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The Ins and Outs of Your iMac's Target Display Mode

A benefit of Apple’s iMac is that users get a monitor and computer in one relatively small package. But unlike a standalone monitor, users were traditionally unable to share the display with another computer or device, leaving the iMac’s large and high quality screen dedicated only to the Mac within.
Apple sought to address this shortcoming in 2009 with the release of a new feature called “Target Display Mode.” Available initially only on the 27-inch Late 2009 iMac, Target Display Mode (TDM) allowed users to plug a compatible device into their iMac’s Mini DisplayPort and gain exclusive use of the iMac’s display. With the proper adapters, DisplayPort can accept DVI and HDMI sources, meaning that practically any computer or video device using these standards could work with TDM, including PCs, game consoles, and even other Macs.
Target Display Mode quickly became a much-loved feature of the 27-inch 2009 iMac, and it persisted with the 27-inch 2010 model. With the introduction of Thunderbolt on the 2011 iMacs, however, things suddenly became far more complicated.
Prior to Thunderbolt, the iMac’s Mini DisplayPort connection was used exclusively for video and audio. Thunderbolt changed all of that by bringing data I/O into the mix. Now, users could not only add displays to their Mac, they could also daisy chain all manner of hard drives, storage arrays, card readers, and other external devices. Even though Thunderbolt also handled DisplayPort video, the new complexities of the Thunderbolt controller meant that Target Display Mode would be far more restrictive.
With Thunderbolt-capable iMacs – the Mid 2011 models and up – Target Display Mode will only work with other Thunderbolt-capable devices. This means that connecting another Thunderbolt Mac to your iMac, such as a 2012 MacBook Air, will work just fine, but devices that only output HDMI or DVI, such as the Xbox One, won’t work.
This limitation disappointed many users. While it’s great to still be able to use TDM with newer Macs, most who took advantage of the feature connected non-Apple devices such as gaming PCs or consoles, especially in small workspaces where having a second display for these other devices was impractical or undesired. All things considered, we wouldn’t trade the benefits of Thunderbolt for the return of broader support for TDM, but those hoping to use the feature should be aware of its limitations.
That said, here’s a simple breakdown of the various iMac models that support TDM, and the limitations for each. For the chart, “Source Output” refers to the device that you want to connect to the iMac’s display, and “Connection Cable” is the cable type required to make the connection between the two devices.

Model Source Output Connection Cable
Late 2009 27-inch Mini DisplayPort or Thunderbolt Mini DisplayPort
Mid 2010 27-inch Mini DisplayPort or Thunderbolt Mini DisplayPort
Mid 2011 21.5-inch Thunderbolt Thunderbolt
Mid 2011 27-inch Thunderbolt Thunderbolt
Late 2012 21.5-inch Thunderbolt Thunderbolt
Late 2012 27-inch Thunderbolt Thunderbolt
Late 2013 21.5-inch Thunderbolt Thunderbolt
Late 2013 27-inch Thunderbolt Thunderbolt

As you can see, because Thunderbolt outputs DisplayPort video, you can use a new Thunderbolt-equipped Mac to connect to the display of an older iMac via a Mini DisplayPort cable, but not the other way around. For any iMac after the 2011-era, it’s Thunderbolt all the way.

How to Use Target Display Mode

If your hardware meets the requirements above, and your host iMac is running OS X 10.6.1 or higher, here’s how to use Target Display Mode.

  1. Both the iMac and the source computer or device will need to be booted up and awake. Once they’re ready, use the appropriate Mini DisplayPort or Thunderbolt cable to make the connection between the two.
  2. Using the host iMac’s keyboard, press Command-F2 to trigger Target Display Mode. You’ll see the iMac’s screen go black for a second or two, and then switch over to acting as the display for the source computer or device. Note that even though the iMac’s display is now in use by the source device, the iMac itself will continue to hum along in the background. Any running tasks or apps will continue without interruption, and you can even remotely log into the iMac from another computer to use it while the display is busy.
  3. When you’re ready to switch control of the display back to the iMac, simply press Command-F2 again. Alternatively, you can shut down the source device or disconnect the display cable; if the iMac in TDM stops receiving an active video signal from a source device for any reason, it automatically switches the display back to default.

Target Display Mode Tips & Caveats

As long as your hardware meets your expectations, TDM can be a great feature, but there are some tips and caveats you’ll need to be aware of.

  1. Target Display Mode won’t give you a “free” Apple Thunderbolt Display. What we mean by this is that when you connect a computer to your iMac, don’t expect to gain any hub functions like those found the Cinema and Thunderbolt Displays. Your source Mac won’t be able to see or use the card readers, USB ports, iSight cameras, or microphones of the host iMac. It’s only video and audio, folks.
  2. You can use more than one TDM Mac with a single source device. Target Display Mode basically turns your iMac into simple monitor, so if you have two iMacs and, let’s say, a new Mac Pro [1], you can put both iMacs into TDM, connect them to the Mac Pro, and have two displays for your new Mac workstation. Note, however, that you’ll need to connect each display directly and individually to the source; you can’t daisy chain iMacs in Target Display Mode.
  3. While in Target Display Mode, you should be able to change the brightness of the iMac’s display or the volume of the speakers using the iMac’s keyboard. However, some users have reported difficulty with these functions since the introduction of Thunderbolt in Snow Leopard. If you have difficulty controlling the brightness in Target Display Mode, check out third party solutions like the app Shades [2], which offers fine-tuned brightness controls for any Mac, not just those in TDM.
  4. Some users report difficulty simply getting their iMacs into Target Display Mode. Be sure to check the integrity of your Mini DisplayPort or Thunderbolt cables, and make sure that the actual ports on each device are working. If you’re connecting a third-party device, such as a game console, via an HDMI to Mini DisplayPort adapter, also be sure to independently verify that the adapter is functioning properly. If all else fails (and we wish we didn’t have to say this), some users on Apple’s support forums report success with repeated presses of the Command-F2 keyboard combination. We’ve never encountered that issue on our end but, hey, it’s worth a shot.
  5. You don’t need to worry about your host iMac sleeping and breaking the connection. While in Target Display Mode, the host iMac automatically ignores any scheduled sleep commands and keeps the system running as long as the source’s video signal is flowing. If your source device sleeps, however, it will break the connection and the host iMac will revert to the internal display.
  6. While 2011 model iMacs and up are practically limited to serving as external monitors for other Macs (due to the Thunderbolt source requirement), those using 2009 and 2010 iMacs with devices other than computers should note that there are some input resolution restrictions. By default, the iMacs can only accept DisplayPort input at 720p or native resolution (which, in the case of the 27-inch iMac, is 2560-by–1440). This means that if you attach an Xbox console, for example, via one of the HDMI to Mini DisplayPort adapters [3] you’ll get your console’s output at 720p, and it will then scale to fill the screen, producing a full-sized but less sharp image. However, there are some more expensive products [4] that have built-in scalers and can take a device’s 720p or 1080p output and scale all the way up to 2560-by–1440.

Apple’s Target Display Mode is certainly not as flexible as many users might like, especially after the Thunderbolt transition, but it’s still a great feature that ensures that your iMac’s big beautiful display won’t be entirely locked down to the components inside. So if you need a display for your MacBook in a pinch, or you’re hoping to repurpose an old iMac as a second monitor for your new Mac, Target Display Mode is the way to go.