Max refresh rate not detected through DP Alt Mode port

I’m running a 2020 M1 Macbook Pro 13" with 3 external displays connected via the Pluggable UD-ULTC4K. The MBP is connected to the dock via the included Pluggable USBC cable.

Two of the displays are Dell S2716DG (2560x1440 native, 144hz native) and one display is Dell Alienware AW3423DWF (3440x1440p native, 165hz native). All three monitors are connected via DisplayPort cables (specifically FIBBR fiber optic cables rated up to DP 2.1)

The Dell Alienware ultrawide is connected to display port 1 (DP Alt Mode port) and the two Dell S2716DG are connected to ports 2 & 3. The non-alt-mode connected Dells are detected as 1440p native @ 144hz. The Alienware via alt mode is detected as 3440x1440 native @ 100hz.

Why is my Mac unable to detect the max refresh rate of the Alienware as 165Hz when connected via the dock?

If I bypass the dock and connect the Alienware ultrawide directly to my Macbook Pro using a DisplayPort → USBC cable (also rated up to DP 2.1), the display is detected as 3440x1440p @ 165hz native. The other Dell monitors remain connected via the dock and maintain 1440p @ 144hz via DisplayLink. The direct connection from monitor → Mac also functions via DP alt mode just like the dock does for display port #1.

It’s also worth noting the dock is detecting and passing through the HDR capability of the Alienware, for what that’s worth, and DDC signaling is reaching the macbook as my Lunar app can change the brightness on the Alienware when connected through the dock.

Hi Jeff,

Thank you for contacting Plugable! I’m sorry the refresh rate of the Alienware display isn’t as high as expected when connecting through the UD-ULTC4K. My name is Sam and I’ll be happy to assist.

Unfortunately the behaviour you’re seeing is the result of a limitation of the amount of bandwidth available to the DP Alt mode-based video output ports (labelled DISPLAY 1) on UD-ULTC4K. The DisplayPort output is a DisplayPort 1.2 port which supports a maximum resolution of 3840x2160 at 60Hz. The amount of bandwidth used by this resolution is very similar to the amount used by 3440x1440 at 100Hz, however the maximum refresh rate at this same resolution exceeds the bandwidth limitation by quite a bit. With this said, I’m afraid that the UD-ULTC4K would not be able to achieve 3440x1440 at 165Hz on a connected display.

In order to use this display at its full potential, an output port which supports DisplayPort revision 1.4 or higher would be required.

Please let me know if you have any additional questions and I’ll be happy to assist. My apologies for the frustration. Thank you for giving us the chance to help.

Plugable Technologies

Thanks for the prompt follow-up and detailed explanation Sam. Please consider this topic closed.

Hi Jeff,

Thank you for your response. Please feel free to reach out if you have any additional questions.

Have a great day!

Plugable Technologies

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