Plugable UD-3900PDZ: why only one HDMI port works

A concrete desk: ports and cables

Three independent office screens with mixed output limits. HDMI 1 uses native video; HDMI 2 and 3 use DisplayLink. This is not a triple-4K dock. Verify that the software is permitted on your Mac.

Source scope: Plugable: UD-3900PDZ output-specific specifications · Apple: MacBook Air display support · DisplayLink: Manager installation and scope · DisplayLink: macOS screen-recording permission

What you need for this arrangement

• 1
  UD-3900PDZ power supply and supplied host cable

  Use the rear host connection.

• 3
  HDMI cables and HDMI-input monitors

  Match each monitor to its output-specific limit.

• Required
  DisplayLink Manager and Mac permission

  HDMI 2 and 3 depend on software even if HDMI 1 already works.

Check the box contents and the cables you already own before ordering. Monitor and dock model names alone do not identify a cable’s mode rating or direction.

If this exact arrangement fails

HDMI 1 works; HDMI 2 and 3 are dark

Check DisplayLink software and permission before replacing all three cables.

HDMI 2 and 3 work; HDMI 1 is dark

Check the native USB-C video route, supplied host cable and screen 1 input.

The 4K screen only offers 30 Hz

That matches HDMI 1’s limit. Move to a different documented dock if 4K60 is mandatory; settings cannot add bandwidth.

1. Connect only screen 1 and record its working input and mode.
2. Add the next screen on the labelled route. Confirm separate windows on each screen.
3. Check every selected mode, a sleep/wake cycle and a reconnect. Record any failure before changing another component.

Read the output map first

These are different capabilities inside one dock. Plugable also lists some 1920 × 1200 modes in its detailed table; verify that exact mode if you need it. “Triple display” does not mean three 4K60 outputs. Plugable: UD-3900PDZ output-specific specifications

Use the working port as evidence

If HDMI 1 works while HDMI 2 and 3 remain dark, check DisplayLink installation, whether the application is running, and the Mac permission. If the software-driven outputs work but HDMI 1 does not, investigate the host’s native USB-C video support and cable.

This split is more useful than repeatedly swapping all three screens. Keep one known-good monitor and cable as the control, and move that pair between labelled outputs. Record the result for each socket.

Set it up in a controlled order

1. Power the dock and connect the laptop to the rear USB-C host port using the supplied host cable. A hub or accessory port is not the computer connection.
2. Attach one monitor to HDMI 1 and select its HDMI input. Verify the laptop port supports native USB-C video.
3. On Windows, confirm DisplayLink installation completes; it is typically automatic. On macOS, install DisplayLink Manager and grant its required Screen Recording permission. Use the linked official instructions and involve IT if installation is restricted.
4. Add HDMI 2, verify an independent desktop, then add HDMI 3.
5. Select supported modes separately for each output. Do not apply HDMI 1’s 4K setting to the other two.

The rear host connection and the different software requirements are documented by Plugable. Plugable: UD-3900PDZ output-specific specifications For the Mac installation and permission steps, use DisplayLink: Manager installation and scope and DisplayLink: macOS screen-recording permission.

Decide whether the limits fit your desk

This arrangement can be useful for office work when two secondary Full HD displays are sufficient. It is a poor match for a requirement written as “three independent 4K60 monitors”. On a managed computer, software approval is part of compatibility, not an optional installation detail.

If only protected streaming is black while the desktop works, continue with the protected-video guide. If the entire display disappears, finish the output and software checks first.