U4320Q, MST Daisy Chaining?

Question

Hello.

The monitor has just arrived. I was assured by the customer support that MST is supported. Please find part of the conversation below:

03/04/2021 03:38:27PM Agent (): "Yes the it does support the Daisy chaining as the monitor come with a Dp port and cable"
03/04/2021 03:39:09PM Me: "ok, because there is no info about it in tech spec"
03/04/2021 03:40:05PM Agent (): "the dp port and cable will help you connect multiple monitors via daisy chaining"

monitor connected, but I don't see any MST option in the Menu.

jphughan · Accepted Answer

@johnnyrr I've always found that reading the actual User Guide of a product is more reliable than relying on tech specs on a product page that's mostly focused on marketing or relying on answers from sales/support reps who tend not to have a lot of technical training or expertise. Real-world experiences are even better if you can find them because sometimes things work or don't work even when the specs and documentation would suggest otherwise.

Anyhow, the U4320Q does not have a DisplayPort output in order to start a daisy chain to another display. That's not unusual for a 4K 60 Hz display because based on the capabilities of most systems on the market today, a 4K 60 Hz display would consume almost all video bandwidth available on the link anyway, leaving very little for another display of any useful resolution. The only exception would be using the DisplayPort input (not USB-C) from a system that supported DisplayPort HBR3, in which case you could in theory run a second 4K 60 Hz display on that daisy chain, but that's more typically found on desktops, in which case there's generally another physical DisplayPort output anyway. Laptop support for HBR3 is still not widespread, and if you connect a laptop via USB-C where the link is also running USB 3.x, your video bandwidth gets cut in half, at which point you're limited to a single 4K 60 Hz display. So Dell may be trying to avoid dealing with the user confusion, frustration, and returns generated by consumers buying a 4K 60 Hz display with an MST output expecting to set up a daisy chain and then learning that their particular system and/or cabling setup wouldn't allow it. I see topics here on a regular basis where customers bought something expecting it would do something without reading the specs to realize that i wouldn't do that or at least wouldn't do it with their particular system, and I would imagine that product returns and customer frustration have both tangible and less tangible costs to Dell, even when those are the result of the customer's own carelessness.

If on the other hand you were expecting to split up the U4320Q into multiple 1080p tiles rather than a single large 4K canvas and use MST so that you could manage those tiles individually over a single physical link, I don't think the U4320Q supports that either since I recently had a discussion about this with someone else. They ended up running separate cables for each tile they wanted to use. The alternative would be to use Dell Display Manager to carve the display up virtually, but that isn't quite the same in all respects as having the system believe that there are 4 completely separate displays connected.

So unfortunately it seems you got bad information. Equally unfortunately, that's not all that uncommon. I've helped users here who had been told by Dell sales/support reps that a laptop can only run a 4K external display if it has a 4K built-in display, or that a particular laptop was a high-end model and therefore could only run external displays through a full docking station rather than the simple dongle that a user was trying to use unsuccessfully (which turned out to be a BIOS bug). But if it's any consolation at all, depending on your system setup, you might not have been able to run a daisy chain anyway due to bandwidth concerns.

jphughan · Answer

@johnnyrr  One follow-on note.  In theory if you had some other display that had a DisplayPort output to start a daisy chain and that display's output supported DisplayPort HBR3 rather than the much more common HBR2, then you could potentially connect the U4320Q to that display in order to have a daisy chain that way.  (If you only had HBR2 bandwidth levels, you'd likely be limited to 4K 30 Hz on the U4320Q due to upstream bandwidth consumption.)  So in that sense, the agent you chatted with was correct that you could use the included DP cable to connect the U4320Q to a daisy chain.  Although I realize that's very likely not what you had in mind.

johnnyrr · Answer

Thx a lot for your elaborate response. I was kinda tricked, or better saying I tricked myself believing in that response unfortunately
what can I say - DELL owes me MST Hub

it seems that's my only option now, and actually I really wanted to avoid that

My plan was to connect 3 separate monitors. I checked and handling 1x4k and 2xWQHD is really not a problem (definitely bandwidth is not a problem)

I am truly disappointed because the monitor is really great, but it was supposed to work as a hub for all other stuff including monitors, keyboard, mouse, audio.

jphughan · Answer

@johnnyrr Sorry about that experience. 4K 60 Hz + 2x QHD would require a full bandwidth DisplayPort HBR3 interface. But if you're connecting via USB-C in order to carry keyboard, mouse, audio, etc. all over a single cable, then the display would have to configure that link to limit itself to USB 2.0 data speeds in order to keep full video bandwidth available rather than cutting it in half to run USB 3.x. I'm not sure how that particular display behaves in that regard. Some Dell displays have a USB-C Prioritization option to allow you to configure this so that you can for example optimize for more video bandwidth to run a daisy chain, at the cost of USB speed, or run USB 3.x and only enough bandwidth for a single display.

If on the other hand you'd be using the DisplayPort input for pure video and then addressing USB data using a separate, additional USB cable, then an MST hub would get the job done. Cable Matters has a triple display MST hub that supports DisplayPort HBR3 and has 2x DP and 1x HDMI outputs. They have one version that plugs into USB-C on the host side and another that plugs into Mini-DisplayPort (with an adapter for full-size DisplayPort). I'm not sure what PC you're using, but with an MST hub you'll still essentially have cables connected very near the PC rather than along a daisy chain, so if your PC has enough outputs to run your displays directly, the MST hub doesn't buy you very much. But if you have a USB-C output that supports HBR3, then this might be the way to run your desired display setup -- although again USB data would need to be carried over a separate cable since when using an MST hub, the USB-C link will only be carrying video. Good luck!

johnnyrr · Answer

thx again for your input and very valuable comments

Currently I have:

I still haven’t bought a third monitor, but as already mentioned I was planning to put U4320Q central and as a hub. In theory U2719D supports MST, but which version of HBR? this is already dark magic for me:)

My XPS is connected via USB-C (up to my knowledge it is like DP 1.4 regarding bandwidth) and charging directly from the monitor.
I will try to go through the U2719D then to see the result.

Initially I was thinking to buy another U2719D, but as you mentioned bandwidth will not be enough here – right?
Plus question that comes to my mind is: can I still use the 43 as primary monitor, also when 27 is first in a row and switched off?

Additionally, I am also having Thunderbolt3 at my XPS, so if I’d go with another 27’ with Thunderbolt3 support… But that’s gonna be a lot of money I suppose. Maybe it would be just easier to buy a docking station (the thing I wanted to avoid)

jphughan · Answer

@johnnyrr The XPS 15 9500 only supports DisplayPort 1.2/HBR2 because of limitations of the Intel GPU used in the CPUs of that system. And in that system, as in many systems, the Intel GPU has direct control of all outputs, even if you have an NVIDIA GPU. For a while, the specs of that system claimed that the non-Thunderbolt port on the right supported DisplayPort 1.4/HBR3, but that is incorrect. So you'd be stuck for that reason alone.

I'm not sure what HBR level the MST output on the U2719D supports. But if you're connecting via USB-C, you'd have to make sure that the display's USB-C Prioritization was set to High Resolution so that USB speeds are limited to 2.0 anyway.

On an unrelated note, don't rely on the U2719D or the U4320Q as a power source for an XPS 15. If your XPS 15 only includes an Intel GPU, it will be designed for a 90W power source. If it DOES have an NVIDIA GPU, it will be designed for 130W. I don't remember offhand, but I'm pretty sure those displays only supply up to 65W of power over USB-C. They definitely won't support 130W. While the XPS 15 can run from an undersized power source, you'll see slower battery charging and throttled performance in that scenario.

I believe that if you completely power off an upstream display, then the downstream displays will lose their signal. At least that's how it works with my U2717D displays that are in a daisy chain.

Anyhow, since you're dealing with a DP 1.2 system, you can't run 4K 60 Hz + dual QHD through an MST hub. The closest you can come would be a Thunderbolt 3 to Dual DisplayPort adapter. That will tap into two full DP 1.2 interfaces on your system (compared to an MST hub that only taps into one interface), and two DP 1.2 interfaces is enough for dual 4K 60 Hz or 4K 60 Hz + 2x QHD. But that only gets you two displays. I'm not sure it's possible to run a daisy chain from one of the outputs of a TB3 to Dual DP adapter. I've just never tried it.

Yes, a docking station would be the easy solution here, since in addition to running all of your displays AND USB 3.x over a single cable, you'd ALSO be able to get enough power to run your system optimally through that cable, even if your system requires 130W. The dock for that purpose would be the WD19TBS.

jphughan · Answer

@johnnyrr  One last thing I forgot to add.  If you do decide to go with the WD19TBS, be aware that it allocates video bandwidth from the system across its outputs only in certain ways, and those ways depend on the HBR level support of the source system.  With an HBR2 system, half of the total bandwidth is on its 'downstream TB3' port.  So you'll need to connect your 4K 60 Hz display to that.  The other two QHD displays can be connected to other ports.  If you try any other port combination, your displays won't all have access to the amount of bandwidth they require.

samuelbarros · Answer

@johnnyrr​  Hi, have you figured out the issue? Which models did you choose? I intend to do something similar, three monitors, with a large central screen in the landscape position and two smaller ones on the side in the portrait position. However, if the central option is a 42' ~ 43' model, I believe the ideal would be 24' monitors on the sides for visual continuity, so the edges of all screens would almost coincide, similar to this image. https://farm8.staticflickr.com/7435/16382179341_43283b821a_h.jpg

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U4320Q, MST Daisy Chaining?

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