iMac Retina - Target Display Mode

iMac retina 5K, and target display mode – Solution?

Posted On Mar-14

iMacs since 2009 have all supported ‘target display mode’, which basically lets you hook up your macbook pro using thunderbolt to your iMac and use the iMac as an external monitor for the macbook.

Well.. I was in for a not-so-pleasant surprise , when I realized that this wasn’t supported in Apple’s most recent iMac retina 5K.

A little digging , revealed Apple’s reasoning behind this. Thunderbolt 2 just didn’t have enough bandwidth  to support 5K. It currently maxes out at 4K. Thunderbolt 3 (expected to come out with Intel’s skylake ) might support this. But who knows.

target display mode – Alternative?

Until then a nifty little compromise ( as I do ) is to just use thunderbolt2 to network the iMac and MacBook. Assign an IP statically to both endpoints, and run Mac’s sharing over this interface ( Thunderbolt 2 runs at 20Gbps ) instead of your typical 1G network interface.

  • kinowethan

    Why not wifi instead of thunderbridge

    • wifi doesn’t typically give you the 20Gbps bandwidth you can get off thunderbolt. Of course, other lesser limits like disk-i/o come into play when you copy to disk to test it

  • kinowethan

    Why not wifi instead of thunderbridge

    • wifi doesn’t typically give you the 20Gbps bandwidth you can get off thunderbolt. Of course, other lesser limits like disk-i/o come into play when you copy to disk to test it

  • janet harrison

    how to configure ip in thunderbolt-2. i cannot find ip settings page. could you giv details. i am trying becos i already bought iMac retina , and now on wifi sharing is not clear

    • i’ll see if i can explain it in detail. Give me a couple of days

  • janet harrison

    how to configure ip in thunderbolt-2. i cannot find ip settings page. could you giv details. i am trying becos i already bought iMac retina , and now on wifi sharing is not clear

    • i’ll see if i can explain it in detail. Give me a couple of days

  • Janet – i’ll see if i can explain it in detail. Give me a couple of days

    • bub_mario

      Ajay, this seems like a really nifty solution indeed! Especially if someone were to, in theory, look at setting up a Mac Pro (cylinder) to treat an iMac 5k retina as a display. Did you get around to posting how this would be configured?

    • bub_mario

      Ajay – it has been roughly 7 months since you said you would explain this set-up. I’m hoping you are able to get to it eventually! Also, interested to read online about the new 5K retina refreshes just released today, and whether or not TDM is included or your current workaround can still be used for it.

      • Indeed. I still use this setup. And I remember we got it working for Janet too. Janet, could you please post the email I sent over.
        The 5k refreshes are nice. Need to swing by the 5th Avenue store to check it out.

  • Janet – i’ll see if i can explain it in detail. Give me a couple of days

    • bub_mario

      Ajay, this seems like a really nifty solution indeed! Especially if someone were to, in theory, look at setting up a Mac Pro (cylinder) to treat an iMac 5k retina as a display. Did you get around to posting how this would be configured?

  • Christian

    Hi Ajay,
    i just came across your blog from googling 5k iMac as external monitor…
    I just bought the new iMac released this week and learned about this “target display” issue…
    Could you provide more details about the alternative mode?
    Would be awesome.
    regards,
    chris

    • Chris – see Mike’s post above. Let me know if you still have questions

      • Christian

        Cool finally i got it working 🙂 and compared to the gigabit lan, it works really smooth!
        BUT i can only use half of my imac screen. probably, because i can’t increase my mbp13 resolution, right? Thats a bit sad.

        • Thats true. However, go into the display settings in your MBP, and set it to use the largest resolution. That will increase the size of the displayed area on your iMac

          • Alexander M

            Hi Ajay, thanks a lot for this post. it seems you are the only person on the internet to offer a solution for this. is there a way to use the full size of my iMac 5k 27’s screen when connecting to my 13′ MBP? or will it never get bigger than a 13′ display? I think my MBP is already on max resolution. I tried switchResX but couldn’t make it bigger. Am I missing a trick? Thanks a lot for your help. Cheers from the UK

          • I have the same 13′ MBP. But I can get a display much larger than that though. Even without switchresx, can you try going into displays(from spotlight), and changing resolution from /default/ to /scaled/. Then click on more space.
            After that, go back to the iMac, and drag to stretch the edges of the display.

          • Chris

            I the meantime i have switched back to a airplay server to view the second screen… Its not the best solution, but i mainly need it for big excels etc… So it works fine.

            If you want to have a bigger screen, make sure, that you not only set the resolution to max. you also need to set the display size font etc to the minimum. Its on the same settingspage as the resolution.

  • For those looking for details, plug in the thunderbolt cable between the two computers, and look under the network settings for the “Thunderbolt Bridge” device. Assign each one a private IP different than each other — even in the “self-assigned IP” space i.e. 169.254.35.10 and 192.168.35.11 – Click on “Advanced” and make sure one of the interfaces shows “Active”

    From there on the higher-res display, you can VNC over to the other one: Finder -> Go -> “Connect to Server” and enter: vnc://169.254.35.10 or whatever the IP of the other computer is.

    To further optimize resolution you can use the program switchResX to select a resolution in line with your display.

    • ajaysdesk

      Michael – thanks. Saved me the burden of writing this out. But really, its just that simple indeed. I still use this setup at home. Need my iMac workspace 🙂

      • Mike

        So this would work between the new 5K iMac as the “target” display and a non-retina MBA?

        Many thanks!

        • Yeah. Doesnt matter if you are trying from a retina or non-retina mac. The whole concept is to take advantage of the bandwidth of thunderbolt interface to view displays across devices. So all you need is a device with a thunderbolt.

  • Brandon Burrows

    Ajay,
    this looks like exactly what i need to get my iMac to work with my MBP… I believe i’ve followed your instructions below, but not able to get them to connect. I feel l’m missing something… Thunderbolt Bridge is active on both computers, i’ve moved it to the top priority connection, i’ve provided dedicate IP addresses for each computer’s bridge, but when i try to VNC over it searches and then times out. I can VNC through the IP address on my WiFi network (just being sure it works), but not TB. Any ideas? I hope you can help!

    Thank you,
    Brandon

    • Hey Brandon – do you know if you have statically assigned IPs yet? If so, what range are you using. Are they part of the same subnet? Bring up terminal, and ping across from the macbook to the static IP you gave to the iMac’s thumderbolt, and vice-versa. Are the pings successful?

      • Brandon Burrows

        (this is outside my realm of expertise, so could have made a dumb mistake)

        On both computers i set the option “Configure IPv4” to “Manually” and i used somewhat arbitrary IP Addresses for each (172.154.34.12 for iMac and 134.125.17.4 for MBP)… i tried a ping from each side and neither went through.

        Worth noting that the subnet mask for each is at 255.255.255.255

        • Ahh – that explains it 🙂
          Those are 2 completely different subnets. Wont work.
          Try using 192.168.200.1 on one side, and 192.168.200.2 on the other , with a subnet mask of 24 (255.255.255.0) for both. This is just one configuration that will work, there are several others.
          Or you can keep one side as 172.154.34.12, and make the other side 172.154.34.13, and make sure that their masks are either /16 (255.255.0.0) or /24(255.255.255.0).
          Try one of the above configurations, and then see if the ping works.

          • Brandon Burrows

            that did it! Greatly appreciated! i’m up and running. I know there are some tricks to grow the size of the window… i did change setting to ‘Scaled’ and ‘More Space’… is this the biggest or is there another unlock?

          • Using scaled + more space on MBP is one way.
            Another is to download a custom resolution app (I dont recall what the name was, but I am sure you can find a few if you google), and then set to a higher resolution than what scaled gives on the MBP.
            You’ve hit the maximize-window on the iMac side already, I assume.
            Have fun!

          • Brandon Burrows

            yep, i pulled down SwitchResX and now it’s working really well! it’s a little laggy, so i’ll dig into that (unless you also know how to do that!), but much faster and better quality than over WiFi.
            I really appreciate the quick help… probably saved me from returning the iMac and waiting for one to ship with TB3… 😉
            Best to you Ajay!

          • AnthonyALR

            Hi Ajay,
            I’ve followed these instructions, using 192.168.200.1 on one side, and 192.168.200.2 on the other , with a subnet mask of 24 (255.255.255.0) for both. The issue I am running into is it is letting me screen share, but I want to use the 192.268.200.2 as an external monitor. Right now, that screen is unaffected, it just appears as an additional window on the 192.168.200.1 screen. How do I make it an external monitor rather than what it is right now?

          • Viraj Master

            I also have this identical problem. I just see the iMac window appearing on my Macbook Pro, and thus screen sharing must be working, BUT it is not working as an external monitor. I must be doing something wrong…may I ask anyone to help? Please?

  • Chinnasamy Arumugam

    Excellent Solution and it worked like a Charm. Thanks Ajay. Just to help out others, I have put a screenshot for the network settings.

  • Phil Zelnar

    Having had first-hand experience with this, if you were in a situation where you needed a new iMac that will often be used as a display for your MBP for design/coding stuff (old iMac conked out), would you try to find a 2014 pre-retina and stick with target display mode, or would you just get the newer iMac and use this method?

  • Arun Prasad

    Hey Ajay – I have a big problem and hopefully you can help me. I need a mac at home with a big screen. When I bring my office macbook pro (MacBook Pro (Retina, 15-inch, Mid 2015) ) back home, I would like to connect my office MBP to the bigger display at home and work. So that limits my options to

    1. new mac mini + thunderbolt display monitor + my office MBP
    2. new macbook pro + thunderbolt display monitor + my office MBP
    3. the new imac 27″ retina + my office MBP -> my preferred.

    So are you saying that I can go ahead and buy the new iMAC 27, a thunderbolt 2.0 cable and make things work ? Am I compromising on the quality ?

    • Chinnasamy Arumugam

      Hey Arun, I would go with option# 3. the new imac 27″ retina + my office MBP -> my preferred.

      Yes, there will be a compromise on the quality between 5K vs Retina 2880×1800. If your work is not a graphics designer and lots of photo editing, you will be good with the screen sharing via thunderbolt.

      I have the same setup iMac 27″ and MBP 15″ Retina and it works like a charm per Ajay’s instructions. I have also put in the screen shots for the setup on my response below.

      I have also seen the discussion on using 2 thunderbolt cable to have 2x20GB throughput via Screen Sharing option. I have not tried this and not sure if anyone had tried this.

      • Arun Prasad

        Thanks so much. Let me try it out.

  • Shane Moore

    Hey Ajay, thanks for posting this! I’m having sort of a weird problem; I can only connect VNC in one direction, and it’s not the useful one (I can view my iMac from my Macbook). Here’s my setup:

    iMac
    ——
    IP Address: 192.168.200.1
    Subnet Mask: 255.255.255.0
    Thunderbolt Bridge is Connected
    Screen Sharing on
    Firewall is off

    Macbook Pro
    —————
    IP Address: 192.168.200.2
    Subnet Mask: 255.255.255.0
    Thunderbolt Bridge is Connected
    Screen Sharing on
    Firewall is off

    I can only ping (and VNC) my iMac from my Macbook, not the other way around. Any ideas?

  • Avalonica

    Pardon my ignorance, but how is all of this any better than regular screen sharing over a network?

    • The bandwidth… and therefore the resolution. On an iMac retina, screen sharing over the network (usually limited to 1.3Gbps even if you are running wireless-ac, assuming an unrealistic efficiency of 100%.) Thunderbolt2 runs to 20Gbps. You see the difference!