Plugable 3900pdh Not detecting Third monitor - Alt Mode

I recently bought a Plugable UD-3900pdh to replace an older Plugable as I needed a third monitor. While it started up right away without issue, it won’t detect the third monitor in Alt mode. I’ve checked all the cables and monitors, and everything functions as it should, unless it’s plugged into Alt mode. I’ve run through any of the troubleshooting I could find online, but to no avail. Need help.

Hi Chris,

Thank you for reaching out directly to us through our support system. I am updating this thread so that we can provide this information to others who may come across this issue in order to help clarify the HDMI port 1 requirement on the UD-3900PDH.

When connecting to video port HDMI 1 on the UD-3900PDH, the host system must provide a USB-C port that can support DisplayPort Alternate Mode. To help expand further, USB-C is capable of providing different data transport technologies. One of those technologies is the ability to transfer DisplayPort signals that are either called USB-C DisplayPort or DP Alt mode. This allows video sources that support DisplayPort to connect to each other through USB-C. In some cases, confusion can arise because not all USB-C ports support DP Alt Mode, and due to the lack of clarity from the laptop manufacturer, it is often difficult to tell if a USB-C port supports DP-Alt Mode without having to look further into the specifications of the system.

We hope this information helps!

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