  Your latest reply was especially confusing because you said you were using a USB-C cable that came with the display and then you said "USB-C over DP to USB-C port".  Please be specific about which cables you are using and exactly how you are connecting them.  For example, you could write things like this:

• Latitude 5480 USB-C port <> USB-C cable <> LG UltraFine 4K USB-C input
• Latitude 5480 USB-C port <> USB-C to DP cable <> LG UltraFine 4K DP input

Those types of descriptions make it very clear what cables you're using, and which end of the cable is being connected to which device.  And again, clarifying the exact LG UltraFine 4K model you're using might help too.  For example, some LG UltraFine 4K models have Thunderbolt 3 ports.  Those use a USB-C connector, but they require your system to support actual Thunderbolt 3, not just regular USB-C, and Thunderbolt 3 was an option on the Latitude 5480, not standard.  If your 5480 doesn't have TB3, then you would not be able to use the TB3 ports on the display if that is the LG UltraFine 4K display model you have.

In terms of the USB-C to HDMI cable, those can only be used to connect a USB-C port on a system to an HDMI input on a display.  You cannot use it to connect the HDMI output on a system to a USB-C input on a display.  I don't know if that's what you did since it wasn't clear from your post how you used the USB-C to HDMI cable you mentioned.  Here again, it is useful to clarify exactly what cables you're using and how you're connecting them, rather than just saying "I tried HDMI to USB-C as well".

And lastly, have you made sure that you're selecting the correct input when you try different cables?  Sorry if that sounds like an obvious question, but I've helped people here who said they weren't seeing an image on their display even after using the correct cables, and the reason turned out to be that they didn't realize they had to select the correct input on the display in order to see an image.

@DELL-Cares  The Latitude 5480 does not have a VGA output, and this display does not have a VGA input.  And VGA cannot run 4K resolution anyway.  Whoever designed this bot needs to make sure it doesn't make dumb suggestions like this, because it is embarrassing for Dell.

Thank you for writing back to us. 

Have you tried connecting with VGA cable?

@jphughan Apologies for the confusion and not being specific.

Display I try to connect is LG 24" UltraFine 4K IPS Monitor 24MD4KLB-B that has two Thundebolt 3 ports and three usb-c ports.

I made a typo writing "USB-C over DP". I meant DP over USB-C.

Connections I tried were:

• Latitude 5480 USB-C port <> USB-C cable <> LG UltraFine 4K USB-C input
• Latitude 5480 HDMI port <> HDMI to USB-C cable <> LG UltraFine 4K DP USB-C input

My Latitude 5480 doesn't have Thunderbolt 3. 

Latitude 5480 has only one usb-c port and display has equal usb-c inputs (as far as I know they are equal so it doesn't matter which one I select) so I am sure I selected correct cables.

@jphughan Thank you so much for your help! I was pretty sure that usb-c cable (male) needs to be connected via usb-c port only plus that usb-c ports on display can be used for laptop connection. You have no idea how many hours I spent on various forums to find a solution. 

If anyone has the same issue - connect laptop (port usb-c) via usb-c cable to display (port TB3).

@Miwako  Glad I was able to help. It’s often helpful to read the manual when you need to start troubleshooting rather than searching forums. I’m always surprised how many people would rather search forums and ask a question on forums hoping for a useful answer instead of checking actual documentation provided by the actual vendor.

thanks

@jphughan​ how do I select an input on my display if my laptop isn't detecting it? Thank you for your help!