Demystifying Dell WD19TB supported display resolutions

XPS 9550 should be a much older device with a Skylake processor.

I cannot speak to whether it has 2 DP connections to offer via TB or not, I just read Dells documentation which states it has 2 Lanes (https://www.dell.com/support/kbdoc/en-us/000060905/thunderbolt-3-40gbps-data-transfer-rate)

And according to https://thunderbolttechnology.net/sites/default/files/18-241_ThunderboltController_Brief_HI.pdf

there are 2 Alpine-Ridge controllers that have 4 PCIe Lanes and 2 DP-Ports and 1 controller that only has PCIe 2x and 1 DP-Port.

While it is totally possible, to connect an 4x controller to only 2 lanes, it makes it more probable that they are only using the smaller controller. This has been a good indication with other manufacturers.

@Boo3 you can look up the specific TB controller you have in device manager when something is connected to the TB port.

Another reason could be, that the Intel Onboard Graphics does not have enough outputs and some ports are shared and can never work together, just like MFDP and HDMI on the Dock. This is actually the reason the T14 is limited, because the Comet Lake-U platform only provides 3 display outputs. So it could be possible, that the 2nd DP output only becomes available to the TB controller when the internal display is turned off...

But as the Skylake-H family of processors should have up to 4 display outputs available, there seems to be no reason for the integrated display to share a port with the TB controller...

@Ray519  Yes, the 9550 was Dell’s first XPS 15 to support Thunderbolt 3, and it had dual GPU interfaces wired, which is why if you do some research, you will find multiple real-world confirmations of users running dual 4K 60 Hz displays from that system and newer XPS 15 models using docks like the TB16, WD19TBS, and even TB3 to Dual DP adapters. There’s no need to speculate on that. There’s years of actual experience on those systems at this point.

The fact that Dell may have implemented a TB3 chipset that supports dual GPU interfaces and PCIe x4, does not mean that Dell actually took advantage of both of those capabilities. Again, all Dell systems with TB3 have had dual GPU interfaces wired, even if they only used PCIe x2, regardless of whether the controller would have supported an x4 setup.

You do not need to disable the built-in display to use the second DP interface on the Thunderbolt controller. Here again, there is plenty of real-world confirmation of people running dual 4K 60 Hz displays via TB3 while keeping the built-in display active.

That's well written masterpiece.  Got a question, I have T480 which is TB3/DP1.2/HBR2 I believe, and it is possible to use with BenQ PD2700U MST to achieve dual 4K@60?  I'm thinking T480 - CtoDP - PD2700U - DP - L28u-30, can it run dual 4K@60?

The other typeC is reserved for charging

Thanks 

@BoringBao  Thanks for the kind words! You’re correct that the T480 is DP 1.2/HBR2. The only way to achieve dual 4K 60 Hz in a daisy chain from that type of system is with a Thunderbolt daisy chain, which can tap into both GPU interfaces, not a DisplayPort MST daisy chain that only taps into one — but that requires actual Thunderbolt displays. The other option that would work if you don’t want a full dock would be a Thunderbolt 3 to Dual DisplayPort adapter — not to be confused with a USB-C MST hub. They look similar, but the former will tap into twice as much bandwidth as the latter. StarTech, Sonnet, Plugable, and Sabrent all make TB3 to Dual DP adapters, and all have a solid reputation for quality products.

Thanks @jphughan 

One more question, what if I'd like to run dual 4K@60 + Dell P2720DC (1440p@60)?

I've learnt that on T480, one of the USBC is shared with HDMI so I can't use both of them - would it work if I plugin the Dell P2720DC on the USBC (for charging as well); then a sarbrent TB3 to dual DP adaptor connecting to the two 4K@60?  (L28u-30 and PD2700U)?

And the laptop display will be off as i put it on a stand all time 

Thanks a lot 

@BoringBao  Happy to help.  I don't have a T480 to test with, so I can't say for certain, but as long as the USB-C and TB3 ports aren't sharing GPU interfaces, then I would expect your proposed setup to work.

@abescully  Glad you found my post useful!  In terms of your setup, the MacBook Pro may be a special case.  Dell has a KB article specifically for Macs and the WD19TB(S) over here.  As you can see, it advises that in any dual display setup, one of the displays should be connected to the downstream TB3 port.  This is related to the fact that macOS still does not support DisplayPort MST, and therefore it cannot run multiple independent displays that are driven from a common GPU source interface.  At least with a DP 1.2/HBR2 system, the way to get each display on separate source GPU interfaces is to put one of them on the downstream TB3 port.  But that then begs the question of what happens when you want to run a dual 4K 60 Hz setup from a Mac that a) must have one of the displays on downstream TB3 because it can't run multiple displays from a single source GPU interface, even if enough bandwidth is available, and b) supports DisplayPort 1.4/HBR3, which should cause the dock not to allocate enough bandwidth to run a second 4K 60 Hz display via downstream TB3.  It would seem that you'd be stuck.  I don't have the equipment to test this, but my guess is that in the specific case of Mac hosts, the WD19TB(S) will limit itself to DP 1.2/HBR2 mode, forcing the Mac to run that way, specifically to make it possible to achieve 4K 60 Hz in such a way that the Mac can run two displays and the dock can allocate bandwidth as necessary for that setup.  But I can't know that for sure.

As for being able to run 4K 60 Hz from the downstream TB3 port when that's the only port active, I noted that in my original post, saying that the downstream TB3 port only gets a single HBR3 lane unless it is the only active output.  Note that in the "Display resolution table" in the the WD19TB(S) User Guide, specifically the table for Thunderbolt systems and the row for HBR3, it specifies that if you're only running a single display, then you can run up to 8K 30 Hz from the downstream TB3 port, which requires twice as much bandwidth as 4K 60 Hz.  So being able to run 4K 60 Hz from the downstream TB3 port when using an HBR3 system is expected, as long as that's the only connected display.  As for how you were able to achieve dual 4K 60 Hz by starting with only one display connected via downstream TB3 and then connecting the DisplayPort display later, I have no idea how to account for that.

First, OP, I'd like to thank you for this post. I recently bought a Dell WD19TBS for work. The information in this thread was key to me figuring out how to get my two 4k@60hz monitors up and running with HBR3 computers on a WD19TBS without pulling all my hair out. My past experience with Dell docks was positive. I ran two 4k@60hz monitors with XPS13 9350 and XPS13 9370 laptops running Fedora with Dell TB16 docks for years.

My two work computers are a Dell XPS13 9310 running Fedora 34 and an Apple MacBook Pro 16 manufactured early 2021 running MacOS 11.4, both of which are HBR3 computers. Connected to the dock, I have a 4k@60hz monitor connected to DisplayPort and a 4k@60hz monitor plugged in to the downstream Thunderbolt port.

If both monitors are connected to the dock when I connect either laptop, the DisplayPort monitor works fine at 4k@60hz but the Thunderbolt monitor will only run 4k@30hz. Both monitors flicker a bit and seem to have other minor trouble when this happens.

According to this thread (thanks again OP), it's happening because not enough DisplayPort lanes get allocated to the Thunderbolt port. Four DisplayPort lanes (32.4 Gbit/s, twice what's needed for 4k@60hz) get allocated to the MST hub, so only one lane (8.1 Gbit/s, less than what's needed for 4k@60hz) is left over for the Thunderbolt port.

With that info, I gave the new WD19TBS another try. I unplugged the DisplayPort monitor from the dock. I then can connect either computer and the Thunderbolt monitor runs fine at 4k@60hz. If I then reconnect the DisplayPort monitor to the dock, it also works just fine at 4k@60hz. So the Dell dock is perfectly capable of using less than 4 DisplayPort lanes for the MST hub when a Thunderbolt monitor is connected, but you have to jump through hoops to get the lanes allocated in a sane way if you have an HBR3 computer.

Woohoo! Dual 4k@60hz with a Thunderbolt monitor and a DisplayPort monitor! Just like the documentation at https://www.dell.com/support/kbdoc/en-us/000124312/dell-thunderbolt-dock-wd19tb-and-apple-usb-c-hosts says should work. What the documentation doesn't mention is the games you have to play if you have an HBR3 computer.

In spite of getting it to work by doing the monitor-connection-order dance with the dock, I went back to the "Dell Docking Station WD19TBS Thunderbolt User Guide" for a closer read there. Looking at "Table 4. WD19TBS for Thunderbolt systems", row "HBR3", column "Dual Display", you'll see that dual 4k@60hz is not listed for any configuration that includes a monitor connected to "USB Type-C TB".

I feel like Dell's firmware could be a tiny bit smarter about DisplayPort lane allocation when connected to an HBR3 computer over Thunderbolt. In this configuration, when one monitor is plugged into the MST hub and one into the Thunderbolt port, allocating 2 lanes to the Thunderbolt port instead of just 1 seems like it would fix my setup and not break any other supported configuration in the table. Allocating 3 lanes to Thunderbolt might even allow for 5K@60hz Thunderbolt + 4k@60hz Displayport, again, without breaking any other supported configuration. Anyway, I'm a firmware engineer and know things aren't always that easy.

I hope my experience saves some folks from the frustration I had with my monitor/computer setup. It is *so* close to just working! ugh.

The really wasteful and expensive solution (and only) I know of:

Use Dell WD19TB to plug into / power XPS.

Daisy-Chain Thunderbolt 4 Hub (or technically any other Thunderbolt-Adapter with 2 Displayports and without MST) of the TB-Out of the Dell Dock and use it to connect the displays. Since Devices like the Anker Thunderbolt 4 Hub / CalDigit Element Hub have no MST functionality built-in, there is no such problem of mapping the connections differently. On the other hand, you can only connect 2 Displays. Any more and you have to add MST-Hubs again.

And yes it works, the WD19TB does not reserve any Displayport connections (at least not with my HBR2 XPS 9500). The whole problem is caused by the MST-Hub inside the dock, that seems to establish an HBR3 connection, even when not needed by its outputs and it seems Dell firmware initializes its connections first (if not forced by plugging in monitors after the dock connection has already been made). In general Thunderbolt controllers seem to hand out / reserve bandwidth simply first-come, first-serve. I just have not yet figured out yet, how that order is determined if you plugin in a whole bunch of monitors simultaneously, whether it is purely by port or also by some property of the display, like response time.

This is the exact scenario we have. Unfortunately it means that if you have flex-desks, it's no longer possible with a WD19TB and newer HBR3 devices. If someone with a HBR3 device connects, they have to swap the cables to get 2x4k60 and viceversa. Since we also have XPS 15's, we are stuck with Dell docks since other docks don't supply 130W.

Basically, unless Dell releases something like a WD22TB, we're stuck in this frustrating situation. If anyone knows another solution, please let me know.

I regret ever buying any 4k "monitors".   IMO they need to be at least 50 inches to make most fonts visible and useful for most people at native resolution and native zoom on Windows or MacOS.  And a 50 inch "monitor" would have to be curved like crazy.  These don't exist.  I've become a big fan of WQHD monitors.  The ones that are 1440 high and all kinds of crazy wide.  You get one or two 34 or 37 inch of those with a bit of curve, they are CHEAP, and you can run totally native resolution and the experience is comfortable, immersive, and easy on the eyes.

@jay613  4K resolution reaches a pixel density that becomes usable at standard scaling around 43”, which yields 102 ppi. For reference, a 24” 1080p display is 94 ppi and a 27” QHD display is 108 ppi. And there are 43” 4K displays that are not TVs. Dell even makes one, namely the U4320Q. It even has a mode that allows it to be used as four 1080p tiles by up to four separate source devices. But it’s certainly a niche product.

But 4K at 50” would result in 88 ppi. That’s lower than the reference standard of Windows (96) and macOS (100), but still better than 27” 1080p (81), although I consider a 27” 1080p display to be unacceptably grainy especially on text. 

@jphugam it's a real pleasure to "meet" someone so knowledgeable and sharing.  I LOVE your articles.

I do not have a 50" display, that was a guess, so I'll take your word for it at 43 is good enough.  I bought a 28 inch 4K display because it was on sale for about $200 ... and it was a big mistake.  I still believe though that at 43 inches, a monitor to be used for work needs to be deeply curved and I have seen no such thing in 4K.  I'd much rather have several smaller curved QHDs or WHDs.   Part of my reasoning is price.

BTW what brought me here is your amazing articles and notes on the Dell docking stations.  My employer just rolled out a new office equipped with these, and various sorts of monitors, and I have a Macbook.  Trying to get it working with two external monitors.   I haven't QUITE tried everything you've recommended however.   Notably --- to use the Thunderbolt out port for the second external monitor rather than the second DP port, two things have to happen on the same day: I have to bring a USB-C port to work that is not a piece of junk (I keep one in my bag that's only suitable for charging) and I have to get a desk with monitors that have USB-C ports.   I'll get there.    Colleagues told me to use DisplaLink .. which I now understand to be wrong.  

Thanks again

@jay613 Thanks for the kind words!  I actually owned a 40" Philips 4K display for a day, before I decided that it was just too overwhelming for me.  Apparently it's fantastic for people who actually have a use for all of that real estate, as well as for gamers due to the immersion, but I got it mostly out of tech geekery and I quickly realized I had bitten off more than I really wanted to chew.  That said, its lack of curvature wasn't an issue for me.  In a 16:9 aspect ratio, the width of a 40" display is right on the edge, but I think it's still doable -- although I can certainly see a case for curvature.  The height though can be a bit of a challenge.  I had bought an Ergotron desk mount arm to allow me to have the display panel rest on the desk, and even then the top of the display still felt a bit high up.  Maybe if I had a deeper desk that allowed me to sit farther back it would have been better, but it just wasn't the right setup for me.  I'm currently using a pair of 27" QHD displays.  I get more real estate than typical ultra-wides, and I prefer the flexibility of two independent QHD displays over something like Dell's U4919DW that is essentially two 27" QHD displays fused together.  I have one display directly facing me and the second display angled toward me a bit.  I can't achieve that setup with a single "super-ultrawide" display, and since I tend to multitask rather than working on a single application that could use all of that real estate as a video editor undoubtedly could, I find that having two separate displays helps me physically compartmentalize the way I'm already mentally compartmentalizing.  And it doesn't hurt that two 27" QHD displays is about half as expensive as a 49" dual QHD display.

But I absolutely hate 27-28" 4K displays.  Their pixel density of 163 ppi places them in a nasty section of the density spectrum, since that's high enough that you need to use scaling but not high enough to give you that incredibly sharp image.  And that setup is an especially bad choice for Mac users because macOS is designed for standard displays (100-110 ppi) and Retina displays (200-220 ppi).  So 163 falls right between those two design targets, which makes it an equally bad fit with both.  The only way to make it livable on a Mac is to use an intermediate Scaled mode, but that causes macOS to render in Retina mode and then downscale the rendered image afterward, which doesn't look great.

Regarding the docks, if you're using a Mac, then placing one display on the downstream TB3 port at the edge of the dock for one display is crucial.  If the displays currently connected to the dock are using the dock's two DisplayPort outputs, then a simpler solution in your case would be to bring a USB-C to DisplayPort dongle, in which case you can just plug that into the downstream TB3 port and temporarily move the existing DP cable over to the dongle, and then put it back when you're done.  Much less of a hassle than dealing with an entire alternative cable.

DisplayLink is a pain.  Since you like my articles, I wrote one about its various drawbacks in the post marked as the answer in this thread.  That said, if your MacBook is an M1-based system, then that's your only option because the M1 only supports two total displays, which on MacBooks (as opposed to the Mac Mini) means only one external displays, because the built-in display always counts even when you're not using it.  Any additional displays would need to be driven by DisplayLink.  The M1 Pro supports three total displays, and the M1 Max supports five.  If you have an Intel-based MacBook, then it would support three or five external displays, depending on the exact size and generation, unless it's VERY old.