MacBook Pro 2017 -- MST not working -- only mirroring displays

I have a brand new MacBook Pro (2017 model) and a Plugable USB-C to DisplayPort cable. Plugable cable is connected to DP-in on a Dell U2415. Second cable goes from miniDP-out to second Dell U2415, DP-in. First monitor is configured for DP 1.2 (enabled), second monitor has it disabled. These are the instructions that I have found on the Dell web site.

Plugable describes the cable as supporting MST. Apple says the hardware/software supports MST. But the second display only mirrors the first display. However, the first display is extended from the laptop display.

If I disconnect the Plugable cable and connect a miniDP to DP cable to the first monitor, and change no settings, then connect it to a miniDP port on a windows laptop, MST works fine.

I’m trying to determine if the plugable cable is the culprit, or it if something else. Apple is trying to look into this. Dell tool.

@PlugableSupport – any suggestions?


According to Apple, the newer MacBook Pro laptops support MST.

Also, according to Apple, the new 2017 MacBook Pro supports multiple monitors:

Hi Gene,

Thanks for contacting us, I am sorry but Apple does not support MST for multiple monitor configurations. When Apple expresses support for MST ( such as this page ) it is referring to MST in a single monitor to allow for a higher resolution displays, not spanning to multiple monitors as MST most commonly refers to. This MacRumors thread (… ) covers some setups for multiple monitors and unsuccessful attempts to get MST Multiple Monitors working on MacOS.

The MacBook does support DisplayPort Alternate Mode on the Thunderbolt 3 ports, which can be used to connect multiple monitors to the MacBook. These additional monitors are connected directly to the Thunderbolt 3 ports and not in an Muilti Stream Transport Multiple monitor configuration.

An option to use both of these displays is to use a graphics adapter like our Thunderbolt 3 Dual DisplayPort Adapter for Mac (… ). This does require a supported MacBook product, both the late 2016 and mid 2017 MacBook Pros are supported. If you are interested in this solution, and would like to return the USB-C DisplayPort cable, please let me know your Amazon Order ID ( available from ) and I can start the return process.

I am sorry this setup will not work as desired, please let me know how you would like me to proceed.

Plugable Technologies

@Pat – thanks for the detailed response. Totally makes sense! (I wish someone at Apple could explain it this well.)

If I were to use the product that you mentioned in your response, it includes a unique, external graphics adapter, right? So I would no longer be taking advantage of the Radeon Pro 560 to drive my monitors.

If I were to purchase a second USB-C/T3 to DP cable, my only downside is using a second T3 port on my machine, right?

Thanks again for your help.


Hi Gene,

Thanks for getting back to me! Our TBT3-DP2X-83 (… ) uses DisplayPort Alternate Mode to provide for two DisplayPort outputs for external monitors directly connected to the internal graphics controller through the Thunderbolt 3 Type-C port. We also have a HDMI version of the same adapter, TBT3-HDMI2X-83 (… ). For the fifteen inch MacBooks any of the Thunderbolt 3 ports is capable of connecting this adapter and supporting two displays up to 4K resolution. On some of the smaller notebooks, only Thunderbolt 3 ports on one side of the notebook are capable of two displays. If you let me know the specific model you are using I can let you know if we have tested that specific model.

To get the Macbook model number:

  1. Select the Apple menu from the upper left corner of the screen
  2. Select About This Mac from the drop down menu
    The model should look something like “MacBook Pro 15-inch ( mid 2017 )”

Thank you,

Plugable Technologies

Hey Pat,

Thanks again for all of the help with this inquiry!

Yes, I’m using exactly that model in your example:

MacBook Pro 15-inch (mid 2017)

I have a 2.9 GHz processor and a Radeon Pro 560 w/ 4GB of GDDR5 memory.

I specifically bought this model, with the intention of leveraging the advanced video horsepower, so I’m definitely looking for a solution that helps me take advantage of these capabilities. It sounds like maybe the TBT3-DP2X-83 does this?

I’m even okay taking up two Thunderbolt 3 ports for the “optimal” configuration. I would rather drive two different monitors, using two different T3 ports, than use an external displaylink device.

What about your T3 Docking Stations – do either of those provide true dual monitor support from the video card in the laptop?



Hi Gene,

Thanks for getting back to me!

The TBT3-DP2X-83 leverages the internal graphics card, in this case the Radeon card, this does not utilize DisplayLink. Likewise, our TBT3-UDV Thunderbolt 3 Docking Station ( ) is also compatible with the MacBook Pro and natively supports one DisplayPort output, and additional Type-C to DisplayPort cable can be added for the second screen ( ). This setup also does not use DisplayLink technology, instead relying on the Type-C DisplayPort Alternate Mode to provide the Radeon Pro card direct control over the monitors. The docking station also provides five additional USB 3.0 ports, gigabit Ethernet as well as one 3.5mm TRRS headset jack.

If you do not need the docking station functionality then two USBC-DP cables can be connected from two Thunderbolt 3 ports on the notebook to the monitors directly as well.

Please let me know if you have any additional questions.

Plugable Technologies

Hi Gene,

We’re closing this thread due to inactivity, but should you have any other questions, please feel free to contact and we’ll be happy to help.