Target Display Mode Remains Absent from 2017 iMac Update
Bad news for fans of Apple’s Target Display Mode: the new 2017 iMacs won’t reintroduce support for the feature, according to an Apple support document.
Target Display Mode is the name of a feature that allowed users with certain iMacs to basically use their iMac as an external display for another source, such as their MacBook, a PS4, or even a Windows PC. The mini DisplayPort connection (and later Thunderbolt) that normally served as the output for users who wanted to connect a second monitor to their iMac could instead be used as a video input, as long as the video source sent a compatible signal.
That all changed when Apple introduced the 5K iMacs in late 2014. The resolution of the iMac’s display was so great that it required more bandwidth than the Thunderbolt 2 specification (which transports video via DisplayPort 1.2) could accommodate. This meant that Target Display Mode, even via Thunderbolt, was no longer an option.
Target Display Mode via Thunderbolt 3
The new 2017 iMacs still have that high resolution 5K display, but they also include Thunderbolt 3 for the first time. With a maximum bandwidth of 40Gb/s, Thunderbolt 3 does have the ability to push the iMac’s 5120×2880 resolution, and we can see this capability in action with relatively new monitors like LG’s UltraFine 5K Display, which transports video, USB data, and power over a single Thunderbolt 3 cable.
Despite this, it appears that Apple has decided not to support Target Display Mode in the 2017 iMacs. The company updated its Target Display Mode support page (HT204592) this week to note that “iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can’t be used as Target Display Mode displays” (emphasis added). Previously, Apple’s note did not include the “and later” expansion of the limitation.
A test done here at TekRevue with a 2017 27-inch iMac and 2016 13-inch MacBook Pro confirmed that Target Display Mode via Thunderbolt 3 does not work on Apple’s latest iMacs. It’s possible that Apple might reenable this feature in the future via an iMac firmware update, but it’s likely that Apple views Target Display Mode as too niche of a feature to bother with.