rMBP 2014 with dGPU 750M - Using 4K monitor and Thunderbolt Display

Discussion in 'MacBook Pro' started by webdevfreak, Nov 24, 2014.

  1. webdevfreak, Nov 24, 2014
    Last edited: Nov 25, 2014

    webdevfreak macrumors newbie

    Joined:
    Jul 19, 2012
    #1
    Hi, This is probably covered in another thread but I can't find a response that helps me. I have a rMBP 2014 with the dGPU and when i plugin my Asus PB287Q in one of the Thunderbolt Port I cannot seem to get my Apple Thunderbolt display to work as well. I think when running at 4k60 the 2nd TB port is disabled but I am not sure. So if disabled what would leave me with the HDMI port for the Apple TB Display but I haven't seen any adapters to go from TB Display to HDMI. Any recommendations to help me run both the 4K display and the TB Display? Thanks!
     
  2. webdevfreak thread starter macrumors newbie

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    Jul 19, 2012
  3. Freyqq, Nov 25, 2014
    Last edited: Nov 25, 2014

    Freyqq macrumors 68040

    Joined:
    Dec 13, 2004
    #3
    When you plug in a 4k into thunderbolt, you get 60hz. At the same time, the 2nd thunderbolt port is disabled. HDMI will still work, which supports up to 4k at 30hz.

    Thunderbolt 2 is Displayport 1.2 with an extension for data transfer. You can plug a displayport monitor into a thunderbolt port. However, you can only plug a thunderbolt-only monitor into a thunderbolt port.

    So, the best option for you is probably to plug the thunderbolt monitor into the thunderbolt port and the 4k monitor into the HDMI, but you'll only get 30 hz (might not even notice the difference, depending on usage).

    There is also a way to install a 3rd party program (switchresx) and a custom setting on the 4k at ~50hz on thunderbolt, which would be the optimal setup. This would not disable the second thunderbolt port, as you are using the maximum bandwidth for one thunderbolt port without dipping into the other one. Search the forums, as this method has been explained in detail before.
     
  4. brdeveloper macrumors 68020

    brdeveloper

    Joined:
    Apr 21, 2010
    Location:
    Brasil
    #4
    I thought 20Gbps would be sufficient for 4k@60Hz. That is, you need roughly 3840 columns * 2160 lines * 4 (1byte/channel + 1 byte alpha information) * 60 Hz = 1.990.656.000 bytes.

    This represents approx. 16Gbps, which is a bit far from saturating a 20Gbps TB2 channel. Ok, maybe there is some overhead from headers and control signals, but are these signals using 4Gbps?
     
  5. Freyqq macrumors 68040

    Joined:
    Dec 13, 2004
    #5
    Thunderbolt 2 is actually 16gbps
     
  6. brdeveloper macrumors 68020

    brdeveloper

    Joined:
    Apr 21, 2010
    Location:
    Brasil
    #6
    Source? From Wikipedia:

    At the physical level, the bandwidth of Thunderbolt 1 and Thunderbolt 2 are identical, and Thunderbolt 1 cabling is thus compatible with Thunderbolt 2 interfaces. At the logical level, Thunderbolt 2 enables channel aggregation, whereby the two previously separate 10 Gbit/s channels can be combined into a single logical 20 Gbit/s channel.
    http://en.wikipedia.org/wiki/Thunderbolt_(interface)#Thunderbolt_2
     
  7. Freyqq macrumors 68040

    Joined:
    Dec 13, 2004
    #7
    Logical 20gbps. It actually only has 16gbps of bandwidth.
     

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