iMac 2020 with 2 LG Ultrafine 5k monitors?

[rkuo]

So past iMac's apparently haven't been able to drive two external 5k monitors due to both Thunderbolt ports being driven off of a single bus.

HOWEVER, the new iMac specs say it can drive up to 2 6K external monitors. Does this also translate to the 5K's? Would love to hear of hands on confirmation if possible!

 

[Cookie18]

No hands on confirmation here but if it can run two 6K displays then it can run two 5K displays as they would require less power to do so.

 

[rkuo]

No hands on confirmation here but if it can run two 6K displays then it can run two 5K displays as they would require less power to do so.

You would think, but with Apple you never know ... sometimes support for certain things is hardcoded!

 

[Cookie18]

You would think, but with Apple you never know ... sometimes support for certain things is hardcoded!

The Apple website specifically says you can connect two 5K displays.

iMac (Retina 5K, 27-inch, 2020) - Technical Specifications - Apple Support

support.apple.com

 

[rkuo]

Thank you! Google wasn't turning up those pages. It's interesting that there is a distinction between the 5700 and the other cards, but as far as 5k monitors go, I think 8 bpc should be OK for me.

 

[Cookie18]

No problem, the 5700 series is massively more powerful than the other 5300 and 5500 so that’s why.

 

[phairphan]

I wonder if there's an error on that page. A 5700-equipped iMac can drive two 6K displays at 10bpc, but can drive two 5K displays at only 8bpc? Something seems...off.

 

[joevt]

A 5700 iMac uses a single DisplayPort 1.4 HBR2 with DSC signal (6016x3384) to drive an XDR display so it can drive two of them no problem.
Other Macs that support DisplayPort 1.4 but not DSC use dual HBR3 (each 3008x3384) to drive an XDR display at 6K.

DSC (Display Stream Compression) effectively triples bandwidth of DisplayPort. That's why HBR2 with DSC (100% * 3) can do the same as dual HBR3 (2 * 150%).

There are two kinds of 5K displays:
1) ones like the iMac 5K Retina display or the LG UltraFine 5K display, or the Dell UP2715K, or HP z27q, etc.. which use dual DisplayPort 1.2 (HBR2) signals.
2) others like the Iiyama Prolite XB2779QQS-S1 that use a single DisplayPort 1.4 (HBR3) signal.

1) The LG UltraFine 5K does not support DisplayPort 1.4 - it requires two DisplayPort 1.2 HBR2 signals (each 2560x2880) for 5K - the iMac's single Thunderbolt controller only has two DisplayPort inputs so only one LG UltraFine 5K display can be supported at 5K. Two HBR2 signals can do 5K 60Hz 10bpc. You can't connect two such displays at 5K by setting them to 8bpc since they don't have a 5K single connection mode. You can probably connect two of these as 4K though since they can use a single connection for that.

2) If you want two 5K displays, then you need one that supports DisplayPort 1.4 (e.g. Iiyama Prolite). An HBR3 signal (150%) is only 50% faster than HBR2 (100%), or 75% of dual HBR2 (200%), so it can only do 5K 60Hz at 8bpc (unless you have a display that supports DSC - I don't think there are any 16:9 5K displays that support DSC unless you count the 6K XDR).

If the 5300 iMac also supports DSC (I assume it does since it's a Navi GPU?), then why can't it support two XDR displays? Maybe it doesn't support DSC or Apple disabled DSC in this case (in which case it uses two DisplayPort 1.4 HBR3 signals - each 3008x3384 - to get 6K), or the XDR has too many pixels to have two supported. Maybe Apple is being conservative - maybe you can connect two XDRs to a 5300 or 5500XT but there might be a performance issue so they say that only one is supported (they did this with the Vega II MPX module - they said it supports two XDRs but people successfully connected three all running at 6K).

 

[rkuo]

A 5700 iMac uses a single DisplayPort 1.4 HBR2 with DSC signal (6016x3384) to drive an XDR display so it can drive two of them no problem.
Other Macs that support DisplayPort 1.4 but not DSC use dual HBR3 (each 3008x3384) to drive an XDR display at 6K.

DSC (Display Stream Compression) effectively triples bandwidth of DisplayPort. That's why HBR2 with DSC (100% * 3) can do the same as dual HBR3 (2 * 150%).

There are two kinds of 5K displays:
1) ones like the iMac 5K Retina display or the LG UltraFine 5K display, or the Dell UP2715K, or HP z27q, etc.. which use dual DisplayPort 1.2 (HBR2) signals.
2) others like the Iiyama Prolite XB2779QQS-S1 that use a single DisplayPort 1.4 (HBR3) signal.

1) The LG UltraFine 5K does not support DisplayPort 1.4 - it requires two DisplayPort 1.2 HBR2 signals (each 2560x2880) for 5K - the iMac's single Thunderbolt controller only has two DisplayPort inputs so only one LG UltraFine 5K display can be supported at 5K. Two HBR2 signals can do 5K 60Hz 10bpc. You can't connect two such displays at 5K by setting them to 8bpc since they don't have a 5K single connection mode. You can probably connect two of these as 4K though since they can use a single connection for that.

2) If you want two 5K displays, then you need one that supports DisplayPort 1.4 (e.g. Iiyama Prolite). An HBR3 signal (150%) is only 50% faster than HBR2 (100%), or 75% of dual HBR2 (200%), so it can only do 5K 60Hz at 8bpc (unless you have a display that supports DSC - I don't think there are any 16:9 5K displays that support DSC unless you count the 6K XDR).

If the 5300 iMac also supports DSC (I assume it does since it's a Navi GPU?), then why can't it support two XDR displays? Maybe it doesn't support DSC or Apple disabled DSC in this case (in which case it uses two DisplayPort 1.4 HBR3 signals - each 3008x3384 - to get 6K), or the XDR has too many pixels to have two supported. Maybe Apple is being conservative - maybe you can connect two XDRs to a 5300 or 5500XT but there might be a performance issue so they say that only one is supported (they did this with the Vega II MPX module - they said it supports two XDRs but people successfully connected three all running at 6K).

Wait, so the iMac 2020 won't support two UltraFine 5K's? I was operating under the assumption that if the UltraFine 5K's can run at 10-bit, they can also run at 8-bit and be compatible. That would be a real bummer if it didn't work that way. I get the dual displayport lanes is an issue, but, if that's so, the support article seems to really miss the point.

 

[joevt]

Wait, so the iMac 2020 won't support two UltraFine 5K's? I was operating under the assumption that if the UltraFine 5K's can run at 10-bit, they can also run at 8-bit and be compatible. That would be a real bummer if it didn't work that way. I get the dual displayport lanes is an issue, but, if that's so, the support article seems to really miss the point.

There are a couple of support pages that mention the dual-link SST nature of the LG UltraFine 5K:

iMac Pro (2017) - Technical Specifications - Apple Support

support.apple.com

Use the LG UltraFine 5K Display with your Mac or iPad - Apple Support

Learn about the LG UltraFine 5K Display and how to use it with your Mac or iPad.

support.apple.com

There are some additional mentions of MST (which refers to old 4K displays that used two streams of a single DisplayPort connection to drive two halves of the 4K display):

Mac mini (2018) - Technical Specifications - Apple Support

support.apple.com

There are some pages that mention a bpc limit:

Set up and use Apple Pro Display XDR - Apple Support

Learn about Pro Display XDR and how to set up and use it with your Mac.

support.apple.com

MacBook Pro (16-inch, 2019) - Technical Specifications - Apple Support

support.apple.com

MacBook Pro (13-inch, 2020, Two Thunderbolt 3 ports) - Technical Specifications - Apple Support

support.apple.com

MacBook Pro (13-inch, 2020, Four Thunderbolt 3 ports) - Technical Specifications - Apple Support

support.apple.com

The following page makes a distinction between 5K displays that allow 10bpc and those that allow 8bpc:

iMac (Retina 5K, 27-inch, 2020) - Technical Specifications - Apple Support

support.apple.com

The page doesn't mention SST, so there's no way to tell that 10bpc is for DisplayPort 1.2 dual-link SST and that 8bpc is for DisplayPort 1.4 single-link SST.
Of course, with DSC, you could have a DisplayPort 1.4 single-link SST 5K display supporting 10bpc or more. Actually, I think the XDR display has a timing for that.
The LG UltraFine 5K display only supports 5K using DisplayPort 1.2 dual-link SST. It can support 4K using DisplayPort 1.2 single-link SST. I don't think it can support 5K 30Hz using single-link SST.

 

[LJGrant]

There are a couple of support pages that mention the dual-link SST nature of the LG UltraFine 5K:

iMac Pro (2017) - Technical Specifications - Apple Support

support.apple.com

Use the LG UltraFine 5K Display with your Mac or iPad - Apple Support

Learn about the LG UltraFine 5K Display and how to use it with your Mac or iPad.

support.apple.com

There are some additional mentions of MST (which refers to old 4K displays that used two streams of a single DisplayPort connection to drive two halves of the 4K display):

Mac mini (2018) - Technical Specifications - Apple Support

support.apple.com

There are some pages that mention a bpc limit:

Set up and use Apple Pro Display XDR - Apple Support

Learn about Pro Display XDR and how to set up and use it with your Mac.

support.apple.com

MacBook Pro (16-inch, 2019) - Technical Specifications - Apple Support

support.apple.com

MacBook Pro (13-inch, 2020, Two Thunderbolt 3 ports) - Technical Specifications - Apple Support

support.apple.com

MacBook Pro (13-inch, 2020, Four Thunderbolt 3 ports) - Technical Specifications - Apple Support

support.apple.com

The following page makes a distinction between 5K displays that allow 10bpc and those that allow 8bpc:

iMac (Retina 5K, 27-inch, 2020) - Technical Specifications - Apple Support

support.apple.com

The page doesn't mention SST, so there's no way to tell that 10bpc is for DisplayPort 1.2 dual-link SST and that 8bpc is for DisplayPort 1.4 single-link SST.
Of course, with DSC, you could have a DisplayPort 1.4 single-link SST 5K display supporting 10bpc or more. Actually, I think the XDR display has a timing for that.
The LG UltraFine 5K display only supports 5K using DisplayPort 1.2 dual-link SST. It can support 4K using DisplayPort 1.2 single-link SST. I don't think it can support 5K 30Hz using single-link SST.

Sorry to bump an old thread, but you seem to know everything there is to know about this subject. To be very clear, and to keep this simple for my feeble mind: I already own two of the LG 5K monitors, can I link them to a 2020 iMac? If not, could I power one via an egpu?

I think (based on your first link) that a 2016 MBP can power up to four of the monitors at a reduced resolution of 4k: can an iMac power two of them at reduced resolution? I’ve connected my son’s 2019 iMac to test but only one monitor came on.

 

[joevt]

Sorry to bump an old thread, but you seem to know everything there is to know about this subject. To be very clear, and to keep this simple for my feeble mind: I already own two of the LG 5K monitors, can I link them to a 2020 iMac? If not, could I power one via an egpu?

I think (based on your first link) that a 2016 MBP can power up to four of the monitors at a reduced resolution of 4k: can an iMac power two of them at reduced resolution? I’ve connected my son’s 2019 iMac to test but only one monitor came on.

There are two versions of the LG UltraFine 5K:

1. 27MD5KA - the first model can only be connected to Thunderbolt.
2. 27MD5KL - can be connected with Thunderbolt for 5K 60Hz 10bpc with USB 3.0, or USB-C DisplayPort alt mode (4K 60Hz 10bpc with USB 2.0).

What models do you have?


Here are some connection methods:

5K options:

• A) You can connect either to a Thunderbolt port of a Mac or Blackmagic eGPU to get 5K as long as no other displays are connected to the same Thunderbolt bus (the Blackmagic eGPU has its own bus in regards to displays).
• B) To connect an LG UltraFine 5K to a different GPU for 5K, two DisplayPort outputs of the GPU need to be connected to a Thunderbolt 3 add-in card - then the LG UltraFine 5K (or any other Thunderbolt display) can be connected to the add-in card. I won't get into the issues of using a Thunderbolt 3 add-in card here.

4K options: (to connect two LG UltraFine 5K to the same Thunderbolt bus):

• C) To connect the 27MD5KL with DisplayPort alt mode, use a connection that doesn't include Thunderbolt. For example, a bidirectional USB-C to DisplayPort cable. To have USB (for camera, audio, brightness control, USB ports, ...), use a cable/adapter that includes USB, such as the Belkin Charge and Sync cable or Wacom Link Plus.
  

  Page 4 | Does a USB-C monitor support DisplayPort input over...

  Page 4 | I have the new generation of the LG Ultrafine 4k for Mac, which eschews DisplayPort and HDMI inputs for just two (2) Thunderbolt 3 inputs. L...

  egpu.io
• D) Maybe a Thunderbolt connection using a USB-C cable will work - if it connects at Thunderbolt 2 speed (20 Gbps) then only one DisplayPort 1.2 connection will get used. I haven't tried this. If you can't find a USB-C cable that forces 20 Gbps mode, then two Apple Thunderbolt 3 to Thunderbolt 2 adapters can be used with a Thunderbolt 2 cable. With this type of Thunderbolt connection, you should be able to get 4K with USB 3.0.
• E) You might be able to connect two LG UltraFine 5K to an iMac at 4K using Thunderbolt but you need to temporarily connect a display like this (I don't know if this method works):
  1. Connect first display (non-LG).
  2. Connect one LG UltraFine 5K - it should be limited to 4K because of the first display.
  3. Disconnect first display.
  4. If the connected LG UltraFine 5K doesn't automatically switch to 5K, then you can connect the second LG UltraFine 5K.

The C, D, E methods can be used to connect four LG UltraFine 5K displays to the 2016 MBP at 4K.
Since you have two LG UltraFine 5K displays, you can test these methods and let us know if they work.

 

[LJGrant]

There are two versions of the LG UltraFine 5K:

1. 27MD5KA - the first model can only be connected to Thunderbolt.
2. 27MD5KL - can be connected with Thunderbolt for 5K 60Hz 10bpc with USB 3.0, or USB-C DisplayPort alt mode (4K 60Hz 10bpc with USB 2.0).

What models do you have?


Here are some connection methods:

5K options:

• A) You can connect either to a Thunderbolt port of a Mac or Blackmagic eGPU to get 5K as long as no other displays are connected to the same Thunderbolt bus (the Blackmagic eGPU has its own bus in regards to displays).
• B) To connect an LG UltraFine 5K to a different GPU for 5K, two DisplayPort outputs of the GPU need to be connected to a Thunderbolt 3 add-in card - then the LG UltraFine 5K (or any other Thunderbolt display) can be connected to the add-in card. I won't get into the issues of using a Thunderbolt 3 add-in card here.

4K options: (to connect two LG UltraFine 5K to the same Thunderbolt bus):

• C) To connect the 27MD5KL with DisplayPort alt mode, use a connection that doesn't include Thunderbolt. For example, a bidirectional USB-C to DisplayPort cable. To have USB (for camera, audio, brightness control, USB ports, ...), use a cable/adapter that includes USB, such as the Belkin Charge and Sync cable or Wacom Link Plus.
  

  Page 4 | Does a USB-C monitor support DisplayPort input over...

  Page 4 | I have the new generation of the LG Ultrafine 4k for Mac, which eschews DisplayPort and HDMI inputs for just two (2) Thunderbolt 3 inputs. L...

  egpu.io
• D) Maybe a Thunderbolt connection using a USB-C cable will work - if it connects at Thunderbolt 2 speed (20 Gbps) then only one DisplayPort 1.2 connection will get used. I haven't tried this. If you can't find a USB-C cable that forces 20 Gbps mode, then two Apple Thunderbolt 3 to Thunderbolt 2 adapters can be used with a Thunderbolt 2 cable. With this type of Thunderbolt connection, you should be able to get 4K with USB 3.0.
• E) You might be able to connect two LG UltraFine 5K to an iMac at 4K using Thunderbolt but you need to temporarily connect a display like this (I don't know if this method works):
  1. Connect first display (non-LG).
  2. Connect one LG UltraFine 5K - it should be limited to 4K because of the first display.
  3. Disconnect first display.
  4. If the connected LG UltraFine 5K doesn't automatically switch to 5K, then you can connect the second LG UltraFine 5K.

The C, D, E methods can be used to connect four LG UltraFine 5K displays to the 2016 MBP at 4K.
Since you have two LG UltraFine 5K displays, you can test these methods and let us know if they work.

Thank you so much for your help. I have two of the 27MD5KL.

In terms of A, my current MBP can power two displays because despite the four ports it has two buses. If I connect an egpu to a “spare port” and get a third screen, is this ok or would each display need its own egpu?

I have no spare cables, but I’m happy to buy some to test things. If I get a USB-C to DisplayPort, how do I plug this into my screen (which has no DisplayPort to my knowledge). Does an adaptor exist? I know this is mega basic, you’re dealing with someone completely out of his depth here.

 

[joevt]

Thank you so much for your help. I have two of the 27MD5KL.

In terms of A, my current MBP can power two displays because despite the four ports it has two buses. If I connect an egpu to a “spare port” and get a third screen, is this ok or would each display need its own egpu?

A)
Yes. The Blackmagic eGPU has only one Thunderbolt downstream port so it can only connect one LG UltraFine 5K display at 5K.
To connect 2 UltraFine 5K displays at 5K to a single bus of a MBP, connect one to the MBP and the other to the Blackmagic eGPU.
The display connected to the MBP will use the dGPU of the MBP. The display connected to the Blackmagic eGPU will use the GPU of the Blackmagic eGPU.
If you wanted to connect more Blackmagic eGPUs to the same bus or port while keep their downstream Thunderbolt ports remain free for a Thunderbolt display, it may be possible to connect them to a OWC Thunderbolt Hub or OWC Thunderbolt Dock that have three downstream Thunderbolt ports.

B)
Some GPUs have 6 DisplayPort outputs (e.g. Radeon Pro W5700). You could connect three LG UltraFine 5K displays to such a GPU using three Thunderbolt 3 add-in cards (but again, I won't go into the issues of that method). You can make an eGPU with two Thunderbolt ports so you can daisy chain them together.

I have no spare cables, but I’m happy to buy some to test things. If I get a USB-C to DisplayPort, how do I plug this into my screen (which has no DisplayPort to my knowledge). Does an adaptor exist? I know this is mega basic, you’re dealing with someone completely out of his depth here.

C)
With a USB-C to DisplayPort cable, connect the USB-C end to the Thunderbolt port of the display. Connect the DisplayPort end to the MBP - you'll need a USB-C to DisplayPort adapter for that. The USB-C to DisplayPort cable needs to be bi-directional so that it can work as a DisplayPort to USB-C cable.
https://www.amazon.com/dp/B07F17ZHJY
https://www.amazon.com/dp/B082XC4KM8

D)
I think this one is the most interesting possibility (if the method works). It has the benefit of working with 27MD5KA and 27MD5KL and giving USB 3.0 support and all the other features of the displays using a single simple USB-C cable which means it is less expensive than (C). You need a USB-C cable (not Thunderbolt, USB 3.1 gen 2) of sufficient length that it works only at 20 Gb/s Thunderbolt speed.

E)
This method is not as convenient as (D) but I think it would at least allow the same features to work (if the method works).