M1 with LG 5K (27MD5KA-B)

[joevt]

I spent a lot of time with Apple yesterday on the phone, talking to a senior engineer. He is as baffled as I/we are. They are spending the next several days researching, and then we have a scheduled call for next Wednesday. In the meantime, I'm going to try on Monday to go through LG's tech support and see if I can find out another answer there.

No software fix is going to fix this. The only thing to do is connect different stuff to different ports in different ways. You've already tried a couple cables. My next idea is to try a Thunderbolt device such as a dock or hub between the Mac and the display. If you can get a dock or hub to connect at "40 Gbps x1" link width "x2", then the LG UltraFine 5K should be able to connect to the dock or hub with "40 Gbps x1" link width "x2" which will have the necessary bandwidth for the two DisplayPort 1.2 connections allowing full 5K 60Hz capability.

 

[strangeengine]

No software fix is going to fix this. The only thing to do is connect different stuff to different ports in different ways. You've already tried a couple cables. My next idea is to try a Thunderbolt device such as a dock or hub between the Mac and the display. If you can get a dock or hub to connect at "40 Gbps x1" link width "x2", then the LG UltraFine 5K should be able to connect to the dock or hub with "40 Gbps x1" link width "x2" which will have the necessary bandwidth for the two DisplayPort 1.2 connections allowing full 5K 60Hz capability.

Dude, are you still at it?

You told us all repeatedly that the M1 would never run an 5K UltraFine at 5K - COULDN'T run it at 5K - and yet here it is, running them just fine in general.

Now you're sure this isn't fixable in software? Uhh, ok.

 

[joevt]

Dude, are you still at it?

You told us all repeatedly that the M1 would never run an 5K UltraFine at 5K - COULDN'T run it at 5K - and yet here it is, running them just fine in general.

Now you're sure this isn't fixable in software? Uhh, ok.

No dude. I'm not still at it. Try to follow along. I know now that M1 Macs have two DisplayPort connections to Thunderbolt that can be used by dual tile displays to do stuff like 5K 60Hz on an LG UltraFine 5K display. The second DisplayPort connection can only be used by a tiled display. The specs for the M1 Mac did not make sense at the time - why would the M1 Mac's Thunderbolt ports support a dual tile 5K display but not two separate displays? It wasn't until people tried it and reported the ioreg that the answers became clear.

@toddpeperkorn is having an issue with cables or ports or something because he can't get a proper 40 Gbps connection which is required for 5K 60Hz. My suggestion is to try to get a 40 Gbps connection with some other Thunderbolt device, then chain the display to that. Maybe he had bad luck with two cables and a third might work...

Maybe it's a voltage or timing issue that affects some people but not others - like it's on the edge of almost working. Maybe a software fix can fix it - software that tweaks the parameters of the hardware to get over that edge. But why keep a product that is marginally/slightly broken if you can get a replacement that is less broken.

 

[toddpeperkorn]

I appreciate all of your help on this dilemma. I've come to the conclusion that the LG Ultrafine 5k display that I got had physical/mechanical problems.The display stopped working in any way on multiple computers (I also have a 2018 Mini). So I'm returning it. I may try another Ultrafine 5k, but I may also try a nice 4k monitor and see if that will suffice.

Thanks for all your help, everyone!

Todd