P2423DE USB-C Hub with MST Pass through wrong resolution

Question

Hi, we are having an issue with MST pass through using a USB-C hub monitor and a second monitor with Displayport. We have two different scenarios. We have several of these setups and can duplicate the issue on any of them.

Setup 1 - USB-C Monitor P2423DE connected to laptop with USB-C cable. Second monitor is P2422H connected by DisplayPort to the DP-Out connector on the P2423DE.

Setup 2 - USB-C Monitor P2422HE connected to laptop with USB-C cable. Second monitor is P2422H connected by DisplayPort to the DP-Out connector on the P2423DE.

On Setup 1 the P2422H won't show the screen properly:

P2423DE on right, P2422H on left

On Setup 2 the displays are correct, like this:

P2422HE on left, P2422H on right

We tried a number of different things and eventually realized that it depends what model laptop is connected to the USB-C cable. With laptops of a Latitude 5420, 5421 or newer, it does work correctly (all displays look like the second photo). But with an older model like a Latitude 7400, only Setup 2 works right, setup 1 does not. The 7400 in this example does have Thunderbolt 3, Displayport over USB-C, and works fine with a P2422HE and P2422H combined (second photo). The Latitude 5411 does not work correctly in Setup 1 but also works fine in Setup 2. The Latitude 5421 and 5420 work fine with both Setup 1 and Setup 2. A Microsoft Surface tablet works fine with both setups. The issue seems to be related to the older laptops, but they should support both of these screens with MST and the resolutions. How do we proceed investigating this issue? It seems like a graphics card limitation, but according to specs I've found Setup 1 should work fine.

As far as drivers and such, all machines are running Windows 10 and fully up to date with Dell drivers installed from Dell Command Update and a full driver library installation, then updated as much as possible. We also installed the monitor specific drivers for the P2423DE, P2422HE and P2422H to make sure they are recognized properly. What's next?

Thank you

-Derek

drivesmeupawall · Accepted Answer

Hi Chris,

I had the same issue with the lid closed, but your suggestion made me go into advanced properties and double check the Hz - The P2423DE was running at 1920x1080@59.957 Hz, not at 60Hz (even though the monitor showed it was at 60Hz). Changing the resolution to be at 60.0 Hz on both screens allowed me to choose the correct resolutions.

drivesmeupawall · Accepted Answer

Hi all, I was able to resolve this issue. The P2423DE showed on screen that the resolution was 1920x1080 60Hz. This however was misleading. In Windows Display settings, under advanced settings for this screen, the actual Hz was 59.957. Changing it to 60.0Hz didn't make any visual change on the P2423DE, but it fixed some of the issues on the 2nd display, the P2422H (MST DP) - it made the second display stop scrunching the extra screen space with black bars. I was then able to change the Hz on that display from 75Hz back to 60Hz, and then the correct 1920x1080 resolution was available to choose. With both screens forced to 60.0 Hz, everything works normally and all resolution options are available on both screens.

Thanks to the Dell Cares folks for sending me down the right road. The trick of the problem was in the Hz of the display - the display may show "60Hz" when it changes resolutions, but the actual Hz could be something different and 59.957 IS NOT the same as 60.0. Lesson learned.

Thanks for the help and suggestions everyone.

DELL-Cares · Answer

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DELL-Cares · Answer

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DELL-Cares · Answer

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drivesmeupawall · Answer

Hello @DELL-Cares  I've sent a PM. What additional information would be helpful? Thanks

DELL-Chris M · Answer

In the passing, the commonality is running 3 displays =
Laptop LCD
P2422HE 1920x1080 60Hz
P2422H 1920x1080 60Hz

In the fails, the commonality is trying to run 3 displays =
Laptop LCD
P2423DE 2560x1440 60Hz
P2422H ???? (I cannot see the resolution, Hz in picture)

When using the P2423DE, close the Laptop lid to drive only the P2423DE and the P2422H. Do the black bars then disappear on the P2422H? If yes, then the culprit is the GPU on the older laptops.

drivesmeupawall · Answer

Hi Chris,

The P2422H resolution in setup 1 and 2 are the same, 1920x1080@60Hz. Even if I reduce the resolution of the P2423DE to the same 1920x1080@60Hz as the P2422HE uses, (all 4 monitors set to the same resolution and Hz) the result on the MST/daisy chained P2422H is still the squished display.

I will try your suggestion with the lid closed and see what happens. Thank you.

drivesmeupawall · Answer

Hi Chris,

Just as a followup - one of the things that surprised me is in the Device Manager - the P2422HE is shown as a USB Type C device, whereas the P2423DE is shown as a DisplayPort device. Is it possible the driver for the P2423DE uses an incorrect inf file? I opened the file and it describes the monitor as DisplayPort, not USB Type C like the inf file for the P2422HE does. I'm hoping this is a driver issue more than a hardware issue, the hardware should support the two display resolutions on the P2422HE and P2423DE with MST in use. If I force the resolution of the P2423DE to match the P2422HE that should rule out the GPU's capabilities on the issue?

Thanks

Brad L (retired) · Answer

Glad you were able to get this working!

Monitors

P2423DE USB-C Hub with MST Pass through wrong resolution

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