Retina 5K: iMac Target display mode and Thunderbolt display

Two of the questions we're still getting asked a lot is whether the Retina 5K iMac can be used in "target display mode" as an external monitor for a new Mac Pro or MacBook Pro, or whether Apple will be releasing a 5K version of their Thunderbolt display any time soon. Sadly, as I tweeted right after the Apple's October Event, and later explained, the answer to the first question is, sadly, "no", and the answer to the second question is the same. Why is that?

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The simple answer is that there's no port that can properly drive 5K yet. The current Thunderbolt 2 ports on the Mac Pro and MacBook Pro use DisplayPort 1.2 and max out at 4K. To drive 5K, we'll need Thunderbolt 3 and DisplayPort 1.3. That's coming with Intel's Skylake platform... which is coming after Intel's Broadwell platform... which is coming... eventually.

Intel claims that the continued delays with Broadwell won't affect Skylake, but I'll be that when they ship it.

With the iMac, Apple can ignore the standards and fuse the two DisplayPort 1.2 pipes internally with their own custom timing controller (TCON). With external displays, they can't. So, we're capped at 4K until Skylake. After that, we'll still need new Mac Pros and new MacBook Pros with Skylake chipsets and Thunderbolt 3 ports. Then we'll be able to drive 5K and then, hopefully, Apple will release a Retina 5K Thunderbolt display.

It makes for a markedly different dynamic than back in October of 2009, when Apple released the first 27-inch iMac with in-plane switching (IPS). That iMac did support target display mode. Since it was so much bigger than the then 24-inch LED display of the time, some Mac Pro owners bought them simply for their screens. That it took until July of 2010 before the stand-alone 27-inch LED display was released was thus somewhat tempered.

This year, not so much. Even if Apple releases a stand-alone Retina 5K Thunderbolt display next July, there's no target display mode to tide anyone over until then, and no existing Mac will be able to drive it when it ships anyway. What there is is a stand-alone Retina 5K iMac for anyone will to not just switch screens, but Macs. (As some seem ready to do...)

For those who don't want to make the switch but still want desktop Retina, there are 4K displays that work with Thunderbolt 2, including the Dell UltraSharp 24 Ultra HD (opens in new tab) which hits 185 ppi. (That's not quite the Retina 5K iMac's 218 ppi, but it's closer than other 27-inch 4K displays.)

There are also some vendors who are or will be using two DisplayPort buses, each driving 2.5K, and stitching them together to try and simulate one "5K display". It's not the same as what Apple's doing, because it's not done internally and there's no custom TCON like Apple is using. So, there are negative side effects like "tearing" down the middle when the two sections fall out of sync. Some people may not care, but the kind of person who buys a Mac Pro likely will. A lot.

If you haven't bought yet, and you're still trying to decide between the Retina 5K iMac and Mac Pro, you'll either want to go iMac now, or wait and go updated Mac Pro and Retina 5K Thunderbolt display whenever those currently-mythical beasts gets announced.

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Rene Ritchie

Rene Ritchie is one of the most respected Apple analysts in the business, reaching a combined audience of over 40 million readers a month. His YouTube channel, Vector, has over 90 thousand subscribers and 14 million views and his podcasts, including Debug, have been downloaded over 20 million times. He also regularly co-hosts MacBreak Weekly for the TWiT network and co-hosted CES Live! and Talk Mobile. Based in Montreal, Rene is a former director of product marketing, web developer, and graphic designer. He's authored several books and appeared on numerous television and radio segments to discuss Apple and the technology industry. When not working, he likes to cook, grapple, and spend time with his friends and family.

  • Added a section on "dual-link" DisplayPort option.
  • At first I thought this might sound like a stupid statement to make but then realized that I have people ask me questions that are clearly present on the document they are referring to. I was going to ask, "are there really that many people that don't bother checking first to see if there is an answer first before they ask people".
  • I thought the same thing. And then wonder are there people who can't google anything and instead email someone at iMore all their questions? Apparently there are. Makes you wonder if iMore's audience isn't more nontechie than you think.
  • Rene, I know that the Retina 5k iMac cannot be used in Target display mode. But wouldn't it be even better to run a Macbook Pro in target disk mode, hook it up to the iMac, and run the SSD from the Macbook through the even faster hardware on the iMac? That way, people can have both the power of the iMac and the portability of the Macbook. I know Thunderbolt isn't as fast as PCI-E, but it should still be plenty fast.
  • Rene - the question nobody is addressing in regards to the Thunderbolt Display, is why not update the USB to 3.0 slim down the body like the 2012 iMacs - in fact, they could make a beautiful Thunderbolt Display in that it would not have the bulge in the back. Not everyone will want or need the 5K thunderbolt display or afford it for that matter. So why not update the ports & case?
  • ..."internally with their own customer timing controller (TCON)". Customer?? Maybe it's custom.
  • Apple was so cheap they couldn't even design a new keyboard
  • What's wrong with the one they have right now? Sent from the iMore App
  • If you have to ask...
  • If it will make folks feel better, remember gaming at 640x480 :-) Lets be happy about how far we've come and think that many people are still stuck at 1080p, lol. Rene, how are you liking the 5k iMac a few weeks in? Was it worth the 2500$?
  • I'm more curious why they decided on 5K instead of the 'industry standard' 4K? I don't really get the advantage, in this case it's really a disadvantage cause you can't use the iMac in target mode.
  • Its so you can show a full-res 4k screen, with space to fit editing timelines etc. around the edges
  • Am I wrong in thinking that we will later, once thunderbolt 3 is out, be able to use the new 5k iMac as external for another device?
  • I have the iMac Retina 5k and can confirm that it doesn't support Target Display Mode. However, there's no problem with using thunderbolt bridge and screen sharing instead, i.e. "same same but different" ;)
  • If you look at the ifixit teardown, they point out the TCON (DP665 timing controller) and mention that Apple did a custom job on it. Worth noting is that the manufacturer (Parade Tech) specs of the non-custom TCON model (DP663) that it is based on, lists eDP 1.3 compliance, even though it is not rated for the higher resolution of 5k and up. What that means is that Apple and Parade probably designed it before the displayport 1.3 spec was fully approved on 9/2014.
  • Rene ~ What I'm wondering--for future proofing purposes, as I'm about to drop over $3k on one of these--is: Do you or others you might check in with think it's possible and therefore probable that a third party or perhaps software upgrade from Apple will allow this iMac to be used in *4k* (yes, 4, not 5) Target Display Mode at some point? That'd at least be a nice consolation prize for having bought early and getting to use it with computer at 5k now, knowing that at some point--even with the DP 1.2, unlikely to ever be upgradable--that this could be used as a secondary 4k display, at the least? I'd love to know the answer to that question. I may cross-post this same Q to Anandtech as well. Thanks for researching, if possible!
  • I was thinking the EXACT same thing. Perhaps there will be an external hardware device that works in sync with some application that would effectively allow you to pipe in 4k from another machine. This scenario would still require the iMac be booted on an running, but it would work similar to running parallels...perhaps boot up a virtual machine that is just a viewer for an external device that takes 4k via HDMI input and exports it as Thunderbolt 2.
  • Display mode question: Why can't Apple simply detect that the thunderbolt supports for 4K and scale the screen down to match? Worst case they can show a smaller screen and only stream a fraction of the pixels through the cable. if I can change the resolution on a standard cheap monitor, why couldn't I do the same in this case?