Two external displays on a 2014 iMac 5K

Discussion in 'iMac' started by antibolo, Oct 19, 2018.

  1. antibolo macrumors member

    Joined:
    Sep 27, 2017
    #1
    Is it supposed to work? I'm trying to output to two DisplayPort monitors using the two Thunderbolt ports on my Late-2014 iMac 27 inch, but I can only get the first connected monitor to be seen, plugging a second one only gets it ignored.

    The official documentation for this model mentions support for two simultaneous Thunderbolt displays. I'm guessing that it only works if they're specifically Thunderbolt monitors or something? That's a real bummer if true...

    Does anyone here successfully use two DisplayPort (not Thunderbolt) monitors on this specific iMac model?
     
  2. mikehalloran macrumors 65816

    Joined:
    Oct 14, 2018
    Location:
    The Sillie Con Valley
    #2
    Not enough information. 2 standard backlit LCD displays through el cheapo mini DVI–HDMI adapters are no problem. I've got a pair hooked up to a 2011 i5 right now.

    For anything more exotic, it depends, Start here.

    https://support.apple.com/en-us/HT206587

    You can run a pair of 4K HDMI displays such as the $350 LGs on Amazon but you have to pay attention to the refresh rates per the article above.

    BTW, used Apple TB displays are going for $150–$400 here in the silicon valley.
     
  3. FreakinEurekan macrumors 68040

    FreakinEurekan

    Joined:
    Sep 8, 2011
    Location:
    Eureka Springs, Arkansas
    #3
    According to the tech specs https://support.apple.com/kb/sp707

    "Simultaneously supports full native resolution on the built-in display and up to 3840 by 2160 pixels on an external display"

    So, officially, just one display but it could be a 4k.
     
  4. antibolo thread starter macrumors member

    Joined:
    Sep 27, 2017
    #4
    OK I figured out what's wrong.

    Seems the 2014 iMac only supports two DisplayPort channels total. The thing is, my Dell UP2414Q uses MST for 4K at 60Hz, which means it eats up two DisplayPort channels, leaving none available for the second port. I tried shutting down MST support (which drops it to 30Hz, ewww) and then both displays work.

    This is a real bummer but oh well. Seems there's not much else I can do.
     
  5. mikehalloran macrumors 65816

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    Oct 14, 2018
    Location:
    The Sillie Con Valley
    #5
    While you're not wrong, the Apple support bulletin is more correct. Unfortunately, that results in

    So, just how bad is a 4K monitor at 30Hz?

    Enquiring minds want to know!
     
  6. antibolo thread starter macrumors member

    Joined:
    Sep 27, 2017
    #6
    It's a waste of what I paid for, so no, I'm not going to downgrade to that.

    Anyway, the second display I was trying to connect is actually a Luna Display dongle (https://lunadisplay.com) for use with Astropad. I'll just unplug my Dell whenever I want to use it. It's unfortunate but not that big of a deal.
     
  7. mikehalloran macrumors 65816

    Joined:
    Oct 14, 2018
    Location:
    The Sillie Con Valley
    #7
    eGPU such as your Luna does not work over OS 10.13.4–6 (unless over TB3, not an issue in your case) as Apple disabled that functionality. They re-enabled it in Mojave. Unfortunately, there was no 10.13.7 where they re-enabled it in High Sierra.

    I don't know if your Luna is 10.14 compatible.
     
  8. antibolo thread starter macrumors member

    Joined:
    Sep 27, 2017
    #8
    The Luna is not a eGPU, it interacts as a physical display connected to DisplayPort. This has nothing to do with eGPU support.
     
  9. FreakinEurekan macrumors 68040

    FreakinEurekan

    Joined:
    Sep 8, 2011
    Location:
    Eureka Springs, Arkansas
    #9
    There are two issues - what's supported, and what works. The document I posted shows what's supported, a single display of up to 3840x2160. "Could" it do two displays, in some way shape or form? Maybe. But it's not supported, so if it doesn't work Apple will fall back on https://support.apple.com/kb/sp707
     

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8 October 19, 2018