5K ultrafine + 4K over TB dock issues

[MiBook84]

Maybe not specifically for MacBook Pro but since I have a thunderbolt 3 dock in tandem with my LG ultra fine 5K screen and a LG Ultra HD 4K screen I'd like to share my experiences.

I have a 14" MacBook Pro M1 Max 10/32 and use an legato thunderbolt 3 Pro dock which I've had my 5K connected to via thunderbolt and the 4K screen over the display port connector on the same dock.

Doing this works well - but with some issues I first thought was Monterey bugs. The 5K screen will represent itself as a 4096x2304 pixel screen instead of 5120x2880. I thought this was just a weird scaling so I set it up to 5K resolution in the scaling dialogue. This works well and in system profiler it looks all good. BUT. It isn't. *insert drama*.

So, what seems to happen is that bandwidth is constricted scaling down the data for the 5K screen to "real" 4K or something (4096 pixels instead of 3840). Anyway the screen gets a wonky resolution and it is not really super easy to detect at first as the screen has so many pixels and the scaling difference is not very, very visible. However - it is there.

It took me some time to realise but after some experimentation I now know I have to run both screens from one port of the computer each. So I decided to run the 5K straight to the laptop and the 4K through the docking station. Doing so also changes the scaling dialogues for the 5K screen so there is a correct default resolution.

Now both screens looks really sharp and good!

Im including a screenshot of the system profiler data versus the display data in about my Mac to show how confusing things are. And to add to the confusion, the built in screen, if used, will never show the "UI looks like" just the screen resolution, so it is hard to know if the default mode is actually correct. I had to measure the amount of pixels with the screen grab tool to correctly see. I guess this is due to Apple running their laptop screens in a not pixel-matched resolution as default between 2016-2020 pre m1 pro/max. A bit silly to have to hide that fact though.. PS, macOS seem to throw around the names of the screens too but whatever. I am already too confused!

 

[IngoX]

A single Thunderbolt 4 port kan only connect a single 5k or 8k monitor, but two 4k monitors.

How many monitors can I connect through Thunderbolt?

 

[joevt]

LG UltraFine 5K requires two DisplayPort signals over Thunderbolt to get 5K60. Each Thunderbolt port can only have two DisplayPort signals. So connecting the LG 4K means there's one less DisplayPort signal for the LG 5K. The LG 5K can only get 4K60 when it has only one DisplayPort connection over Thunderbolt (or when connected with USB-C not -Thunderbolt).

But you could maybe get a 5K39 timing on the LG 5K with the single DisplayPort connection (at least on an Intel Mac - I don't know how to get custom timings on M1 Macs).

You can edit the names of the displays using a utility like SwitchResX. SwitchResX can be used to create custom scaled resolutions. It can create custom timings as well but I don't think they work on M1 Macs.

 

[killawat]

agree with what @joevt posted above. Thunderbolt can push a lot of bandwidth around, but its easy to get oversubscribed when pushing it to its limits. And bandwidth issues show up in subtle, frustrating ways. As a general rule, bypass the dock entirely for 5k displays and eGPUs.

 

[WilliamG]

I'm having trouble with a similar setup. I have a 16" MacBook Pro M1 Max connected via a Caldigit TS3 Plus to my LG 5K Display. I then have a second 27" 1440p monitor that I connected a DisplayPort to DisplayPort cable from the Caldigit to the monitor (Acer Predator). For some reason, I get no display unless I first unplug the LG's power cord, and then reconnect it. Then I get the Acer come on (at a glorious 144hz) followed by the LG 5K display. Everything then works great. However, as soon as the computer sleeps, I lose the Acer display again unless I follow the same procedure.

I located a DisplayPort to HDMI cable (going from the DisplayPort on the dock to the HDMI on the monitor) and same issue!

So then I located an HDMI to HDMI cable, and no such problem, while maintaining the connection to the LG 5K display. I grabbed a HyperDrive Thunderbolt 3 dock with DisplayPort, and it has the same issues I mentioned earlier.

Anyone have any ideas why I can't get DisplayPort's connection to reconnect or just work when I connect it without having to fudge with my LG Display first?

 

[joevt]

I'm having trouble with a similar setup. I have a 16" MacBook Pro M1 Max connected via a Caldigit TS3 Plus to my LG 5K Display. I then have a second 27" 1440p monitor that I connected a DisplayPort to DisplayPort cable from the Caldigit to the monitor (Acer Predator). For some reason, I get no display unless I first unplug the LG's power cord, and then reconnect it. Then I get the Acer come on (at a glorious 144hz) followed by the LG 5K display. Everything then works great. However, as soon as the computer sleeps, I lose the Acer display again unless I follow the same procedure.

I located a DisplayPort to HDMI cable (going from the DisplayPort on the dock to the HDMI on the monitor) and same issue!

So then I located an HDMI to HDMI cable, and no such problem, while maintaining the connection to the LG 5K display. I grabbed a HyperDrive Thunderbolt 3 dock with DisplayPort, and it has the same issues I mentioned earlier.

Anyone have any ideas why I can't get DisplayPort's connection to reconnect or just work when I connect it without having to fudge with my LG Display first?

In your case, the LG5K is getting both DisplayPort connections over Thunderbolt, so there's none left over for the 1440p monitor.
If you connect the 1440p monitor first, then it gets one DisplayPort connection. When you connect the LG5K after that, there's only one remaining DisplayPort connection available over Thunderbolt, so the LG5K is limited to 4K60 (nowhere in macOS does it show that it is scaling 5K downto 4K - use SwitchResX to see that).

Now, if you don't mind that you can't get the LG5K to do more than 4K when another display is connected to the same Thunderbolt port, then a method to work around the problem where the other display won't turn on after boot or wake is to limit the LG5K to 20 Gbps using a USB-C (not-Thunderbolt) cable or a Thunderbolt cable that only supports 20 Gbps. That way the LG5K can't take both DisplayPort connections from Thunderbolt - it will always only take one DisplayPort.

 

[WilliamG]

In your case, the LG5K is getting both DisplayPort connections over Thunderbolt, so there's none left over for the 1440p monitor.
If you connect the 1440p monitor first, then it gets one DisplayPort connection. When you connect the LG5K after that, there's only one remaining DisplayPort connection available over Thunderbolt, so the LG5K is limited to 4K60 (nowhere in macOS does it show that it is scaling 5K downto 4K - use SwitchResX to see that).

Now, if you don't mind that you can't get the LG5K to do more than 4K when another display is connected to the same Thunderbolt port, then a method to work around the problem where the other display won't turn on after boot or wake is to limit the LG5K to 20 Gbps using a USB-C (not-Thunderbolt) cable or a Thunderbolt cable that only supports 20 Gbps. That way the LG5K can't take both DisplayPort connections from Thunderbolt - it will always only take one DisplayPort.

Interesting info. You're exactly right. I knew my display didn't look as crisp but couldn't put my finger on it, when both the 5K display and the 1440p display were running via a single connection. I'm just going to deal with using a separate USB Type C to DisplayPort cable on the other side of the MacBook Pro. That will solve it.

Thank you for your help - much appreciated.