M4 Mini dual monitor refresh rate issues

[MrGimper]

OK So I have a mac mini M4 (non-pro chip) with 32GB RAM and 1TB running Tahoe 26.3 but this happened on Sequoia too .....

My understanding from the Apple Website:

Two external displays​

Supports two displays in the following configuration:

• Two displays up to a native resolution of 6K (6144 x 3456) at 60 Hz or 4K (3840 x 2160) at 144 Hz over Thunderbolt or HDMI

I have 2x Koorui S2741LM screens. Screen A connected via HDMI to the HDMI port of the mac. Screen B connected to the mac via either a UGreen USB-C to DisplayPort adapter, OR a UGreen USB-C to HDMI adapter, I've tried both. These cables are connected to a TB/USB port on the back of the mac, one at a time

The monitors support 4K at 160Hz

Screen A can be set to pretty much any comfortable HiDPI resolution, 1920x1080, 2304x1296 (which I use), and 2560x1440 up to 144Hz. Zero issues.

The Screen B acts the same when connected to the mac HDMI port.

When Screen B is connected (as well as Screen A on the mac's HDMI port) either via USB-C to HDMI or USB-C to DisplayPort, I can match those resolutions no problem, however I CANNOT go above 60Hz. The only way I can get above 60Hz is if I drop the resolution to 1280x720.

Nothing on Screen B changes if I physically unplug Screen A from the mac HDMI port so thst it's the only connected screen.

Oddly, I also have a 16" M3 Max Macbook Pro, and if I plug Screen B in using the exact same USB-C to DisplayPort cable, I can get 1920x1080, 2304x1296, and 2560x1440 HiDPI resolutions up to 144Hz on the MBP

I've tried messing with the configuration with and without BetterDisplay, I own the Pro version.

Cables being used:

USB-C to HDMI 2.1 (48Gbps)

https://www.amazon.co.uk/dp/B0BXWBSJ77

USB-C to DisplayPort

https://www.amazon.co.uk/dp/B0CDW73QH7

Any ideas on how to resolve this or if it's an M4 limitation.

Thanks

 

[Bigwaff]

Have you tried 120Hz using USB-C to DP alone, w/out 2nd monitor attached?

 

[MrGimper]

Have you tried 120Hz using USB-C to DP alone, w/out 2nd monitor attached?

Think I tried USB-C to HDMI 2.1 alone, but not DP. Will give that a go later.

 

[MrGimper]

OK if I detach Screen A from Mac by pulling the HDMI cable from the internal HDMI port, and have ONLY Screen B attached via USB-C to DP, then Screen B works with all refresh rates after a reboot. It’s obviously seen as the main screen. If I then plug in Screen A again via internal HDMI then it exhibits the limited refresh rate issue.

So this is an M4 limitation then?

 

[MrGimper]

So to add to this further….. If I connect only a single screen, using any connection (so either internal HDMI port, USB-C to HDMI, or USB-C to DP) I get the full range of resolutions and refresh rates on that single screen.

If I add a second screen using any connection (so internal HDMI if free, or another USB-C to HDMI/DP inc if using one for first screen) then the screen is restricted. If I remove the first screen and reboot so the second screen is the ONLY screen, it gets the full range.

In every instance the only way to get a refresh rate above 60 on any restricted screen is to drop the resolution to 1280x720 HiDPI.

So despite having an HDMI 2.1 port which clearly can handle at least 40Gbps, and thunderbolt/USB-C that can handle 40Gbps, you can’t use 2 of those together at full bandwidth.

 

[1105]

The mini definitely has some type of issue with dual screens or port limitations.

I had a maxed out m4 pro and ran dual screens with a kvm thru usb-c / thunderbolt. Frequently the machine would drop the second display and give no indication that the second was added or plugged in. I'd have to unplug both, plug in A, then plug in B to get it to work again.

Upgraded to the m4 studio and three weeks in, using the same cables and kvm, it hasn't dropped it once.

 

[Bigwaff]

Appears first display attaching at max 160 refresh rate (you say full range) by default. Are you able to set the 120 or lower refresh rate using display onboard controls before attaching to M4 mini?

 

[MrGimper]

Just to circle back to this. I had to unplug both displays and plug them both back in at the same time. Once I did this they both worked at all resolutions at 144hz