Running two LG UltraFine displays from M1 Pro 16" late 2021 Macbook Pro

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Running two LG UltraFine displays from M1 Pro 16" late 2021 Macbook Pro

Hello, I have a MK193LL/A (M1 Pro 16" late 2021 Macbook Pro), the specs for this model state "This model supports full native resolution on the interior display in addition to up to two external displays up to 6016x3384 (6K) at 60 Hz".

I want to connect two LG UltraFine 27MD5KL-B displays as an extended desktop. The MK193LL/A has three Thunderbolt 4 ports. The displays have one Thunderbolt 3 port with up to 94W power supply and 3 USB Type C ports.
I can connect both displays from the laptop Thunderbolt ports and they both light up. Only one mirrors the laptop screen, the other shows the screensaver image. The cursor will only work on the mirror screen. If I open the laptop, both screens show the screensaver and the cursor will move seamlessly across both screens, but no log in field is visible.
as per attached screenshot I can see both external displays and the built-in laptop display in system settings, and I can drag them to different positions, but these things are only visible / possible on the built-in screen.
I contacted LG chat support and was told that it is not possible to connect two of these displays to one Mac. That seemed very improbable to me so I am here ... Grateful for any help!

Posted on Jul 29, 2024 7:15 PM

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Answer 1

Jul 31, 2024 5:07 PM in response to Pduncanj

Pduncanj wrote:

Further to previous: am wondering whether it's worth trying a HDMI to Thunderbolt cable to run one of the displays? Saw this on Macworld 'the M1 and M2 Mac Mini does natively support up to two external monitors—one via the HDMI port and a second via USB-C', so thought perhaps it might work for my MBP

I've never heard of a HDMI to Thunderbolt cable - an adapter that takes HDMI video input, and that generates Thunderbolt-encapsulated DisplayPort output. I don't think such a thing exists.

You're probably thinking of a USB-C to HDMI adapter cable. Those work with USB-C (DisplayPort) input, not USB-C (Thunderbolt) input. I believe they're meant for use with the USB-C side plugged into a computer (or dock), and the HDMI side plugged into a monitor – not the reverse.

They're going to be a no-go as far as connecting a LG UltraFine 27MD5KL-B monitor, with 5120 x 2880 pixel resolution, that expects Thunderbolt 3 input.

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Answer 2

Jul 30, 2024 10:33 AM in response to Pduncanj

<< Only one mirrors the laptop screen, the other shows the screensaver image. >.

if you get ANY picture, the displays are working as designed, you are not done setting them up yet.

Use the (Arrange) button to specify where the displays are located in physical space on your bench. Then the mouse can move across the boundary between physical displays as one large "extended desktop"

...

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Answer 3

Pduncanj Author

Level 1

12 points

Aug 1, 2024 4:49 AM in response to Grant Bennet-Alder

Thank you once again. So I was finally able to get the setup working, but I am not confident in the ongoing reliability of this, as what I had to do was so strange ...

I tried plugging the cables into the MBP after rebooting and with the lid closed, and then designating the main and extended displays. I then had both working, but the cursor would only move to the extended display from the wrong side of the screen. I tried various way of configuring each monitor and no change. I eventually got it to work correctly by switching the physical position of the monitors, which makes absolutely no sense to me, but there it is.

Thanks again for ongoing advice!

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Answer 4

Jul 30, 2024 11:53 AM in response to Pduncanj

<< and it has no effect on the external displays - they continue to behave as I described. >>

it is possible you may have to select each display and explicitly specify "use as Extended display" rather than "use aa mirrored display".

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Using DisplayLink when you are not FORCED to use DisplayLink is a Bad Idea™

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Answer 5

Aug 1, 2024 7:06 AM in response to Pduncanj

<< Graphic&Displays screenshot showing NO serial numbers for either display >>

if the displays do not present their serial numbers in the correct field, according to VESA standards, the Mac may stumble when trying to tell the two displays apart.

If you had two different Make&Model displays, Users have not reported such issues.

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Answer 6

Jul 31, 2024 4:49 PM in response to Pduncanj

Pduncanj wrote:

• I contacted LG chat support and was told that it is not possible to connect two of these displays to one Mac. That seemed very improbable to me so I am here ... Grateful for any help!

I don't know if this is applicable, but I'll mention it just in case.

Each display is supposed to have its own serial number, part of its Extended Display ID structure. However, when manufacturers are assembling monitors, sometimes they take shortcuts. Instead of customizing the firmware for each individual monitor to include a unique ID, they'll flash a whole batch of monitors with the SAME "unique" ID.

If you get two such monitors from the same batch, and attach them to a computer that's depending on the unique IDs to actually be, you know, unique, there can be problems.

Again, I have no idea whether this applies to your LG monitors, but their "improbable" tech support answer might make more sense if some off their monitors actually suffer from this issue.

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Answer 7

Jul 31, 2024 5:05 PM in response to Servant of Cats

modern displays reveal their serial numbers in the system report:

about this Mac > (system Report) > Graphics&Displays

...

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Answer 8

Pduncanj Author

Level 1

12 points

Jul 30, 2024 11:16 AM in response to Grant Bennet-Alder

Thank you Grant for taking the time to reply. I already tried various ways of arranging the displays in the way you describe, but I can only do this on the laptop screen and it has no effect on the external displays - they continue to behave as I described.

I'm thinking I may have to resort to using DisplayLink with an adaptor, but I understand that this may compromise the screen resolution so I'm not keen on this pathway.

Thanks again

Peter

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Answer 9

Pduncanj Author

Level 1

12 points

Jul 31, 2024 3:14 PM in response to Grant Bennet-Alder

thanks again for the ongoing advice. The display settings panel on my machine appears differently from your version, mine doesn't have tabs for 'Display', 'Arrangement', and 'Color'. However after updating from Sonoma 14.5 to 14.6, mine now shows options with a right click on each display in settings, per attached screenshots. Once this came up, on one occasion I had both displays working normally with one as the main display and one as extended. However when I unplugged the displays and tried again, I was never able to replicate this. There are all sorts of weird artefacts ... e.g. when I connect the second display, it immediately selects itself as the main display, the first display (which was designated main) shows the screensaver, and the cursor only appears on the first display, so nothing can be done until the second display is disconnected.

Even setting up to use one display as the main display and the MBP screen as an extended display (no second external display connected) will not work. I can use one external display, only if it is designated main display.

I'm with you on DisplayLink - last resort. Seems like I'm close to that point.

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Answer 10

Pduncanj Author

Level 1

12 points

Jul 31, 2024 3:33 PM in response to Grant Bennet-Alder

Further to previous: am wondering whether it's worth trying a HDMI to Thunderbolt cable to run one of the displays? Saw this on Macworld 'the M1 and M2 Mac Mini does natively support up to two external monitors—one via the HDMI port and a second via USB-C', so thought perhaps it might work for my MBP

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Answer 11

Pduncanj Author

Level 1

12 points

Aug 1, 2024 4:35 AM in response to Servant of Cats

Thank you for the thoughtful reply - that is quite bizarre but there it is. Per screenshot below, my monitor s/n do not appear. However they would not be from the same batch - very slight difference in model number, altho specs are identical. And I bought one new and one used.

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