U3415W, MHL cable

@catorg  If you're already looking at the manual, the diagram on Page 10 and the table describing each port that appears directly below said diagram should make this clear.  The display has MHL, HDMI, DisplayPort, and Mini-DisplayPort inputs, followed by a DisplayPort MST output that can be used for daisy chaining.  If you have an HDMI output, the easiest connection would be a regular HDMI cable to the display's HDMI input.  If the HDMI input is occupied by something else, then there are MHL to HDMI adapters, and they require a USB connection because MHL connectors are designed to carry both power and video, and in the adapter scenario the power is drawn from the USB connection.  The idea of MHL was that if you wanted to connect a smartphone to a display, or for that matter something like a Fire TV Stick, you could send video to the display and draw power to run that device from the display all over a single cable, rather than needing a video cable and a separate USB or wall power cable for power.  MHL to HDMI adapters therefore are more typically used when your source device has an MHL connector but your display only has a regular HDMI input.  In that case, the display's HDMI input wouldn't be able to send power to the MHL-equipped source device, but adding the adapter would allow that to happen.  Of course in your case, the MHL connector is on the display side and your unspecified device that has a regular HDMI output wouldn't be trying to draw power from the display through that port, but that's why MHL to HDMI adapters have that USB connection.  MHL never really caught on though, and now we have USB-C that can handle video, power, and USB data all simultaneously over a single cable.

@catorg  Quick add-on to my earlier post.  If you were thinking about connecting your HDMI output to the DisplayPort or Mini-DisplayPort inputs using a DisplayPort/Mini-DisplayPort to HDMI adapter or cable, don't.  Those cables are designed to connect DP/Mini-DP sources to HDMI inputs and can't be used in the opposite direction that you'd need in this case.  Read my post here if you really want to understand why, but basically if the device you have only offers HDMI for video output, then you need to use the HDMI or MHL input on the display, unless you want to buy a rather expensive and specialized "reverse" HDMI to DisplayPort adapter as I noted in that post I linked.

Thank you - I made a mistake trying to read upside down and backwards with a mirror and misidentified the DPout port - So, a reset:  the MHL port looks like an HDMI (positively identified now) - so I need to figure out how to convert the HDMI output from a device to MHL (and it sounds like a regular HDMI cable isn't the answer..) - if that conversion isn't possible, then I'll buy an HDMI switch and use two devices on the HDMI input.

@catorg  Happy to help, and glad things are becoming a bit clearer.  In your case it sounds like the best option is an HDMI to MHL adapter as I alluded to above -- or yes a 2x1 HDMI switch would get the job done too.  That's unless one of your devices would be capable of feeding either the DisplayPort or Mini-DisplayPort inputs on the display, but I don't have a way to know that since you haven't specified either of the source devices you're working with.

Thanks for the reply - I appreciate the info - yeah, I'm greedy - I 4 inputs on the monitor and I want to use them all ha ha.. Already have 3 computers hooked up to the mDP, DP, and HDMI ports so since I have a 4th device with HDMI output, that's why I was interested in HDMI to MHL.. Didn't really know anything about MHL (as was obvious) so at least now I have a better idea of how to handle it.. A $20 HDMI switch might be the easiest way to do it since I can't find and HDMI to MHL (with large port as needed by the monitor input)..

thanks again!