I got a USB-C to DisplayPort cable from Amazon and connected it to my first Dell UltraSharp U2515H.

The display finally mirrored my MBP, but I had to move USB-C ports seemingly to “wake up” the monitor or MBP. Then I activated the DP1.2 mode on the U2515H since by default DP1.1a is the factory setting.

Then I connnected the video out of the first UltraSharp to the video in of the second UltraSharp. For this, I used the Dell-provided DisplayPort to mini-Display Port cable. I then activated DP1.2 on the mDP port on the second monitor (not sure if I had to do this but thought it made sense).

The second UltraSharp came to life and displayed a mirror of the first UltraSharp. My laptop screen showed one desktop while both UltraSharps displayed a second (mirrored) desktop. The MBP recognized one external monitor, not both.

I also tried using a DP-to-DP cable to interconnect the two DP1.2-compatible monitors together, but that didn’t change the results (yes, I activated DP1.2 for the monitor’s DisplayPort). My MBP is running macOS Sierra 10.12.5.

Now the Dell manual states that only a single external display with resolution of 2560×1440/60Hz is supported by MST, and that’s the native resolution of the Delll UltraSharp U2515H. So there’s that. I tried reducing the resolution of the Ultrasharps seeing if that had any effect, but nope.

So either MST isn’t supported/implemented in the Mac OS X software or Mac hardware or I need to try this again using monitors with lower resolutions. Or most likely, I need to forget MST and go the old fashion route of using two video connections to two monitors.

Bye VGA, bye DVI. Got Dell UltraSharp U2515H displays that only have HDMI and DisplayPort inputs. But there’s this “weird” DisplayPort that wasn’t working for me…until I learned that it’s an ouput for DisplayPort, not an input. But why would a monitor need an output port? So you can daisy chain them.

Yup, I learned that monitors that support DisplayPort 1.2 can actually be daisy chained to each other and still work as independent displays. Of course, your graphics card has to be able to keep up, pushing out all them pixels. But wow, this is convenient and with less cable clutter on your computer, especially a laptop. But… for these UltraSharp U2515, you need to enable DisplayPort 1.2. By default, it’s disabled to the more commonly used version 1.1, indicating that daisy chaining isn’t a common consumer feature.

On the Mac front however, my older MacBook Air doesn’t support daisy chaining aka Multi-Stream Transport (MST). I kinda expected that from a mid-2012 computer, but my Air does flawlessly power the U2515’s 2560 x 1440 resolution via the ThunderBolt port that shares the same form factor as a miniDisplayPort (mDP). So yay!

Next, make a return to the MacBook Pro line…