A while ago I wrote a “Demystifying USB-C and Thunderbolt” thread here, which addressed how USB-C and Thunderbolt worked, including in docking station scenarios, and how that related to supported display setups. Since then, Dell has released the WD19 dock family that includes support for the newer HBR3 standard, and the WD19TB dock in particular has some limitations on maximum resolutions with various display output combinations that might seem strange. So I decided to write this thread for anyone who was simply curious from a technical perspective about why those exist.
First of all, it’s important to note that although the WD19 dock family can take advantage of HBR3 support (DisplayPort 1.3 or 1.4) if the system has it available through its USB-C/TB3 port, the vast majority of systems on the market here in May 2019 still only support HBR2, for the simple reason that Intel GPUs today still only support HBR2 (DisplayPort 1.2). Even among systems that also have discrete NVIDIA/AMD GPUs, the USB-C/TB3 port is often still physically wired to the Intel GPU and is therefore subject to its limitations -- so at the moment, the only systems that have HBR3 support on USB-C/TB3 are those that have those ports driven directly by a discrete GPU. However, Intel’s upcoming “Ice Lake” family of CPUs will incorporate a new GPU that supports DisplayPort 1.4 and therefore HBR3. Those CPUs are slated to begin arriving in late 2019, as of this writing.
The main focus of this thread, however, is that the WD19TB has altered how it allocates display bandwidth to its various outputs compared to the TB16 that it replaces. That’s why if you look at the manual’s Display Resolution Table for a Thunderbolt system, you’ll find some limitations that might seem unintuitive or arbitrary. For example, when using an HBR2 system, running dual 4K 60 Hz displays requires that one of them be connected to the dock’s “downstream” Thunderbolt port, a limitation that didn’t exist on the older TB16 dock. But on an HBR3 system, that same Thunderbolt 3 port is limited to just QHD resolution whenever any other output is also in use. So what’s going on here?
There are two underlying causes for these limitations. The simple one is that the WD19 simply doesn’t support using its HDMI port and USB-C port for video output at the same time (although using the latter for a data device while using HDMI for video seems to be fine.) The second and much less obvious reason is that the WD19 family only allocates 4 of the incoming HBR lanes from the system to be shared across all of its “core” display outputs, i.e. all outputs except the Thunderbolt 3 port built into the removable attachment module. Any remaining HBR lanes coming from the system are only available to that Thunderbolt 3 port, regardless of whether it’s actually being used. This ends up accounting for both of the unintuitive and seemingly contradictory limitations relating to the Thunderbolt 3 port I mentioned earlier.
For the HBR2 system scenario, on a system that has two GPU outputs wired to its Thunderbolt 3 port (which to my knowledge all Dell systems have), an HBR2 connection over TB3 includes 8 HBR lanes, since a full DisplayPort link has always been defined as 4 HBR lanes, even before USB-C/TB3 arrived. But since the “core” display outputs only have access to half of those, which is equivalent to the bandwidth of a single full DisplayPort 1.2 link, you can only use those ports for display setups that fall within those bandwidth limits. That’s why even though the system is providing enough total bandwidth for dual 4K 60 Hz displays, for example, you’re limited to QHD if you want both displays on “core” outputs. However, if you instead connect only one display to a “core” port and the other to the Thunderbolt 3 port where the other 4 lanes are available, you can run dual 4K 60 Hz just fine.
For the HBR3 system scenario, there are at most 5 lanes coming from the system. The reason for this is that two full DisplayPort connections (i.e. 8 lanes) at HBR3 would require 64.8 Gbps of bandwidth, which is well beyond the 40 Gbps of Thunderbolt 3, and that’s before even considering any non-display data you might want to send across your Thunderbolt 3 connection to the dock, such as USB data for external hard drives, Ethernet data, etc. (If you're wondering, Thunderbolt 3 always prioritizes display traffic and throttles everything else when there isn't enough bandwidth to run everything at max performance. However, Thunderbolt 3 supports 40 Gbps in each direction simultaneously, and display traffic only ever runs one way, so depending on what else you're doing, high-bandwidth display setups might not bottleneck you.) In an HBR3 scenario where only 5 lanes are available, the first 4 get allocated to the “core” outputs, and then the Thunderbolt 3 port only gets access to that single remaining HBR3 lane – which is why it’s limited to QHD. The only exception seems to be if the Thunderbolt 3 port is the only one being used for display traffic, in which case it gets access to all 5 lanes, since the manual specifies that a single 8K 30 Hz display can be used from that port, just like all other ports.
One question not addressed by the manual is whether the dock supports DisplayPort DSC, i.e. Display Stream Compression. That’s part of the DisplayPort 1.4 spec, but I don’t know if it’s mandatory. But if the system and dock both support it, then higher-end display setups than indicated in the manual would be possible -- OR a given display setup would require less bandwidth, which would especially benefit Thunderbolt 3 connections because that would open up more bandwidth for other traffic. (On regular USB-C, currently half of the high speed lanes are allocated to video and half are allocated to USB, so reducing display bandwidth consumption doesn't benefit USB traffic -- although USB4 will be changing that to allow dynamic bandwidth allocation.) The higher-end display setup option could potentially even be achievable if the displays themselves didn’t support DSC as long the WD19 could “decompress” the DSC signal from the system and output a standard DisplayPort 1.4 signal to the attached display(s). And if the attached displays DID support DSC, then assuming all of the aforementioned support was still in place, even the maximum per-display resolution would increase, because at that point even the "normal" constraints on the DisplayPort 1.4 link between the dock and display could be exceeded. Hopefully we’ll find out through some testing once suitable systems and displays are more widely available.
Solved! Go to Solution.
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.
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-host... 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.
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.
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.
@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.
This is a tad off topic considering all the excellent information posted on this thread about HBR3 and the issues we're running into. However, I do want to say that most of my "dual high resolution monitor with WD19TB" problems went away when I setup my new employees with dual 1440p @ 120hz monitors.
I know, it's not a "fix" per se, but I came to this conclusion when I noticed the vast majority of my users equipped with 4k @ 60hz hardware, were also running Windows at 175%+ scaling.
I have some CAD/GIS users that I decided to buy them 27" ASUS TUF IPS panel gaming monitors, which are 1440p @ 140hz. They were astounded at the image quality and of course the silky smooth operation and accuracy of the mouse and moving elements around because they were running at double the refresh rate. They didn't even notice or care that they weren't at 4k resolution.
I bought a few other workstations these 1440p monitors and the users (accountants, designers) are running them at 100% Windows 10 display scaling and also have not noticed that they're running at a resolution less than 4k, because the text/icons/experience is roughly the same.
You can't run two of them with a WD19TB at 140hz, but you can run each of them at 120hz. I have my users connected to the WD19TB via DP and/or DP to USB-C cables.
We still had a handful of power users that were truly utilizing the entire 4k "workspace", but for the rest of them, they were effectively not quite using the 4k resolution on monitors we gave them. That kind of puts all my efforts to get them dual 4k @ 60hz to naught.
My solution isn't for everyone, especially if you already own your 4k monitors, but it may be something to consider.
@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.
@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.