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U2520D, U3818DW, MST daisy chain?

Hello,

I currently own a 27” LG monitor and a U3818DW, but would like to replace the LG with a Dell for the Display Manager and daisy chain. My question is about affects in resolution by running my Lenovo T480 -> TB3/USB-C -> a lower resolution monitor like the U2520D’s 2560X1440 and then DP/mDP -> a higher resolution 3840X1600. Would the primary monitor cap the resolution to its native resolution, or would it pass through as much as possible?

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Community Manager
Community Manager

Re: U2520D, U3818DW, MST daisy chain?

That configuration was never tested by us, so unknown. We never even thought about such a combination. The U3818DW User's Guide doesn't show any multiple monitor configurations. The U2520D User's Guide page 34 has this stipulation =  The maximum number of supported monitors via MST is subjected to the bandwidth of the USB-C source.

 

IF the Lenovo Lenovo T480 TB3/USB-C can provide the necessary bandwidth, then possible. You will need to post on the Lenovo Forum and ask the users there if the Lenovo T480 TB3/USB-C could actually push the U2520D 2560X1440 60Hz plus the U3818DW 4K 3840x1600 60Hz.


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3 Argentium

Re: U2520D, U3818DW, MST daisy chain?

FWIW, I have a Lenovo C940 daisy chained via TBT3 to a U2520D and a U4320Q. Both monitors run at native resolution: 2560x1440 and 3840x2160.

Not exactly the same config as your T480 which is, I think, an 8G Intel w. discrete TBT3 controller where the C940 is 10G w. integrated TBT3, but I don't know if that changes the total bandwidth Chris asked about.

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Re: U2520D, U3818DW, MST daisy chain?

As always, thanks for the assist!

 

In addition to the USB Type-C to C cable, does your Lenovo C940 have its own AC power adapter attached? Or is it getting all power from the U2520D?


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Re: U2520D, U3818DW, MST daisy chain?

It has its own power adapter, which connects to 1 of the 2 TBT3 ports, but it can be powered by the U2520 as well.

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Re: U2520D, U3818DW, MST daisy chain?

@JitsuStew  (I updated this post to remove some information based on findings that I posted in subsequent replies.  Keeping some general information here.)  Your proposed display setup would probably be a borderline case.  I'm not sure whether it would work.  First, be aware that when you connect to a display over USB-C, it might configure the USB-C link to carry both video and USB 3.x data simultaneously, but in that setup you lose half of your available video bandwidth right out of the gate, which of course makes it less likely that you'll have enough bandwidth to run a second display in a daisy chain.  Some displays allow you to configure them to set up the USB-C link to allocate all high speed lanes in the USB-C link to video, in which case you'd get a full DisplayPort 1.2 interface out of the system, although in that situation you only get USB 2.0 speeds through the USB-C cable.  But when working with very high resolution displays, sometimes even that doesn't leave enough bandwidth left over to run a second fairly high resolution display.  This is all why high resolution displays like the U2818DW don't offer DisplayPort outputs to facilitate creating a daisy chain, even though other lower resolution displays in Dell's Ultrasharp line offer them, including models made years before the U3818DW.

@swamped207if you're running a daisy chain of QHD and 4K, then it's very likely that your 4K display is only running at 30 Hz rather than the traditional 60 Hz, but 30 Hz is not an acceptable experience for most users since even the mouse cursor is noticeably laggy at that refresh rate.  The only way you'd be able to run QHD 60 Hz and 4K 60 Hz via daisy chaining would be if your system and all displays in the chain supported DisplayPort HBR3 rather than the much more common HBR2 and the USB-C link to the QHD display was set up for pure video (plus USB 2.0) rather than video plus USB 3.x.  As for Thunderbolt 3, the controller design doesn't change the available bandwidth, and actually Thunderbolt 3 is irrelevant here given that the displays in question here don't support native Thunderbolt anyway.  They're just using DisplayPort Alt Mode, which means you have at most a single DisplayPort HBR2 or HBR3 interface available.  With Thunderbolt, you can either get dual DisplayPort HBR2 interfaces (8 HBR2 lanes) or slightly more than a single DisplayPort HBR3 interface (5 HBR3 lanes) because two full DisplayPort HBR3 interfaces would exceed the 40 Gbps maximum bandwidth of Thunderbolt 3 itself.

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Re: U2520D, U3818DW, MST daisy chain?

As I mentioned, my platform was a C940, which has a 10G ICL CPU: the U4320Q is running at 3840x2160 @ 60Hz linked off the U2520D running at 2560x1440 @ 60Hz. As you point out, an 8G Intel CPU will have lower total bandwidth and to be sure the same monitors daisy changed off an HP x360 w. 8G Kaby Lake - which is closer to the op's T480 - sees the U2520D running at 2560x1440 @ 60Hz and the U4320Q running at 3840x2160 @ 30Hz. Whether that is acceptable or not is subjective, but even the T480 will run both monitors at native resolution.

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Re: U2520D, U3818DW, MST daisy chain?

@swamped207  ah ok, you didn't mention Ice Lake the first time.  Not all Core 10th Gen CPUs use the Ice Lake architecture that comes with the new Gen 11 GPU that supports HBR3 over USB-C.  That's only the CPUs with model names ending in a G followed by a number.  The Core 10th Gen CPUs that end in a U are just a refresh of the previous architecture that's been getting minor refreshes since Broadwell, and they come with the old GPU.

In that case yes 4K 60 Hz + QHD 60 Hz would be possible from your system so long as your USB-C high speed links are all set up for video (plus USB 2.0) rather than split between video and USB 3.x.  And yes, even without using a Thunderbolt-based solution, the T480 could run both displays at native resolution, but 30 Hz is not what most people expect since 60 Hz is pretty much assumed on PC displays unless explicitly otherwise stated.  So if you told someone, "Yes this will all work on your PC at native resolution", and then they asked why their experience wasn't what they were expecting, and you then replied, "I told you it would all work at native resolution, which it does....but I never said anything about running at the 60 Hz refresh rate that's always assumed unless otherwise specified," I think most people would consider your original statement misleading.  Best case there is that yes you were technically correct in what you said, but the person you were trying to help is disappointed/angry, and definitely not feeling helped.  I try to avoid that type of outcome.

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Re: U2520D, U3818DW, MST daisy chain?

@JitsuStew  upon further reading, the U2520D does support DisplayPort 1.4 (HBR3), and it does offer a "USB-C Prioritization" setting to specify whether the USB-C link should maximize video bandwidth and only run USB 2.0 (the High Resolution option) or cut video bandwidth in half to support carrying USB 3.x over USB-C simultaneously (the High Data Speed option).  If you were using a system that supported DisplayPort HBR3 and you set the first display to High Resolution, then you would very likely be golden.  The only potential snag I can see here is that the U3818DW only supports DisplayPort 1.2, i.e. HBR2.  I don't think that would matter in this setup where it would be at the end of the chain and DisplayPort 1.2 is fast enough to handle just that display's resolution, but I'm not 100% sure on that.  It's possible that using an HBR3 and HBR2 display together sets the whole chain down to HBR2.  I doubt it, but again I'm not sure.

However, the issue is that the T480 only supports DisplayPort HBR2, for the simple reason that the vast majority of Intel GPUs on the market even today still only support HBR2.  The only exception are the new "Ice Lake" CPUs.  The only other way to get HBR3 would be to have a laptop that had a discrete GPU that had direct control of the display output you're using, but even among laptops that have discrete GPUs, many of them do not have direct control of the display outputs.

So your only hope would be if QHD + 3840x1600 fits within a single DisplayPort 1.2 interface.  Again, I'm not entirely sure about that.  I know that dual QHD would definitely work and dual 4K 60 Hz definitely wouldn't, but 3840x1600 is a "niche" resolution that sits almost precisely between QHD and 4K in terms of total pixel count, so I don't know its precise bandwidth requirements, and that resolution isn't covered in tables that describe supported display setups for various hardware configurations.  But if it somehow does all fit within a single DisplayPort 1.2 interface, then if you set your U2520D's USB-C Prioritization setting to "High Resolution", you would be good to go.  Otherwise, your only option would be to sacrifice display refresh rate, but that typically isn't an acceptable experience.

If you decide to test this, please do report back!

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Re: U2520D, U3818DW, MST daisy chain?

@JitsuStew  sorry for the triple reply here, but I may have more hopeful news for you.  I noticed that the manual of a Dell dock includes some bandwidth figures that I was able to work with.  According to the table:

- QHD requires 5.6 Gbps
- 4K 60 Hz requires 12.5 Gbps
- DisplayPort 1.2/HBR2 offers 4.3 Gbps of effective bandwidth per lane, i.e. after subtracting bandwidth lost to overhead.  A USB-C link dedicated to carrying video would provide 4 lanes, or 17.2 Gbps of usable bandwidth overall.

Doing the math on the QHD and 4K 60 Hz bandwidth requirements above indicates that you can push ~658-663K pixels per Gb of DisplayPort bandwidth.  The discrepancy between the two might come down to different requirements for the "blanking interval".  Anyhow, even if we're conservative here and assume that you can only get 658K pixels per Gb of bandwidth, a 3840x1600 display is 6,144,000 pixels, which means its bandwidth requirement would be about 9.3 Gbps.  That plus QHD's 5.6 Gbps works out to 14.9 Gbps, which is below the 17.2 Gbps that's available, with a decent margin.

So if the numbers I'm working with are right, it suggests that the daisy chain you're considering would work even from an HBR2 system, so long as you set the USB-C Prioritization setting to High Resolution.  But again, I'm just working with numbers here, so I can't be totally certain.  But it would certainly be worth trying, and again please do report back if you do this.  Absolute worst case, a Thunderbolt dock or Thunderbolt to Dual DisplayPort adapter would definitely get both displays working off your T480's USB-C/TB3 port, although the former of course adds cost and the latter would mean you'd lose power and USB data over that port.

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