4k@60Hz via USB-C, have to set monitor to USB 2 mode?

[iamsen47]

I've just recently bought the 2018 mac mini and a 4K monitor (Philips 328P6VUBREB). The monitor has a setting to switch to either USB 2 or USB 3 modes, with USB 3 as the default.

Using the usb-c cable that came with the monitor, I managed to get 4k@60Hz when connected to a 2017 MacBook. No settings were changed on the monitor.

However, using the same cable on the mac mini, I only get 30Hz unless I switched the monitor into USB 2 mode.

Can anyone explain what's going on?
I'm curious why the same cable performs differently.

I remember reading on here something about usb-c not having enough bandwidth for both USB 3 and 4k@60Hz, but I can't find the thread no similar topic turned up on google.

I tested the frame rates using this link that I found from one of the threads in here.
https://www.testufo.com

 

[Stephen.R]

Yes, when you use USB-C in alt-mode (for DisplayPort) it essentially uses the extra wires that would normally provide USB3 speeds, to carry the DP signal, and the remaining wires can only carry a USB2.0 signal.

I wonder if some difference in the host-side controller meant that on the MBP it was automatically dropping back to USB2.0 mode? Did you actually check that the ports operate as USB3 when connected to the MBP?

 

[iamsen47]

Yes, when you use USB-C in alt-mode (for DisplayPort) it essentially uses the extra wires that would normally provide USB3 speeds, to carry the DP signal, and the remaining wires can only carry a USB2.0 signal.

I wonder if some difference in the host-side controller meant that on the MBP it was automatically dropping back to USB2.0 mode? Did you actually check that the ports operate as USB3 when connected to the MBP?

It's not an MBP, it's the Retina MacBook, no other ports for me to check.
The monitor does come with its own USB hub but I didn't check the speeds there since I didn't have anything on hand to. I'll give it a try later today.

Would it make any difference if I switched to a TB3 cable even though the monitor doesn't have a TB3 port?

 

[Fishrrman]

My opinion only.

If it works and gives you the correct 60hz 4k display -- even if the USB ports on the back of the display are running only at USB2 speed -- just use it as it is and enjoy it.

The ports aren't in the best spot for easy access anyway.
I'd use them (perhaps in conjunction with a USB hub or "USB extender cable") for my USB2 devices (keyboard, older flash drives, wireless mouse transceiver, etc.).

I still have plenty of that stuff around. No use to clog up a USB3 A or C port with them.

 

[Stephen.R]

If it works and gives you the correct 60hz 4k display -- even if the USB ports on the back of the display are running only at USB2 speed -- just use it as it is and enjoy it.

I would agree with this - if it ever said USB3 mode was active, I'd suggest that's a bug anyway.

Would it make any difference if I switched to a TB3 cable even though the monitor doesn't have a TB3 port?

That would depend. An active TB3 cable would actually not work at all, and a passive one wouldn't change anything.

 

[iamsen47]

Oh I'm ok with this setup. This is purely academic curiosity.