Applicable To
- UD-3900C4
The UD-3900C4 uses two different technologies to support its four HDMI outputs. If displays connected to HDMI 3 and HDMI 4 are mirrored or only one display works, the host computer may not support the DisplayPort Multi-Stream Transport (MST) functionality required by those ports.
Troubleshooting Steps
- Confirm that the affected displays are connected to HDMI 3 and HDMI 4 on the UD-3900C4.
- Check the specifications for the USB-C, USB4, or Thunderbolt port being used to connect the dock to the computer.
- Confirm that the host connection supports Alt Mode video and DisplayPort Multi-Stream Transport (MST).
- If you are using Windows or ChromeOS and the computer supports MST, configure HDMI 3 and HDMI 4 as extended displays within the operating system's display settings.
- If you are using macOS, be aware that macOS does not support DisplayPort MST for multiple independent displays from a single DisplayPort connection.
On a Mac, HDMI 3 and HDMI 4 cannot both operate as unique extended displays. The UD-3900C4 can support up to three unique extended displays on macOS, with the fourth display mirrored.
Plugable has also produced a video demonstrating this macOS MST limitation:
https://www.youtube.com/watch?v=HNoGjqrrSok
For more information about determining whether your computer supports MST:
https://kb.plugable.com/1390562
Expected Result
On a compatible Windows or ChromeOS computer that supports Alt Mode and MST, displays connected to HDMI 3 and HDMI 4 should operate as separate extended displays.
On macOS, HDMI 3 and HDMI 4 mirroring is expected because MST is not supported. A maximum of three unique extended displays is supported with the UD-3900C4 on macOS.
If the Issue Persists
If you are using Windows or ChromeOS and have confirmed that the host computer supports DisplayPort Alt Mode and MST, but HDMI 3 and HDMI 4 still do not work as expected, contact Plugable Support at support@plugable.com.
If your computer's specifications do not clearly state whether its USB-C port supports DisplayPort Alt Mode and MST, contact the computer manufacturer for confirmation.
