While in Target Display Mode, only the keyboards display brightness, volume, and Target Display Mode key combinations are active.

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The 27-inch iMacs introduced in late 2009 included the first version of Target Display Mode, a special feature that allowed iMacs to be used as displays for other devices.

Apple originally hinted at the iMac being used with DVD and Blu-ray players as an HDTV display, and even as a display for another computer.

But in the end, Target Display Mode became an Apple-only technology that allowed Mac users to drive an iMacs display from another Mac.

Still, it can be quite compelling to see your Mac mini making use of your older 27-inch iMac as a display, or for troubleshooting an iMac having display issues.

The 27-inch iMac has a bi-directional Mini DisplayPort or a Thunderbolt port (depending on the model) that can be used to drive a second monitor.

The same Mini DisplayPort or Thunderbolt port can be used as a video input that allows your iMac to serve as a monitor for another Mac.

All you need are the proper ports and cables to make the connection between the two Macs.

The Mini DisplayPort or Thunderbolt-equipped iMac can only receive DisplayPort-compatible video and audio.

It cant receive analog video or audio sources, such as those from a VGA connector.

Both the iMac that will be used as the display and the Mac that will be the source should be turned on, then connect either the Mini DisplayPort cable or the Thunderbolt cable to each Mac.

Each iMac used as a display counts against the simultaneously connected displays supported by the Mac youre using as the source.

Your iMac should automatically recognize the presence of a digital video signal at the Mini DisplayPort or Thunderbolt port and enter Target Display Mode.

If your iMac doesnt automatically enter Target Display Mode, press Command F2 on the iMac you wish to use as a display to manually enter Target Display Mode.

If Target Display Mode isnt working to connect your iMac as an external display, there are a few things you can try that might help make the connection.

Make sure the Mini DisplayPort or Thunderbolt cable is properly connected.

If the iMac being used as a display is currently booted from a Windows volume, restart it from the normal Mac startup drive.

If youre currently logged into the iMac you intend to use as a display, try logging out, returning to just the normal login screen.

There are a few third-party keyboards that will not send the Command F2 correctly.

Try using another keyboard, or the original keyboard that came with your Mac.

You can manually turn off Target Display Mode by pressing the Command F2 keyboard combination, or by disconnecting or turning off the video device connected to your iMac.

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