Question Getting Low Resolution on Second Monitor (1440p) in MST Daisy Chain ?

[coastcoaster]

Hello, I am hoping for some assistance with low resolution on a MST-daisy-chained dual monitor setup.

Laptop: Dell Latitude 7210 2-in-1
Monitor 1: Dell P2422HE USB-C hub monitor max resolution 1920x1080
Monitor 2: ViewSonic VX2758-2KP-MHD max resolution 2560 x 1440

I have these connected as follows:

From Latitude USB 3.2 Gen 1 Type C port to P2422HE USB-C 3.1 Gen 1 upstream/DisplayPort 1.2 port. This connection is made with a USB-C cable marked "USB 3.1"

From the P2422HE's DisplayPort output (MST) port to the ViewSonic VX2758 DisplayPort 1.2 port via a DisplayPort cable.

In this setup, Windows 11 gives a maximum monitor resolution option of 1920x1080 for both the Dell P2422HE and the ViewSonic monitors. But I want the ViewSonic to display in its native resolution of 2560x1440 (while running the Dell P2422HE at its native 1920x1080). When in Windows 11 settings I disconnect the Dell P2422HE, Windows opens up the option of setting the ViewSonic resolution to its native 2560x1440. Of course, though, I just have a black screen on the Dell P2422HE. I have also tried my spouse's slightly newer laptop with USB4 ports, and it has the same result: the ViewSonic limited to 1920x1080 when running both monitors at once. This suggests that the limitation is not the USB 3.2 output port on my Dell Latitude.

How can I best determine what the limiting factor is? So far, I suspect (1) the USB 3.1 cable from the laptop to the Dell P2422HE; or (2) the fact that the Dell P2422HE hub is limited to USB3.1 with DisplayPort 1.2.

I have learned what I can about the USB 3.1 and 3.2 protocols along with the DisplayPort 1.2 protocol. And it seems this relatively modest dual monitor setup should be achievable with this hardware. Do I really need USB 3.2 or DisplayPort 1.4 (or both) in the hub monitor to achieve this?

Other options are (3) the Intel UHD Graphics 620 video card in the Dell Latitude, but from what research I've done, it seems doubtful this is the limitation.

EDIT: I tested using a Dell WD19 USB-C dock, plugging the WD19 dock's USB-C cable into the Latitude 7210, and then connecting from the WD19 dock to the P2422HE via HDMI and to the ViewSonic via DisplayPort and it is running as I would like (ViewSonic at 2560x1440 and Dell P2422HE at 1920x1080). My understanding is that this dock is a USB 3.1 dock with DisplayPort 1.4 protocol. But when I purchased the Dell P2422HE, I had hoped its internal hub hardware would allow me to do away with the separate, bulky WD19 dock, so still hoping I can make this work through the existing monitor setup.

Before I go and buy more cables or hardware to test, I thought I would query the considerable knowledge on this forum. Thank you for any suggestions!

 

[helpstar]

check the monitor where the dp out is located. can you change a setting inside this monitor from dp1.1 to 1.2?

 

[Glenwing]

USB-C has 4 high-speed lanes. In a normal USB-C to DisplayPort adapter, all 4 lanes are used for DisplayPort, just like a native DisplayPort connection. In a USB-C hub or docking station that also has SuperSpeed USB ports, gigabit ethernet, and/or other functionality, 2 lanes are dedicated to USB/other functions, and 2 lanes are given to DisplayPort. So you only have half the capability of a normal DisplayPort connection, which will lead to limited resolutions/refresh rates. A 4-lane HBR2 (DP 1.2) stream supports four 1080p 60 Hz monitors, a 2-lane stream will be half that (two 1080p max). The other docking station may have some other configuration that allows more lanes to be allocated, or HDMI gets its own lane or something. Can only guess.

EDIT: I see you are using the integrated hub/MST capability built into the monitor, not an external docking station, I misunderstood. I'll have to think about it a bit more.

 

[coastcoaster]

So you only have half the capability of a normal DisplayPort connection, which will lead to limited resolutions/refresh rates. A 4-lane HBR2 (DP 1.2) stream supports four 1080p 60 Hz monitors, a 2-lane stream will be half that (two 1080p max). The other docking station may have some other configuration that allows more lanes to be allocated, or HDMI gets its own lane or something. Can only guess.

I believe this is the culprit. I did some more reviewing of the manual of the WD19 dock. At pages 17-18, it says that a 2-lane HBR2 (DP 1.2) can support maximum of 2x 1920x1080 displays. I believe the internal hub/MST capability in the P2422HE is just two lanes of HBR2, and therefore that is the maximum.

EDIT: I see you are using the integrated hub/MST capability built into the monitor, not an external docking station, I misunderstood. I'll have to think about it a bit more.

That's right. When I used my standalone WD19 dock rather than the internal MST/hub capability built into the P2422HE, I was able to get 2560x1440 on the ViewSonic and 1920x1080 on the Dell P2422HE. I believe this is because the WD19 dock is HBR3/DP 1.4. While it still only offers two lanes of HBR3, they have higher bandwidth than the internal MST/hub/dock of the P2422 HE (which is only HBR2).

As a side note, I have read up on various Dell models, that Dell offers a "USB-C Prioritization" feature in the On-Screen Display of certain monitors which allows the user to prioritize higher resolution over higher data speeds when using a single USB-C connection for data, video, and power. Unfortunately, Dell did not include this feature on the P2422HE, only on its higher-range models, although it does appear to be offered on the more recent P2723DE. Alas.

 

[coastcoaster]

check the monitor where the dp out is located. can you change a setting inside this monitor from dp1.1 to 1.2?

I did not find such a setting, and believe it already to be set to DP1.2 as its maximum. As I noted in another post, I think the culprit is the limitation to two lanes of DP1.2/HBR2 in the monitor MST/hub and that I would need DP1.4/HBR3 in order to accomplish this dual-monitor resolution setup using just USB-C.

 

[helpstar]

ok, just read the manual. in mst mode it states:
Maximum external monitor resolution supported is 1920 x 1080 60Hz only
page 16
https://dl.dell.com/manuals/all-pro...s/dell-p2422he-monitor_user's-guide_en-us.pdf

 

[coastcoaster]

ok, just read the manual. in mst mode it states:
Maximum external monitor resolution supported is 1920 x 1080 60Hz only
page 16
https://dl.dell.com/manuals/all-products/esuprt_electronics_accessories/esuprt_electronics_accessories_monitor/esuprt_monitor_p_series/dell-p2422he-monitor_user's-guide_en-us.pdf

Thanks for tracking that down. It is just a hardware limitation in the P2422HE monitor's DisplayPort 1.2 then.

For now in case it's useful to a future reader of this thread, the WD19 dock, with its DisplayPort 1.4 protocol, successfully provides full native resolution on both displays (2560x1440 and 1920x1080).