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bassomaco89

macrumors newbie
Original poster
Apr 10, 2021
12
4
Hi guys, I'm new to this forum, hope this post is in the right topic list. I own a macbook pro (macOs: Big Sur 11.2.3) to which I connected a simple external monitor via HDMI cable to extend the macbook destop. Also I have an old 27 "iMac from 2012 (macOs High Sierra) that I would like to use as an additional external monitor. To do this I bought a thunderbolt cable to connect two macs. So when the macbook pro is on I tried to connect it to the iMac as well, and then again from the iMac I pressed "cmd + F2" (both during startup, and with the iMac already on) to put the iMac in "Target Display Mode". The iMac however does not recognize this command and therefore I cannot use it as a second external monitor.
I tell you right away that the two Macs recognize the connection via thunderbolt, and I also tried the combination "cmd + fn + F2".
Can you give me some advice to solve the problem?
Thanks!
 

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That answers the question whether you can ever have two external monitors - most intel MacBooks can, but not the newer M1 based machines. I have never connected an iMac so I'll leave that to others.
 
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I think it should work:
https://support.apple.com/kb/sp715?locale=en_CA
Code:
Graphics and Video Support
- Intel Iris Graphics 6100
- Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 3840 by 2160 pixels on up to two external displays, both at millions of colors.
- Thunderbolt digital video output
- HDMI video output
For Thunderbolt Target Display Mode, did you try an earlier version of macOS on the MacBook Pro?

Apple removed the documentation about Target Display Mode?
https://support.apple.com/en-us/HT204592
https://forums.macrumors.com/threads/macos-10-15-catalina-on-unsupported-macs.2183772/post-28164916
 
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I think it should work:
https://support.apple.com/kb/sp715?locale=en_CA
Code:
Graphics and Video Support
- Intel Iris Graphics 6100
- Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 3840 by 2160 pixels on up to two external displays, both at millions of colors.
- Thunderbolt digital video output
- HDMI video output
For Thunderbolt Target Display Mode, did you try an earlier version of macOS on the MacBook Pro?

Apple removed the documentation about Target Display Mode?
https://support.apple.com/en-us/HT204592
https://forums.macrumors.com/threads/macos-10-15-catalina-on-unsupported-macs.2183772/post-28164916
In the article that you have attached, Apple sad that Target Display Mode is no longer supported by macOS mojave or later. But I found some videos on youtube where they use this Display Mode with the latest macbook pro.
I went to an authorized apple center and they advised me to use a wired keyboard. They also told me to press cmd + F2 while the iMac is starting up, and not when the iMac is already on.
However I have not tried an earlier version of macOS.
 
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from the iMac I pressed "cmd + F2
What keyboard are you using with the iMac?

I ask because that command is actually Cmd + Brightness Up, which just happens to be Cmd + F2 on any reasonably recent Apple keyboard.
 
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In the article that you have attached, Apple sad that Target Display Mode is no longer supported by macOS mojave or later.
That only refers to the iMac you want to use as a monitor afaik.

They also told me to press cmd + F2 while the iMac is starting up, and not when the iMac is already on.
macOS needs to be fully booted up (at least: showing the login screen) for Target Display Mode to work. It won't work while it's still starting up.
 
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macOS needs to be fully booted up (at least: showing the login screen) for Target Display Mode to work. It won't work while it's still starting up.
Right. Sending and receiving display data over Thunderbolt requires both Macs to be booted into macOS where the Thunderbolt drivers that perform these tasks exist.

Target Display Mode is a macOS driver for both computers.

On the other hand, Target Disk Mode on the target is an EFI driver. USB and FireWire Target Disk Mode I think pretend to be normal USB / FireWire mass storage devices (I've connected to FireWire Target Disk Mode in Ubuntu). Thunderbolt Target Disk Mode is probably a special macOS only driver.
 
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That only refers to the iMac you want to use as a monitor afaik.


macOS needs to be fully booted up (at least: showing the login screen) for Target Display Mode to work. It won't work while it's still starting up.
Right. Sending and receiving display data over Thunderbolt requires both Macs to be booted into macOS where the Thunderbolt drivers that perform these tasks exist.

Target Display Mode is a macOS driver for both computers.

On the other hand, Target Disk Mode on the target is an EFI driver. USB and FireWire Target Disk Mode I think pretend to be normal USB / FireWire mass storage devices (I've connected to FireWire Target Disk Mode in Ubuntu). Thunderbolt Target Disk Mode is probably a special macOS only driver.
I have a Logitech K480. But I tried also to use this display mode in my Macbook Pro. So I started the iMac and then the macbook too. Then I tried to use cmd + F2 on the macbook in order to use the laptop as an external monitor, but nothing works. So I don't think it's a keyboard problem but it's a compatibility between macOS
 
You have to use an Apple keyboard. The Logitech keyboard isn't sending the correct keycode when you press Cmd+F2.


MacBooks don't support Target Display Mode. Only iMacs do.
Ok thanks for your support. Asap I will try with the apple keyboard. I'll let you know if it works.
 
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Ok thanks for your support. Asap I will try with the apple keyboard. I'll let you know if it works.
So I'm trying with the Apple keyboard. It appears that both the iMac and the Macbook recognize the cmd + F2 command. But when I use this combination in the iMac, it gets me out of the account. When I log in again, nothing happens.
 
Does this also happen when the iMac is not connected to the MBP?
Finally I made it!
Looking at other forums I saw that it was necessary to disable the automatic brightness on the iMac and now it works perfectly for me. Thank you very much for helping.
Now I have another doubt: do you know if you can use the speakers of the iMac rather than those of the macbook?
 
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Now I have another doubt: do you know if you can use the speakers of the iMac rather than those of the macbook?
Do the iMac's speakers appear as output devices on the MBP? They definitely work on my 2010 iMac via Target Display Mode (that's using Displayport though).
 
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