After connecting a M1 MacBook Pro 16" to a 5K2K display, how many additional 4K displays can I connect to?

[hajime]

Hi, from the product page of the MacBook Pro 16" 2021, it states that "Up to two external displays with up to 6K resolution at 60Hz at over a billion colors (M1 Pro) or Up to three external displays with up to 6K resolution and one external display with up to 4K resolution at 60Hz at over a billion colors (M1 Max)"

If I want to connect a MacBook Pro to one 5K2K (5120x2160) display, how many 4K displays can I connect to in the case of M1 Pro and in the case of M1 Max? I want all to run at at least 60Hz.

 

[joevt]

The 5K2K has a tiled connection mode (dual HBR2 over Thunderbolt) and a non-tiled connection mode (HBR3) but for M1 Macs, it doesn't matter what kind of connection the display has. For M1 Macs, every display (including tiled displays like the LG UltraFine 5K) counts as a single display.

M1 Macs have extra DisplayPort connections to support tiled displays but those extra DisplayPort connections are not available for single tiled displays. For example, the M1 Max MacBook Pro has 8 DisplayPort connections (one for the built-in display, one for HDMI, and two for each of the three Thunderbolt ports). You can connect three LG UltraFine 5K displays and an HDMI display to use up all 8 DisplayPort connections. You are not able to connect more displays by replacing an LG UltraFine 5K (which uses two DisplayPort connections) with a 4K display (which uses one DisplayPort connection) even if you add Thunderbolt docks.

So in any case:
- The M1 Pro MBP can connect two external displays.
- The M1 Max MBP can connect four external displays.
- You can connect all the displays to Thunderbolt or have a mix of Thunderbolt and HDMI.

It is unknown what happens if you try to connect four 6K displays to the M1 Max MBP (which should be possible using at least one Thunderbolt 4 hub/dock to allow connecting two displays to a single Thunderbolt port).

 

[hajime]

I am confused about your summary. Do you mean regardless of refresh rate and resolutions, M1 Pro MBP can connect two external displays while M1 Max MBP can connect for? For example, if there were a 5K2K display with 120Hz and a 4K display at 60Hz, can both M1 Pro and M1 Max MBP handle it? How about two 5K displays running at 60Hz or above?

 

[Sanpete]

People have reportedly had more than four displays running from the Max, and more than two from the Pro, using adapters and docks designed for that. You can figure out the bandwidth available with some simple math and go from there.

 

[joevt]

I am confused about your summary. Do you mean regardless of refresh rate and resolutions, M1 Pro MBP can connect two external displays while M1 Max MBP can connect for? For example, if there were a 5K2K display with 120Hz and a 4K display at 60Hz, can both M1 Pro and M1 Max MBP handle it? How about two 5K displays running at 60Hz or above?

Yes. If such display exists and can connect as the only external display, then the M1 should allow connecting multiple of them up to the external display count limit.
One person did a test with three LG UltraFine 5K (483 MHz x 2 = 966 MHz) and a HDMI display (594 MHz).

There are limits imposed by the software or hardware that we don't know about. I don't think I've seen anyone achieve 8K60 (2360 MHz) from an M1 Mac yet. 6K60 is 1286 MHz.

People have reportedly had more than four displays running from the Max, and more than two from the Pro, using adapters and docks designed for that. You can figure out the bandwidth available with some simple math and go from there.

The adapters and docks that allow extra displays to be connected use DisplayLink. DisplayLink drives the displays from USB using video compression (≈1 Gbps for 4K60), not from the GPU (≈16 Gbps for 4K60).

 

[Sanpete]

The adapters and docks that allow extra displays to be connected use DisplayLink. DisplayLink drives the displays from USB using video compression (≈1 Gbps for 4K60), not from the GPU (≈16 Gbps for 4K60).

Could well be, I haven't paid close attention since I don't use them. I think some of the monitors used that way haven't been 4K either. But for many purposes that's fine.

 

[hajime]

According to Apple:

"Up to two external displays with up to 6K resolution at 60Hz at over a billion colours (M1 Pro) or
Up to three external displays with up to 6K resolution and one external display with up to 4K resolution at 60Hz at over a billion colours (M1 Max)"

How many K is 5K4K?

 

[joevt]

How many K is 5K4K?

There's no display with 5Kx4K resolution. Doesn't matter. It would count as one display if it works.

 

[hajime]

There's no display with 5Kx4K resolution. Doesn't matter. It would count as one display if it works.

Sorry I meant 5K2K

 

[joevt]

5K is less than 6K so you should be able to connect as many 5K displays as you can 6K displays.

 

[bill-p]

If I want to connect a MacBook Pro to one 5K2K (5120x2160) display, how many 4K displays can I connect to in the case of M1 Pro and in the case of M1 Max? I want all to run at at least 60Hz.

I have a M1 Pro 14" and a 5K2K display (LG). The computer is able to handle the 5K2K display plus another 4K display just fine. Connecting a 3rd display to the remaining Thunderbolt or HDMI ports did not work. It stopped at 2.

 

[hajime]

I have a M1 Pro 14" and a 5K2K display (LG). The computer is able to handle the 5K2K display plus another 4K display just fine. Connecting a 3rd display to the remaining Thunderbolt or HDMI ports did not work. It stopped at 2.

Thanks. Can they run at extended display mode? At what Hz are they running at? Is the failure to connect to the 3rd display is that the M1 Pro only supports two displays? On YouTube, there are people who could connect to several 4K/5K displays using older Intel MacBook Pro. So in this aspect the M1 MacBook Pro is not as good?

Now I recall M1 Macs have no VRAM. For unifying memory, does that mean the more 4K/5K/5K2K displays I connect to the MacBook Pro, the less memory I have to run memory intensive tasks?

 

[joevt]

Can they run at extended display mode?

Yes.

At what Hz are they running at?

The displays mentioned here are usually 60Hz. Some displays can do more - like 120Hz.

Is the failure to connect to the 3rd display is that the M1 Pro only supports two displays?

M1 Pro supports only two displays. Get an M1 Max if you want more displays.

On YouTube, there are people who could connect to several 4K/5K displays using older Intel MacBook Pro.

Up to four, like the M1 Max. Or more if you add an eGPU (which M1 Macs do not support).

So in this aspect the M1 MacBook Pro is not as good?

Depends on which M1 MacBook Pro you're talking about.
One advantage that M1 Macs have is that they don't count tiled displays like the LG UltraFine 5K as two displays. You can still only connect one tiled display per Thunderbolt port, but you can connect three of them to the M1 Max MBP and only two or one to an Intel MBP. 4K, 6K, and the new Apple Studio Display (5K) are not tiled displays though.

Now I recall M1 Macs have no VRAM. For unifying memory, does that mean the more 4K/5K/5K2K displays I connect to the MacBook Pro, the less memory I have to run memory intensive tasks?

Displays don't take a lot of memory.
3840 x 2160 x 4 bytes per pixel = 32 MB.
6016 x 3384 x 4 bytes per pixel = 78 MB.
7680 x 4320 x 4 bytes per pixel = 127 MB.

 

[hajime]

Thanks for the analysis. For M1 MacBook Pro, we don't need to worry about the speed at which the the displays are on when we try to decide the number of displays they can support?

 

[joevt]

Thanks for the analysis. For M1 MacBook Pro, we don't need to worry about the speed at which the the displays are on when we try to decide the number of displays they can support?

I believe so. I don't recall seeing anyone get more than 6K working on M1 Mac so maybe 8K won't work.

 

[Diablo360]

I was able to connect 4 x 49" (5120 x 1440) monitors. Two plugged into their own USB C port, and one plugged into a Caldigit Element dock via USB C. I was unable to connect another (lower resolution) monitor via the empty HDMI port, or via using another dock. So it looks like there is some limit.

 

[JohnDoe12]

I was able to connect 4 x 49" (5120 x 1440) monitors. Two plugged into their own USB C port, and one plugged into a Caldigit Element dock via USB C. I was unable to connect another (lower resolution) monitor via the empty HDMI port, or via using another dock. So it looks like there is some limit.

Why didn't you connect three monitors via the Element Hub?

 

[joevt]

Why didn't you connect three monitors via the Element Hub?

There are no Thunderbolt docks that support three displays.
Thunderbolt 1 and Thunderbolt 2 docks support 1 display. You need a second dock to support 2 displays.
Thunderbolt 3 docks support 2 displays.
Thunderbolt 4 docks also support 2 displays, even if they have 3 downstream Thunderbolt ports.

Also, remember that Thunderbolt only supports up to 40 Gbps.

5120x1440 60Hz is at least 462 MHz pixel clock which is at least 11 Gbps at 8bpc. So I suppose Thunderbolt could support 3 of them. However, the only way I can think of to get 3 or more DisplayPort connections on the same Thunderbolt cable without using MST (which macOS doesn't support for multiple displays) is to put an eGPU in the Thunderbolt chain - one of the eGPU's that has a downstream Thunderbolt port such as the Blackmagic eGPU or the Sonnet eGPU Breakaway Puck RX 5500XT/5700 (but M1 Macs don't support eGPUs - you need an Intel Mac). With the eGPU, two DisplayPort connections can come from the Mac, and two more can come from the eGPU. I don't think anyone has tested this arrangement and you would probably just use the DisplayPort or HDMI outputs of the eGPU for connecting non-Thunderbolt displays. One thing that would be a problem is that while the display takes only 11 Gbps, Thunderbolt divides its bandwidth for DisplayPort in discrete chunks 1.296, 2.16, 2.592, 4.32, 5.184, 6.48, 8.64, 12.92, 17.28, 25.92 Gbps. For 3 displays, you would want 12.92 Gbps for each which is 2 lanes of HBR3. You can get that with a USB-C dock or cutting some wires in a DisplayPort cable. It would require an MST hub to convert 2 lanes of HBR3 to 4 lanes of HBR2 for the display. The CalDigit SOHO is a USB-C dock with an MST hub so it should be able to do that but you would need one for each display. It is unknown if macOS will allow DisplayPort tunnelling from the Mac to beyond the downstream Thunderbolt port of the eGPU. It is unknown if macOS will allow DisplayPort tunnelling from the Mac to a Thunderbolt port that exists before the eGPU in the Thunderbolt chain.

For the Apple Pro Display XDR, when it is connected to an Intel Mac that supports HBR3 but not DSC, Apple has a trick to allocate two 25.92 Gbps chunks (4 lanes of HBR3 each) to support a dual tile connection mode (one DisplayPort connection for each 3008x3384 half of the display). This works because each half of the display is only ≈19 Gbps and Thunderbolt doesn't transmit the DisplayPort stuffing symbols that are used to fill up the DisplayPort bandwidth.

If you use USB for video (such as with DisplayLink) then there's no limit to the number of displays that can be connected with a single Thunderbolt connection. DisplayLink uses ≈1Gbps for 4K instead of the usual 16 Gbps. That means there's a lot of compression (any video source such as YouTube or Netflix or BlueRay or DVD uses compression).

 

[npjnpj]

Hope it's ok resurrect and ask a related question here...

Would really appreciate some help with this. I've been googling madly but can't find an answer.

I've got an M1 Pro 16" + LG5k2k (34" via Thunderbolt) + Dell 4k (27" via USB-C).

With the MBP lid open, everything works fine.

If I close the lid and go into clamshell mode - the LG 5k2k loses it's connection. (flashes on trying to connect briefly with an image that looks to be lower res and doesn't take up the whole screen). As if the bandwidth drops (is that possible?).

I tried connecting the 4k display differently, via a Displayport cable, via a Thunderbolt hub, which, as mentioned above, must use less bandwidth - because this DOES work with clamshell mode (together with the 5k2k).

I thought the MBP display support would be identical between open and clamshell modes. Does something change when you switch or is this some kind of software/firmware issue?

The displays are a Dell P2721Q (4k) and an LG 34wk95u (5k2k).

Thanks in advance.

 

[npjnpj]

(accidental double post sorry!)

 

[jdoll021]

Hope it's ok resurrect and ask a related question here...

Would really appreciate some help with this. I've been googling madly but can't find an answer.

I've got an M1 Pro 16" + LG5k2k (34" via Thunderbolt) + Dell 4k (27" via USB-C).

With the MBP lid open, everything works fine.

If I close the lid and go into clamshell mode - the LG 5k2k loses it's connection. (flashes on trying to connect briefly with an image that looks to be lower res and doesn't take up the whole screen). As if the bandwidth drops (is that possible?).

I tried connecting the 4k display differently, via a Displayport cable, via a Thunderbolt hub, which, as mentioned above, must use less bandwidth - because this DOES work with clamshell mode (together with the 5k2k).

I thought the MBP display support would be identical between open and clamshell modes. Does something change when you switch or is this some kind of software/firmware issue?

The displays are a Dell P2721Q (4k) and an LG 34wk95u (5k2k).

Thanks in advance.

Hi,

Did you get the clamshell mode issue resolved?

I do have an unrelated question if you don’t mind. I’m looking at a similar setup to what you have, 34” 5k2k with a 27” 4k (or maybe even go bigger with 40/32, same resolutions). But what I’m not sure is how MacOS handles the scaling. Do you run them at scaled resolutions or native resolutions? If scaled, do they both run the same resolution or is each monitor using a different resolution? What resolution are you using (if not native)?

Thanks!