P2422HE, MST second Philips 276B9 resolution

Question

Hi all

I just received a P2422HE to use with my Latitude 5320 (Thunderbolt 4, Intel Iris Xe).

I'd like to use it as a hub and daisy chain my other monitor to it, which is a Phillips 276B9 (QHD 2560x1440 @60hz or 75Hz, DP1.2).

The problem is that this secondary display is limited to a FHD resolution (1920x1080) in Windows 10, no matter what I try.

I disabled the integrated display because I don't use it in docked mode but it doesn't help.

My setupe :

Latitude 5320 TB4 port -> USB-C to USB-C cable (included with my P2422HE display) -> P2422HE USB-C port

P2422HE DP out port -> DP to DP cable (also included with my P2422HE display) -> Philips 276B9 DP IN port

MST is activated in the P2422HE OSD, I double checked (and daisy chaining works, it's just that the resolution is not good).

Is this a hardware limitation of one of the component in the chain ? To my understanding, this should work.

My laptop's TB4 port supports 3 external 4K monitors (I'm far from this bandwidth), the graphic card (Iris XE) supports 3 external QHD monitors (also far from this bandwidth), I use the included USB-C to USB-C cable .... So to me everything should be OK.

Any idea on why this doesn't work the way I expect it to, and what I could try ?

BTW, Latitude is fully updated, including bios, and I installed the P2422He driver.

Thanks for helping !

jphughan · Accepted Answer

@thewild Using Thunderbolt requires a Thunderbolt cable as well as support for Thunderbolt within the devices on each end of the connection. There are displays that support actual Thunderbolt, like Dell's U4021QW and some of LG's displays, but they're relatively rare and typically expensive. I don't think I've seen one with a resolution lower than 4K.

Thunderbolt and Alt Mode are two different things. Since this question comes up a lot, I wrote a whole explainer post about the various operating modes of USB-C and Thunderbolt with specific focus on their impact on possible display setups over here if you feel like some reading. Your system even when using regular USB-C can support DisplayPort 1.4 Alt Mode, but in addition to requiring a USB-C cable that carries that, you can only run that mode if the device on the other end supports it as well. If your display(s) only support DisplayPort 1.2, then that's what your system will use -- just like you can have a USB port that supports USB 3.x, but if you connect a device that only supports USB 2.0, that's all you'll get.

I can't find where you're reading the P2422HE technical specs you quoted even after Googling for the specific phrase you quoted, but a dual QHD daisy chain using only DP 1.2 inputs and MST outputs is possible when you have a full bandwidth DP 1.2 interface available to work with. That can be achieved either by a) connecting to the first display using its regular DP input, rather than its USB-C input, or b) using a display that supports being configured to run only USB 2.0 data speeds over the USB-C link rather than USB 3.x, since running USB 3.x is what cuts video bandwidth in half. But you're not using the first display's DP input, and your display doesn't seem to support that particular USB-C mode, which means you only have a half bandwidth DP 1.2 interface to work with. (This incidentally is laid out nicely in that explainer post I linked earlier.) The Dell displays that offer this flexibility call the option "USB-C Prioritization". But if you were for example to connect to your first display using a USB-C to DP dongle like this one, you would get a full bandwidth link to the first display, and you WOULD be able to run that second QHD display just fine. You'd even be able to keep the USB-C cable connected to this dongle to draw power from that display. All you'd lose would be USB data. But if you want that, then you'd need another cable, and if you're going to be dealing with a second cable anyway, then you may as well just connect the secondary QHD display directly to the system instead.

jphughan · Accepted Answer

@thewild Yes you're out of luck with your current equipment, but yes that adapter would get the job done. The only caveat is that the P2422HE's only "upstream" USB data connector is its USB-C input, and that is also the only port that can supply power. So if you were to use that hub, you could either connect the display's USB-C input to the USB-C power passthrough port to keep using it as a power source through the hub, or connect it to one of the USB ports (using a USB-C to USB-A 3.0 cable) to be able to use the USB ports built into the display. I'm not aware of a "breakout cable" that would be able to achieve that. Dell's higher end U Series displays have dual USB upstream interfaces partly for this reason (and also to allow toggling both video and USB device connectivity between two source systems), but the P Series displays are essentially the same display panels and stands but without the same level of connectivity options.

But if you're willing to either give up using the P2422HE's built-in USB ports or connect a separate USB-C power source to the hub's power passthrough port, you should be good to go with that solution. There are also full docking stations like the WD19S that come with their own power source, but they of course cost more.

jphughan · Accepted Answer

@thewild  In that case, I agree that the product you found is a good fit, because yes it keeps maximum video bandwidth by restricting USB data to 2.0, which sounds like it gives you what you want without imposing any limitations that would be a concern for you.  Good luck!

jphughan · Answer

@thewild  I just realized that you specifically mentioned that your Philips display is DP 1.2.  Somehow I forgot about that detail in the midst of my earlier reply. That to me suggests that the Philips display is causing the entire daisy chain to run in DP 1.2/HBR2 mode, and thus not to have enough bandwidth to run the setup you want to run.

jphughan · Answer

@thewild There are definitely some asterisks on the capabilities you mentioned. Triple 4K support over a single TB4 port would require using actual Thunderbolt — not just USB-C as you’re using here— AND it would require the use of DisplayPort DSC, which is not widely supported yet. Without DSC, a single TB4 port would only be able to run dual 4K 60 Hz displays, but again that would require using actual Thunderbolt, not regular USB-C.

In terms of display quantity, I’m not sure where you found that the Xe supports triple QHD. The Xe can run four displays (all four can be external if the built-in display is disabled) and can support up to 8K displays when installed in a system that has the necessary outputs to run that. So that’s actually better than you found.

Based on your problem description, it sounds like at least one of your displays might only support the older DisplayPort 1.2/HBR2 standard, not DisplayPort 1.4/HBR3. And since the U2422HE uses USB-C rather than Thunderbolt, the MAX bandwidth you can have is a single DisplayPort interface of bandwidth. However, since the U2422HE configures the USB-C link to carry both video and USB 3.x, video bandwidth gets cut in half. And if the daisy chain is running at DP 1.2/HBR2, a half-bandwidth HBR2 interface is only enough for dual 1080p 60 Hz. If everything in your chain supported DP 1.4/HBR3, you’d have enough bandwidth for dual QHD 60 Hz, but it sounds like that isn’t available.

In terms of where the culprit is, the U2422HE specs indicate that its USB-C input supports DP 1.4. So it could be the MST output port, where the specs don’t say anything about DP revision, or it could be the DP input on your other display, whose specs I didn’t check. On Dell’s higher resolution USB-C displays, they have an option to limit USB data speeds to USB 2.0 in order to double available video bandwidth specifically to facilitate daisy chain setups, but it looks like the U2422HE doesn’t offer this. It seems that Dell’s displays that have daisy chain outputs only ever account for running a second identical display, not a secondary display with higher bandwidth requirements.

In terms of possible fixes, if you’d be willing to give up USB data connectivity to the display, you could get a USB-C to DP dongle with a USB-C power passthrough port so that you can get max video bandwidth out of that USB-C port to the U2422HE’s DP input, and still have power. But then you have no USB data unless you connect a separate cable. But if you were willing to do that, you could just connect the second display via a separate cable as well. But if you want power, USB data, and both displays running over a single cable, then you’d either have to get a different Dell display that would allow you to run the desired daisy chain (like the U2722DE, which is a 27” QHD display) or else get a docking station like the Dell WD19S.

thewild · Answer

Wow, thats a comprehensive answer, thanks a lot !!
OK, so first my display is P2422HE, not U2422HE, and this display is definitively DP 1.2, so at least this part of the chain is limitting the bandwidth.

Now, trying to dig a bit further into this...
First, you say I'd need actual Thunderbolt to get full bandwidth, not USB-C. Do you mean the cable ? Because what I meant is that since my laptop is TB4, it implies that when using it with a USB cable I have Alt mode DP 1.4. Am I right about that ?

But that's just out of curiosity, because the real point is that, as I said, my display is the "P" version, and contrary to the "U" version it only has DP 1.2, and the USB upstream port also only has Alt Mode with DP 1.2.

Now... I understand my chain is limited to DP 1.2, ... but, reading my P2422HE technical specs, it says "Connect to productivity: Easily daisy chain up to two QHD or FHD monitors with DisplayPort 1.2 output."

So they basically say that DP1.2 is fine for two QHD monitors, right ? But to the contrary, you say taht DP1.2 (so with HBR2) is not enough to run my setup, even though to me this setup need lower bandwidth than two QHD monitors (one QHD and one FHD).

So, what's the check really ? If sharing USB bandwidth is limitting the setup, why do they advertise two QHD displays ?

Thanks again for your very informative help !

thewild · Answer

Thanks a lot, this all makes sense now.

My mistake was that the documentation mentionned two QHD displays ... but using the DP 1.2 input indeed !
So as I understand, I'm a bit out of luck with my config here.
I'll have to purchase another hub if I want to have a "single plug" setup. I guess this one has the specs I'm looking for, right ? It has DP1.4, 100w charging and multiple USB ports, so that would be good I guess.

jphughan · Answer

@thewild  Happy to help!  I'm not sure what other Cable Matters product you're looking at, but be careful there.  Some adapters might only support USB 2.0 data speeds and therefore only offer USB 2.0 ports and/or bottlenecked Gigabit Ethernet, or they might support USB 3.x but only DP 1.2 source signals, in which case you'd end up in exactly the same situation you're facing right now.  But good luck!

thewild · Answer

Actually, I care about having a 'single plug' setup, but I really don't mind giving up on the P2422HE's USB ports since I'd have them on the hub. It's a shame that I need another HUB since this screen was really really close to checking all the boxes, but at least I found a relatively cheap solution. Actually there's even a Cable Matters HUB that's cheaper and would also do the job. I already have a W19s at work and it does work OK (some flickering on HDMI out the first minutes after power on), but it is very expensive and also really big. I think I'll try this option (ie PC to Hub, and hub connected to both displays via DP, plus a connection between the hub and the P2422HE to get power through USBC). I'll save on a separate USB-C power adapter this way. Thanks again for all your help !

thewild · Answer

Hi again @jphughan

I was actually thinking about this one : https://www.amazon.com/Cable-Matters-Multiport-DisplayPort-Ethernet/dp/B0746NKVBN

It's only USB 2.0 and 480Mbps Ethernet indeed, but to me this should guarantee that DP is not bottlenecked, right ?

It's cheaper, I don't need Ethernet, and USB is only for keyboard and mouse in my case. And then I still have the USB 3 ports on the laptop for other uses.

This one should work too, don't you think ?

TBH, the real problem is that the other Hub is out of stock on the french Amazon store, so I found this compromise.

Cheers

Monitors

P2422HE, MST second Philips 276B9 resolution

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