How to Use Your Old iMac as a Monitor

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 iMac's 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.

Connecting Another Mac to Your iMac

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 can’t receive analog video or audio sources, such as those from a VGA connector.

Compatible Macs

* Mac must be running OS X 10.6.1 or later

Making the Connection

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. The connection should then happen automatically.

Multiple iMacs as Displays

It's possible to use more than one iMac as a display provided all Macs––both the iMacs used for display and the source Mac––are using Thunderbolt connectivity.

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

Maximum Connected Thunderbolt Displays

Enable Target Display Mode

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 doesn't automatically enter Target Display Mode, press Command+F2 on the iMac you wish to use as a display to manually enter Target Display Mode.

What to Do If Target Display Mode Doesn't Work

If Target Display Mode isn't working to connect your iMac as an external display, there are a few things you can try that might help make the connection.

1. Try using Command + Fn + F2. This may work for some keyboard types.

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

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

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

5. 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.

Exit Target Display Mode

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.

Things to Consider

• Your iMac will continue to run OS X as well as any applications that were open when it entered Target Display Mode.
• While in Target Display Mode, only the keyboard’s display brightness, volume, and Target Display Mode key combinations are active. Any other keyboard input will be ignored. USB, FireWire, and inputs other than the keyboard will also be ignored.
• The Mac being used as the source for the display can’t make use of any of the display iMac's features, including the built-in iSight camera.
• Target Display Mode can be helpful in many situations, but it's not a full-
• time substitute for having a dedicated display for another Mac.

Should You Use Your iMac as a Display?

If a temporary need arises, sure, why not? But in the long run, it just doesn’t make sense to waste the computing power of an iMac, nor does it make sense to pay for the energy the iMac needs to run when you're only using the display. Remember, the rest of the iMac is still running, consuming electricity and generating heat.

If you need a large display for your Mac, do yourself a favor and grab a decent 27-inch or larger computer monitor. It doesn't need to be a Thunderbolt display; just about any monitor with a DisplayPort or Mini DisplayPort will work very well with any of the Macs listed in this article.