Dell monitor USB ports not working with MacBook Pro

<< I am connecting my macbook pro 2024 to an external monitor Dell u3818dw via an HDMI cable. >>

When you connect a display with an HDMI cable, you get a picture and sound. Nothing more.

If you would like to use the display's USB ports, you can connect to the Mac with a separate USB cable under ONE meter.

That Dell u3818dw display appears to be a 3840 by 1600 display with hiDPI (10 bits/color) capability, essentially 4K with the bottom half missing.

interfaces include:

1 USB-C

2x HDMI 2.0

1 DisplayPort 1.2

a 60 Hz refresh rate is attainable using only a USB-C cable as your main display cable.

One USB cable from a USB port on the Mac, to a USB (upstream) port on the display, allow the USB ports on the display to be connected when using HDMI for display and sound.

Most users would choose to just connect one USB-C cable for everything. Your display can work that way, but if you had an even larger display, sometimes there is no data bandwidth left to run the stuff connected to the display's USB ports.

Grant Bennet-Alder wrote:

One USB cable from a USB port on the Mac, to a USB (upstream) port on the display, allow the USB ports on the display to be connected when using HDMI for display and sound.

Most users would choose to just connect one USB-C cable for everything. Your display can work that way, but if you had an even larger display, sometimes there is no data bandwidth left to run the stuff connected to the display's USB ports.

It looks like with this display, if you hook things up using a single USB-C cable, that will limit downstream ports on the monitor to USB 2.0 speeds.

I've read elsewhere that USB-C has dedicated pins for USB 2, but not for USB 3. To run DisplayPort Alt Mode and USB simultaneously over one "plain" USB-C connection, one must take half of the pins that could carry video and reallocate them to carrying USB 3.

With a 4K display and DisplayPort v1.2, this cuts video bandwidth enough to force dropping the refresh rate from 60 Hz to 30 Hz. The Dell monitor we are talking about here uses DisplayPort v1.2. Some Dell monitors give you a "USB-C Prioritization" menu with "High Resolution" and "High Data Rate" choices, but it appears that this monitor does not have such a menu and that the choice is hard-wired in favor of allocating all of the pins to carrying video.

DisplayPort v1.4 can get more bandwidth out of a given number of pins – enough more to allow having 4K, 60 Hz, and USB 3 speeds at the same time.

Dell – Dell UltraSharp U3818DW Monitor User’s Guide

Page 11 shows four video inputs.

• Two HDMI inputs (ports 3-1 and 3-2). These are for video only.
• One DisplayPort input (port 4). These are for video only.
• One USB-C (DisplayPort) input (port 5). This can carry video, serve as a USB upstream port, and charge a laptop at the same time.

If you connected the monitor via HDMI or DisplayPort, and wanted to hook up the USB downstream ports (ports 8-1 and 8-2), you would need to run a separate USB cable to a USB upstream port (7-1 or 7-2).

See also page 43 ("USB Selection"), which describes the monitor menu that you use to choose which USB port the monitor uses as the USB upstream port (port 5, 7-1, or 7-2) when using the HDMI or DisplayPort inputs.