from:http://computers.tutsplus.com/tutorials/using-an-imac-as-a-second-monitor-in-target-display-mode--mac-51833
One of the things about the iMac, in fact any Apple-made display, is it’s extremely high quality image and colour-depth. It’s not that you’ll necessarily notice it immediately, but if you ever move back to a PC monitor, you’ll become acutely aware of the difference in quality.
Given that the iMac has a superb display, it could be useful to share that with another Mac, on occasion. In this tutorial I will explain a little known feature, named Target Display Mode, and show you how to use it.
Target Display Mode Explained
Target Display Mode is a feature that allows you to use the display of an iMac to view the screen output from a second, external, Mac. For example, I am able to use Target Display Mode so that my late–2009 27" iMac can display the output from my late–2009 Mac mini.
Target Display Mode is achieved through the use of either a Mini DisplayPort cable or a Thunderbolt cable in order to share the screen of an iMac with an external Mac.
Target Display Mode Requirements
Getting Target Display Mode to work depends upon your set up as it does not work with all combinations of Macs. Particular combinations of Macs, and cables, are required for Target Display Mode, as shown in the table below:
| Use this cable | to connect Mac with this port | to this iMac |
| Mini DisplayPort | Mini DisplayPort | iMac (27-inch Late 2009) |
| Mini DisplayPort | Thunderbolt | iMac (27-inch Late 2009) |
| Mini DisplayPort | Mini DisplayPort | iMac (27-inch Mid 2010) |
| Mini DisplayPort | Thunderbolt | iMac (27-inch Mid 2010) |
| Thunderbolt | Thunderbolt | iMac (21.5-inch, Mid 2011) |
| Thunderbolt | Thunderbolt | iMac (27-inch, Mid 2011) |
| Thunderbolt | Thunderbolt | iMac (21.5-inch, Late 2012) |
| Thunderbolt | Thunderbolt | iMac (27-inch, Late 2012) |
As can be seen, there are two possible cables that can be used to connect Macs in Target Display Mode. They are:
- Mini DisplayPort cable
- Thunderbolt cable
How to Use Target Display Mode
In order to use Target Display Mode, both Macs must be turned on and must be awake (ie: not in sleep mode).
In my examples, throughout this tutorial, I shall use one iMac, an external Mac mini and a cable. Though my equipment uses Mini DisplayPort, this example could be Thunderbolt-equipped Macs and a Thunderbolt cable if those are available.
In this tutorial, I am using:
- Late–2009 Mac mini Core 2 Duo with Mini DisplayPort
- Late–2009 27" iMac Core 2 Duo with Mini DisplayPort
- Mini DisplayPort connection cable
With both the Mac mini and the iMac turned on, I can initiate Target Display Mode by pressing Command F2 on the keyboard of the iMac. This has the effect of switching the video display input to be that from the Mac mini.
Cancelling Target Display Mode
There are a number of different ways to exit the Target Display Mode that is displaying, in my example, the video output of the Mac mini to the iMac.
The easiest way is to press Command F2, on the iMac keyboard, to toggle the display input back to the iMac.
Target Display Mode from the Mac mini to the iMac will also be disabled if either Mac goes into sleep mode, is turned off or if the Mini DisplayPort cable is detached.
Tip: Some older Apple keyboards and third-party keyboards may not allow Command F2 to toggle the display inputs. For this reason, Apple recommends that you use either the wired or wireless aluminium Apple keyboards to ensure proper functionality.
Considerations When Using Target Display Mode
Wired for Sound and Vision
Thunderbolt is a recent technology that is capable of transferring huge amounts of data at very high speed when compared to pre-existing technologies. As such, the capabilities of Thunderbolt and Mini DisplayPort are slightly different.
If you are able to connect two Thunderbolt-equipped Macs with a Thunderbolt cable, you will be able to hear the audio of the external Mac on your primary Mac.
If, however, you connect two Mini DisplayPort equipped Macs with a Mini DisplayPort cable, you may be able to transfer the audio from the external Mac to the screen of the host machine.
In this instance, if you want the external display Mac to play it’s audio and video, go to Apple menu > System Preferences then click on Sound, then click Output and select the external display Mac from the list of available devices.
In the example, that I am using in this tutorial, my late-2009 Mac mini is not able to put it's audio through my late-2009 27" iMac.
Running Applications
It is possible to continue to run applications on both machines, regardless of which one is using the iMac’s screen.
For instance, if I have an application running on the iMac, I can allow it to continue running whilst I am in Target Display Mode to view the video output from the Mac mini.
Using Accessories
When running the iMac in Target Display Mode to receive the video output from the Mac mini, the Mac mini will not be able to access the built-in camera, USB or FireWire ports of the iMac.
If you need to use a USB or FireWire accessory, from the iMac, on the Mac mini, it will be necessary to properly disconnect them from the iMac and plug them into the Mac mini.
Screen Brightness
To change the screen brightness levels, whilst using the iMac, use the F1 and F2 keys on your Apple keyboard. In the default OS X installation configuration these keys will adjust the brightness down and up, respectively.
If the function of the iMac keyboard has been modified, in OS X, such that the function keys respond to F1 to F12 inputs instead of the custom Apple functions, then it will be necessary to hold down the Fn key whilst pressing F1 and F2 to adjust the brightness down and up.
Tip: Whilst using the iMac in Target Display Mode, to receive the video output of the Mac mini, it must be remembered that it is still the iMac keyboard that is used to adjust the brightness levels.
Media Controls
These are the F7, F8 and F9 keys on the top row of the Apple keyboard. Their default fuctions are to skip backwards/rewind, play/pause and skip forwards/forward wind respectively.
Sound: Mute and Volume
To mute the sound or adjust volume levels, on the iMac, use the F10, F11 and F12 keys to mute, decrease and increase the volume, respectively.
Whilst the iMac is in Target Display Mode, displaying the video received from the Mac mini, you will still need to use the iMac keyboard to perform these functions.
Tip: If the mute (F10) and volume down and up (F11 and F12) buttons do not achieve the desired effect, it’s possible the default behaviour of the keyboard has been modified, in which case hold down the Fn key in conjunction with the F10–12 keys.
Conclusion
Target Display Mode is a useful yet little often used feature available on Macs. Provided that you have the right cable, Mini DisplayPort or Thunderbolt, for the correctly-equipped Macs, it is possible to share the excellent screen of your iMac with another Mac.
This can be a really useful feature, especially when you do not have a spare screen available for a Mac mini. Let me know of your experiences and uses for Target Display Mode in the comments, below.