User profile for user: Adayapple
Adayapple Author
User level: Level 1
30 points

Dual External Monitors Big Sur Thunderbolt 3: Shifting Positions; 30Hz vs. 60Hz


Hi, Apple Community,


I’m using Big Sur, now 11.2.2 on a 2020 iMac with 2 Samsung UHD/5K monitors connected by Thunderbolt 3, AMD Radeon Pro 5700.  


2 Questions I hope to get some help on:


1)    After shutting down the computer and starting up again, the monitors often shift position (I have them set as Left and Right of the iMac. They'll usually revert to the correct position after another restart or two.


2)    I have both monitors set at 2560 x 1440 so I can see print better, but I’ve noticed that although these are the same model Samsung monitor, the monitors show (confirmed in MacOS System Information, below) that they’re running differently at 30Hz and 60Hz.


Are 1) and 2) "normal" behavior, or not?


If not, how can I correct these behaviors?


Thanks.

_____________________________________________

AMD Radeon Pro 5700:

 

  Chipset Model:               AMD Radeon Pro 5700

  Type:                                GPU

  Bus:                                  PCIe

  PCIe Lane Width:           x16

  VRAM (Total):                  8 GB

  Vendor:                            AMD (0x1002)

  Device ID:                        0x731b

  Revision ID:                     0x0040

  ROM Revision:               113-D1820Q-231

  VBIOS Version:               113-D182A2XG-012

  Option ROM Version:    113-D182A2XG-012

  EFI Driver Version:         01.01.231

  Metal Family:                  Supported, Metal GPUFamily macOS 2

  Displays:

iMac:

  Display Type:                                        Built-In Retina LCD

  Resolution:                                           5120 x 2880 Retina

  Framebuffer Depth:                             30-Bit Color (ARGB2101010)

  Main Display:                                        Yes

  Mirror:                                                    Off

  Online:                                                   Yes

  Automatically Adjust Brightness:      Yes

  Connection Type:                                Internal

LF32TU87:

  Resolution:                                           5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)

  UI Looks like:                                        2560 x 1440 @ 60.00Hz

  Framebuffer Depth:                             30-Bit Color (ARGB2101010)

  Display Serial Number:                       HCPN706837 

  Mirror:                                                    Off

  Online:                                                   Yes

  Rotation:                                               Supported

  Automatically Adjust Brightness:      Yes

  Connection Type:                                Thunderbolt/DisplayPort

LF32TU87:

  Resolution:                                           5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)

  UI Looks like:                                        2560 x 1440 @ 30.00Hz

  Framebuffer Depth:                             30-Bit Color (ARGB2101010)

  Display Serial Number:                       HCPN706805 

  Mirror:                                                    Off

  Online:                                                   Yes

  Rotation:                                               Supported

  Automatically Adjust Brightness:      Yes

  Connection Type:                                Thunderbolt/DisplayPort

____________________________________________


iMac 27″, macOS 11.2

Posted on Mar 8, 2021 9:06 AM

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3 replies
User profile for user: Adayapple
Adayapple Author
User level: Level 1
30 points

Mar 10, 2021 2:46 PM in response to DerekM87

Thanks, DerekM87.


1) Each of the monitors is connected separately to the iMac's Thunderbolt 3 jacks.


2) I have set the resolution of the monitors to "Default for display" and sure enough, the refresh rate changed to 60Hz on each monitor.


And once I changed back to my preferred 2560 x 1440, both monitors remain at 60Hz.


(By the way, I wouldn't be able to tell by looking, and I only know from the displays' Information menus, which show 60Hz, as does "Displays" in Mac's System Report, below).


3) The displays reverse themselves ONLY on Startup (from "Off"). The iMac and the 2 displays show the Big Sur graphic on Startup, so I can't tell UNTIL I log in, but they don't reorient until I Restart again. So if there's something to fix, it would keep me from having to Restart to get the orientation correct.


_________________________________________

  Displays:

iMac:

  Display Type:                                        Built-In Retina LCD

  Resolution:                                           5120 x 2880 Retina

  Framebuffer Depth:                             30-Bit Color (ARGB2101010)

  Main Display:                                        Yes

  Mirror:                                                    Off

  Online:                                                   Yes

  Automatically Adjust Brightness:      Yes

  Connection Type:                                Internal

LF32TU87:

  Resolution:                                           5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)

  UI Looks like:                                        2560 x 1440 @ 60.00Hz

  Framebuffer Depth:                             30-Bit Color (ARGB2101010)

  Display Serial Number:                       HCPN706837 

  Mirror:                                                    Off

  Online:                                                   Yes

  Rotation:                                               Supported

  Automatically Adjust Brightness:      Yes

  Connection Type:                                Thunderbolt/DisplayPort

LF32TU87:

  Resolution:                                           5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)

  UI Looks like:                                        2560 x 1440 @ 60.00Hz

  Framebuffer Depth:                             30-Bit Color (ARGB2101010)

  Display Serial Number:                       HCPN706805 

  Mirror:                                                    Off

  Online:                                                   Yes

  Rotation:                                               Supported

  Automatically Adjust Brightness:      Yes

  Connection Type:                                Thunderbolt/DisplayPort

 


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User profile for user: DerekM87
DerekM87
User level: Community Specialist

Mar 10, 2021 12:44 PM in response to Adayapple

Hi Adayapple,


We see you have a couple of inconsistencies regarding your external monitors connected to your Mac.


To help us better isolate this issue, can you confirm the following?

  • Are these monitors connected by two separate Thunderbolt 3 cables or daisy chained?
  • If you set the resolution back to the native 5k, do the discrepancies with the refresh rate resolve themselves?
  • When the displays reverse themselves is it only on startup? Meaning once you login, do they automatically reorient or do you have to restart again?


Let us know and we'll continue from there.


Cheers!

Reply
User profile for user: sterling r
sterling r
User level: Community Specialist

Mar 10, 2021 5:57 PM in response to Adayapple

Hey Adayapple,


Glad to hear the monitor displays its proper refresh rate after toggling it to full resolution and back again.


Regarding the arrangement, if it's an issue in pre-boot, you may be able to swap the Thunderbolt cables to test if that resolves the issue. Plug monitor 2 into where monitor 1 was plugged in or vice versa.


Regarding macOS itself, the Arrangement section of System Preferences > Displays is how the system allows you to change which monitor goes where. You can simply drag and drop the monitors in that section into whatever arrangement you'd like. See more details here:


If you have done that already and the issue intermittently continues, we'd recommend reaching out to Apple Support for additional assistance.


Take care.

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Dual External Monitors Big Sur Thunderbolt 3: Shifting Positions; 30Hz vs. 60Hz

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