Thunderbolt 2 devices and 4K monitor on the same bus

Discussion in 'Mac Pro' started by theSeb, Oct 24, 2014.

  1. theSeb, Oct 24, 2014
    Last edited: Oct 24, 2014

    theSeb macrumors 604

    theSeb

    Joined:
    Aug 10, 2010
    Location:
    Poole, England
    #1
    I know that his has been discussed before in detail, but I cannot remember those details and I haven't been successful in trying to search for the discussions. I thought that I understood how TB and TB2 works, but I no longer think so, so I would appreciate some thoughts on this.

    Let's say I want to connect a 4K display to the Mac Pro and a Thunderbolt 2 enclosure with two SSDs in RAID 0. Anandtech's 2013 Mac Pro review tells me that daisy chaining the 4K display off the TB2 enclosure is not a good idea. Fair enough and I understand that so far.

    Great. Not on the same chain. Got that. But what happens if I put the TB2 enclosure and the 4K display on the same bus, in other words, on TB bus 1, port 1 connected to the 4K display and port 3 connected to a TB2 enclosure, for example?

    Then let's take this question and say I want to do the same on the latest iMac, which only has one TB bus (unless they have changed this for the riMac, which I don't think they have). Reading through the 2011 iMac review, I am left confuzzled just how much the above scenario will be handled.

     
  2. mintakax macrumors regular

    Joined:
    Dec 19, 2013
    #2
    I would also like to see this answered/discussed or maybe links to more info ?
     
  3. VirtualRain macrumors 603

    VirtualRain

    Joined:
    Aug 1, 2008
    Location:
    Vancouver, BC
    #3
    Each bus has a single TB2 controller which has a DP 1.2 input and a PCIe x4 bus interface on one side and a pair of TB2 ports on the other (20Gbps each). Daisy chaining obviously muxes both into one port and contention can occur if both are running near peak bandwidth.

    Putting the display and PCIe device on separate ports (same bus) should eliminate contention and allow each to operate at full capacity.

    What's not possible is running two 4K DP1.2 displays off one bus as there's only a single internal DP1.2 interface to the TB controller.

    Similarly, running two devices that can both saturate a x4 PCIe bus on the same TB bus (controller) will likely create contention if both are used at the same time.
     
  4. theSeb thread starter macrumors 604

    theSeb

    Joined:
    Aug 10, 2010
    Location:
    Poole, England
    #4
    I got it. Thank you.
     
  5. scottomfg macrumors member

    Joined:
    Oct 21, 2014
    #5
    Hey VirtualRain, that is fantastic news! We've all been stumped over in the iMac forum, and this certainly seems to be a definitive answer. Can Yyou point me to some source or documentation? I have no reason to not trust you, and I really, really want you to be right, but when it comes to plopping down $4k, I prefer certainty. Thanks!
     
  6. MacVidCards Suspended

    Joined:
    Nov 17, 2008
    Location:
    Hollywood, CA
    #6
    I wouldn't get too excited. I don't think the iMac has same setup. They had to use up something to get that 5K display going.
     
  7. scottomfg macrumors member

    Joined:
    Oct 21, 2014
    #7
    What he described wouldn't be affected in any way by the GPU having a unique connection to the 5k display, FYI. Please don't reply unless you actually have something real to add, thanks!
     
  8. MacVidCards Suspended

    Joined:
    Nov 17, 2008
    Location:
    Hollywood, CA
    #8
    Thanks for the tip. I was also telling you that Mac Pro and iMac have very different implementations of TB2. You have far fewer options and bandwidth to play with so what works on a nMP won't necessarily work on an iMac.

    Thank You !
     

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