Dell monitor USB ports not working with MacBook Pro

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.

<< 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.

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.

<< 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. >>

It's a little more complicated than that.

The USB-C plug is reversible, but that only means it can be inserted either way. Either way, (almost) All the pins will still be used, they will just be swapped around by the smart controller.

There are un-shielded pairs in the USB-C interface to carry USB-2 data, separate from the pairs that carry shielded "SuperSpeed" aka USB-3 data.

But because only ONE USB-2 pair is carried all the way through a USB-C cable, so USB-2 will be somewhat slower over a USB-C cable if fully loaded. It will have to be momentarily "turned around" to do inbound instead of outbound. Also, USB-2 is required for USB-C Power Delivery negotiation. The negotiated higher power levels are eventually delivered on the conductors reserved for USB power.

USB-3 speed data:

There are FOUR data pathways for SuperSpeed USB-3. As noted above, ALL are used in either connector orientation. For default uses, Two pathways are intended for Outbound data, and two pathways are intended for Inbound data. The basic speed of each SuperSpeed data pathway is about 5 G bits/sec. So two pathways used together provides the nominal basic 10 G bits/sec of USB-3 ports.

To get higher than 10 G bits/sec overall, the controller must momentarily "turn around" the Inbound data pathways into Outbound data pathways. When you "turn around" the Inbound SuperSpeed data pathways into Outbound data pathways, you get a total throughput that approaches 20 G bits/sec. This is the way higher-resolution Displays work.

For use with Displays, display data has priority over other data, so if almost all bandwidth is used by a display, almost none is available for a peripheral.

Re: “Unfortunately, there is no USB port on my macbook pro 2024 version.”

All of the “USB4 / Thunderbolt” ports on your MacBook Pro are USB ports that can carry USB 2/3, USB4, DisplayPort, Thunderbolt, and USB Power Delivery charging power (to your MacBook Pro).

They just have USB-C connectors (as required by the USB4 standards) instead of USB-A ones. There are various ways to convert between USB-C (USB 2/3) and USB-A (USB 2/3) in either direction.

Re: “Doubtful that this might harm the laptop's battery given it wont be an original cable/power plug, but i guess i need to make a choice.”

If the monitor and cable are compliant with the USB-C Power Delivery standard, there “should” be no risk to your computer (short of something like a lightning strike). USB-C Power Delivery is negotiated, and your MacBook Pro does not have to agree to accept any voltage it does not like.

Charge your MacBook Air or MacBook Pro - Apple Support

Note that the USB-C cable should be rated to support Power Delivery with at least the amount of power your monitor can supply. Part of the standard is that cables must have embedded chips to tell the devices connected to them that it is safe to transmit more than 15W of power.

I found the USB-C connecter pinouts on wikipedia illuminating.

This is the A-side:

and this is the B-side: