[SOLVED] Macbook Pro M3 not detecting second monitor. Have tried all various configurations to no avail

[xbtmatt]

EDIT: Two HDMI 2.1 cables with one running through a VMM 7100 HDMI -> USB-C adapter works flawlessly! I am able to get HDR on both monitors with both at 60 hz (will try one at 100hz later), both at 3008x1692 (although can probably do 4k? didn't try).

Hi guys, I can't seem to get my dual monitor setup to work. I'm not sure if this is specifically BetterDisplay related, but since I'm using it I figured it's best to start here first.

Specs:
I have a Macbook Pro M3, Model Identifier: Mac15,7

I'm using two alienware aw3225qf's (two 3840x2160 monitors)

I'm trying to simply get my Macbook to detect both at the same time at any resolution/refresh rate. Ideally it'd be the rate they describe is possible:

Two external displays
Supports two displays in any one of these configurations:
- Two displays up to 6K at 60Hz over Thunderbolt
- One display up to 6K at 60Hz over Thunderbolt, and one display up to 4K at 144Hz over HDMI

Is this simply a limitation with my Macbook or am I using an incorrect combination of cables? I've tried nearly every combination with various adapters, HDMI or DisplayPort to USB-C, all purportedly Thunderbolt 4 capable.

The current setup I have is a native HDMI 2.1 cable (to the Macbook HDMI port) and then a CableMatters DisplayPort to USB-C cable. I tried HDMI to USB-C too (with the native HDMI port as the other monitor), although it might be HDMI 2.0 and not HDMI 2.1. I'm trying the 2.1 cable tomorrow.

The issue is I can only ever use one at a time. Both work independently, but when I connect the second I always get No <HDMI_OR_DISPLAY_PORT> signal from your device.

As I said, it always works when only one is connected. But not when two are connected. Both can independently get 240hz at 4k res with HDR on.

Is there anything I can do with BetterDisplay (or in general) to force my MacBook to recognize the second monitor? This is driving me nuts.

Thanks.

[waydabber]

I think the entry level M3 supports only one external display with the lid open - or two in clamshell mode with the latest macOS versions. I am not sure how things are configured and whether the lid is closed.

[xbtmatt]

It's the Macbook M3 Pro, so I think it should be able to handle two external displays (according to the Apple specs)

They say that the connection must be through a thunderbolt cord, and when I connect through my ports (all thunderbolt 4) they show up as USB3.1 in System Report, so I suspect that's why. Do you have any idea if I need to buy a thunderbolt cable? Can I even use a thunderbolt cable on non-apple displays?

Sorry for the random question, I really appreciate BetterDisplay, you've basically saved my WFH life and greatly increased my productivity on my Mac! Thanks for all the hard work

[waydabber]

Yes, if it has the Pro chip inside (so not the "MacBook Pro with M3" but the "MacBook Pro with M3 Pro" - what a confusing naming lol), then it should handle two external displays alongside the built-in display. If the display is a Thunderbolt display which can't receive DisplayPort Alt Mode over USB-C, then yes, proper connectivity is important. However a 20GBps USB-C cable can transmit Thunderbolt just fine. But the Alienware display you have has HDMI and DisplayPort afaik - so a simple cheap, direct USB-C to DisplayPort cable should work best. Alternatively you can try some VMM7100 based USB-C to HDMI dongle or simply use a HDMI 2.1 cable and use the built-in HDMI port.

[xbtmatt]

Yes I just tried my last possible solution, the VMM7100 Type C -> HDMI Adapter (CableMatters) and it works!! Absolutely brilliant, super happy.

The Macbook refused to work with any other type of adapter. Specifically, I was using an older CableMatters Type C -> HDMI (tried DisplayPort, too) but apparently it must be the VMM7100 version. I thought it would just downscale my monitor but it wasn't even detecting it.

To be clear, I was using HDMI 2.1 plus another cable. Each one worked independently, but even with closing the Macbook only one at a time would ever be "detected".

Anywho, it works now, so thanks for the VMM 7100 suggestion, that's what would've solved it!

[xbtmatt]

@waydabber Sorry for the tag, but did something change recently with detecting displays? I see there's new options in BetterDisplay specifically related to the VMM 7100 adapter, but suddenly my second monitor no longer is being detected.

Is there any way I can rollback to a previous version, too, just to see? Thanks!

[waydabber]

No, not specifically, but of course you can roll back, all previous versions are available here:

https://github.com/waydabber/BetterDisplay/releases

[xbtmatt]

Awesome, sounds good.

I got it to work, apparently I just had to try connecting in different orders.

Thanks again!

[cgatlin00]

Sorry to bring this thread back up, but I have been running into a very similar problem with my MacBook Pro M3 Pro as well and am using better display. I am currently using two MSI 4k monitors and attempting to connect them to my computer but only one seems to work at a time. I have also tried every cable combination possible

• Two thunderbolt cables
• One thunderbolt one hdmi
• Two HDMI cables

No matter what I seem to do, I cannot get both monitors to be recognized at the same time it is only one or the other. Any other suggestions would be greatly appreciated!