I have a mid-2015 MacBook Pro 15" retina (A1398), and I'm trying to setup external displays as follows:

When I have just the 4k tv hooked up, it works fine at 4k 60hz. When both mDP are in use, it drops to 30hz.

When I have just the MST hub hooked up, both displays work, but are only recognized as a single display in System Preferences. It seems to be working more like a splitter than an MST hub. I thought this might be because only one mDP or the other is 1.2, but this is the situation no matter which it is plugged into.

So, 2 questions, and a plea for help:

  1. Are the mini DisplayPorts on my MacBook Pro 1.2 or 1.0? If they are 1.2 then my MST hub should allow the displays to be recognized separately, right?

  2. Why does my display drop from 60hz to 30hz when both mDP are used?

  3. How can I get this arrangement setup?

  • So you are trying to run 3 external displays on a Mac that supports 2 ! – Ruskes Sep 24 '18 at 3:38
  • It supports more monitors, as long as you have an active connection for any more than 2. Turns out the problem is MacOS's limited MST support. – Matt Flamm Sep 28 '18 at 3:50

Turns out macOS only support MST on thunderbolt and for >4k screens, but not for daisy chaining or multiple displays on mDP 1.2. Point for Windows on this one, I guess.

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