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I'm trying to connect the old generation LG UltraFine 4k Display (USB-C DisplayPort) and the old generation LG UltraFine 5k Display (USB-C Thunderbolt) to a 13" MacBook Pro from 2019 (with 4 Thunderbolt 3 USB-C ports).

Here's what works:

  • If I connect just the 5K display through its Thunderbolt USB-C cable, it works fine.
  • If I connect just the 4K display through its DisplayPort USB-C cable, it works fine.

Here's what doesn't work:

  • I can't seem to daisy-chain the 4K display of the 5K display as there's no incoming DisplayPort/Thunderbolt port on the 5K display.
  • If I connect both displays to the MacBook using two different ports, it just registers and uses the one I plugged in first.

I suspect that the Mac somehow either can't handle or don't understand running both Thunderbolt and DisplayPort at the same time - but maybe it's a configuration issue? I haven't been able to find anything online about this.

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  • What resolutions are the 2 monitors. Looking at the spec that would seem important.
    – Tetsujin
    Aug 8, 2019 at 11:15
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    The 5K display is 27" (5120 x 2880). The 4K display is 21.5" (2304 x 4096). The MacBook Pro it self is 13.3" (2560 x 1600). The graphics card in the MacBook is Intel Iris Plus Graphics 655 1536 MB
    – Thomas Watson
    Aug 8, 2019 at 11:21
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    Then the answer is on the spec sheet you linked - Video Support Simultaneously supports full native resolution on the built-in display at millions of colors and: One display with 5120-by-2880 resolution at 60Hz at over a billion colors Up to two displays with 4096-by-2304 resolution at 60Hz at millions of colors Up to two displays with 3840-by-2160 resolution at 60Hz at over a billion colors
    – Tetsujin
    Aug 8, 2019 at 11:23
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    So, one 5k or 2 4k
    – Tetsujin
    Aug 8, 2019 at 11:23
  • Ah nice catch. So one would assume that if I lowered the resolution of the 1st display I connected, it would be able to register the 2nd one, but if I try that, that doesn't work either. It's like it's not even seeing the 2nd external display. So I think it somehow fails before it even comes to that point. Could it be that the MacBook simply doesn't support running with both a Thunderbolt and a DisplayPort display at the same time - I.e. that they both have to be either Thunderbolt or DisplayPort?
    – Thomas Watson
    Aug 8, 2019 at 11:43

2 Answers 2

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+25

Your model MacBook Pro can power external displays at 60Hz in the following combinations:

  • One display at 5120 x 2880 resolution with over a billion colors
  • Two displays at 3840 x 2160 resolution with over a billion colors
  • Two displays at 4096 x 2304 resolution with millions of colors

So, to use both the 5K and 4K displays you need to be mindful of the resolution they're each at. To change the resolution:

  1. Since you're trying to use a 5K display and a 4K display, I'd start by only connecting the 5K display first, preferably in one of the left ports
  2. Go to Apple > System Preferences... > Displays
  3. Make sure the Display tab is selected
  4. Select the Scaled radio button
  5. Now choose one of the scaled options that appear below (Hint: You can view the selected pixel dimensions below the image of your display on the left)
  6. Once you have your display set at 4096 x 2304 (or below), exit System Preferences
  7. Connect your 4K display to one of the right ports
  8. Now go back to Apple > System Preferences... > Displays
  9. Make sure the Display tab is selected
  10. Now hold down the option key to see the additional scaled options for the second display
  11. Now choose one of the scaled options for the 2nd display
  12. Exit System preferences

See the screenshot below for reference:

enter image description here

REMINDER: If you're wanting to use billions of colours, then at Steps 5 and 11 you need to set your resolution at 3840 x 2160 or lower. Otherwise, if you're happy to use millions of colours, you can set the resolution for each display as high as 4096 x 2304 each. Keep in mind the actual resolution choices you get will depend on your displays and the adapters you're using.

Let me know if you're still running into problems and we can troubleshoot from there.

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Apparently, the MacBook Pro can only power external displays up to 5K resolution. What you can do to use both the monitors is: Connect one of the monitors (preferably the 5k first) and lower it's resolution way down, up to 720p perhaps, by following the steps as illustrated in

https://support.apple.com/kb/ph25175?locale=en_US

Then connect the other monitor(4k), now the combined resolution is below/equal to 5k, depending on your adjustments.

Now adjust the resolutions on both the monitors to suit your needs, while you do take care that the combined resolutions don't exceed 5k. Unfortunately, you will have to make some compromises with the resolutions with both the monitors.

I personally haven't tried this out as I don't have the hardware available, so do tell me if it works.

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