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Demystifying Dell WD19TB supported display resolutions
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.
jphughan
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March 22nd, 2021 15:00
@decker12 I remember you posting about that in the past, but unfortunately the viability of the "dual USB-C" option depends on the system and display setup you're running. If you have dual 4K 60 Hz displays for example, that setup only works for systems that use DisplayPort HBR2, because that's the only scenario where the WD19TB allocates enough bandwidth to its downstream TB3 port to run a 4K 60 Hz display over there. With a DisplayPort HBR3 system, you'd only have enough bandwidth for a 1440p 60 Hz display on that port, and dual 4K 60 Hz would require you to use some combination of the other ports. Explaining that and other points was the entire point of my original post that started this thread. (And in the case of the XPS 13, the XPS 13 9310, 9300, and 7390 2-in-1, but not the regular 7390, are all DisplayPort HBR3 systems.)
Tone112232
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March 22nd, 2021 16:00
I have a XPS 13 9310 / I7 using USB C to connect to dock, dock then connects to 2 x monitors using 1x HDMI and 1 x DisplayPort.
I really am trying to give you the info you have asked for! I apologise if I am not achieving this.
Will a MacBook Pro 13 inch Intel chip be able to use the same connections and produce the 3 screen solution (laptop and 2 monitors).?
Thank you again for patiently bearing with me and replying. You are so very kind.
This is what it boils down to for me:
I want the 3 x screen solution with a 13 inch MacBook Pro or Air. 16GB RAM / 512 SSD.
I either pull the trigger on an Intel version or risk the M1. M1 is obviously attractive but extremely disappointing as per your previous comments. What a massive disappointment from Apple.
How risky is it to go for the M1 and dual screen DisplayPort scenario?
I am trying to asses the risk here - any feedback?
jphughan
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March 22nd, 2021 17:00
@Tone112232 You still have not said what exact displays you are using.
Tone112232
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March 22nd, 2021 17:00
2 x Dell 24 Monitor - S2421HS
https://www.dell.com/en-au/shop/dell-24-monitor-s2421hs/apd/210-AXIK/monitors-monitor-accessories
Tone112232
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March 22nd, 2021 18:00
That's massive progress - thank you so much. At least it gets me a solution with the 2013 MacBook. That's my fallback position with that solution.
Can I improve on it? eg getting power to the MacBook 2013?
eg Is there any way I can use the 2013 MacBook with the WD19TB dock (eg get an adaptor to convert the USB into USB C).
I know this dock solution is not great but it provides me with the option to use the 2103 MacBook whilst I wait for Apple to finally produce an M1 MBP that allows dual screens.
jphughan
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March 22nd, 2021 18:00
@Tone112232 That's a DisplayLink-based dock like Dell's Dxxxx dock models. The one you linked only supports dual HD displays though, so it might use a pretty old generation of the DisplayLink chipset like the old Dell D3000, which has since been replaced by the D3100 and then the D6000. There are now DisplayLink docks that can run dual 4K 60 Hz displays, so if you want something that would remain useful if you ever upgraded to higher-end displays, you might want a different option. But yes that dock would work with your 2013 MacBook Pro -- except it won't charge the system when connected via USB-A, of course. And since that dock also supports USB-C and can provide up to 85W of power, it would be able to charge your XPS 13 and newer MacBook systems that actually have USB-C/TB3 connectors. As an alternative, the Dell D6000 also supports USB-A and USB-C and supports dual 4K 60 Hz displays (or dual 4K 60 Hz + 4K 30 Hz when connected via USB-C), although it only supports up to 65W of power when connected via USB-C rather than 85W. But unless you'll be looking at 15-16" MacBook Pros, 65W is enough. Or if that dock you linked is less expensive and/or easier to get and you're ok with it being limited to dual HD displays, then it seems like it might be a good option as long as DisplayLink will work for you.
Tone112232
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March 22nd, 2021 18:00
You are absolutely awesome - thank you.
Would this dock work for the MacBook Pro 2013 tp get dual screen? It enables the Macbook to connect using the MacBook's USB port.
https://www.jbhifi.com.au/products/alogic-universal-twin-hd-pro-docking-station?queryID=9042f20d7af0ebebef0cc748b6f2972b&objectID=434015
https://www.alogic.co/us/alogic-universal-twin-hd-pro-docking-station-with-85w-power-delivery-and-usb-c-usb-a-compatibility-dual-display-1080pat60hz.html
https://support.apple.com/kb/sp690?locale=en_AU
I would connect the Macbook to it using the USB port, then leads to the 2 monitors. Would 2 x HDMI's do it?
jphughan
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March 22nd, 2021 18:00
@Tone112232 Ok, so you're running dual 1080p 60 Hz displays. In that case, yes you could move one of those displays over the downstream TB3 port at the edge of the dock and connect it using a USB-C to DisplayPort adapter. That is the cabling setup that you'll need in order for non-M1 Mac systems to use the two displays in Extend mode instead of Mirror mode, at least according to Dell's KB article on this topic. I haven't tested that myself. But that different cabling setup will still allow your XPS 13 9310 to use the displays the same way it is right now. If you had been running 4K displays, you might have had a problem.
In terms of the risk of DisplayLink, I'm really not sure what more I can say beyond what I wrote in the thread I linked and what I said the last time you asked me that question here. The only way to know for sure how DisplayLink will perform in your case would be to actually try it out, which you can do with your XPS 13 if you want. Just get an actual DisplayLink adapter, like this one, connect it to a USB port on the WD19TB, and connect one of your displays through that adapter instead of the dock's normal DisplayPort output. Install the DisplayLink software on your system (get it from DisplayLink.com) and see how it performs. If it works the way you need it to, then fantastic. If not, then now you'll know for sure.
Tone112232
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March 22nd, 2021 19:00
Would the D6000 work with Macbook Pro 2013? I've just checked it and I previously thought you could only connect a laptop that had USB C. it would appear that I can connect a USB A laptop to it - is that correct?
And it has DisplayLink - correct?
Tone112232
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March 22nd, 2021 19:00
Thank you for being so amazingly helpful and extremely patient. I have a way forward now - thanks to you.
jphughan
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March 22nd, 2021 19:00
@Tone112232 No. The 2013 MacBook Pro doesn't support any industry standard method of receiving power, because there WASN'T an industry standard method until USB PD, but that is only supported over USB-C, not USB-A. I don't know of any single dock that support providing power both via USB PD and Apple's MagSafe 2 standard that is used by your 2013 MacBook Pro.
And no, you can't use the WD19TB through a USB-A connection because as I explained in the thread I linked where I discussed DisplayLink, the WD19 dock family models, like most USB-C/TB3 docks, rely on DisplayPort Alt Mode. That is a native GPU output wired to a USB-C/TB3 port. But that's only possible on USB-C/TB3. You cannot have a native GPU output wired to a USB-A port, and therefore you would not be able to send a video signal to a DisplayPort Alt Mode dock over a USB-A connection. The reason docks that rely on DisplayLink or similar "indirect display" technologies can work is because they involve using software within the system to convert video to regular USB data, which of course CAN be carried over a USB-A port. That is why the dock you found and the Dell Dxxxx series docks can work through a USB-A port.
Bottom line here: There is no single dock model that will perform optimally with a 2013 MacBook Pro, an XPS 13 9310, and an M1 MacBook. If DisplayLink's drawbacks are not an issue for you, then a DisplayLink-based dock that supports both USB-C and USB-A is probably your best option, because at that point all you need to do is connect power separately for your 2013 MacBook Pro. But if you plan to replace that somewhat soon anyway, then that shouldn't be a major issue.
jphughan
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March 22nd, 2021 19:00
@Tone112232 Yes the D6000 can be connected via USB-A, so yes it would work with your 2013 MacBook Pro as long once you install the DisplayLink software, and yes the D6000 uses DisplayLink for the first two displays you attach to it. If you connect a third display to the D6000, then the display that is connected via HDMI switches over to relying on DisplayPort Alt Mode instead (since the DisplayLink chipset in the dock can only run two displays), but a triple display setup is only possible when connected to a system via USB-C. But since you only have two displays, both of them will use DisplayLink, even if one of them is on HDMI.
jphughan
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April 12th, 2021 13:00
@mafj2 For the USB-C connection, make sure you're using the downstream TB3 port at the edge of the dock, not the USB-C port near the HDMI output. For dual 4K 60 Hz from HBR2, one of the displays has to be on that downstream TB3 port. But yes, several users have gotten dual 4K 60 Hz displays running from HBR2 systems. @decker12 supports a business where that's a pretty common setup, in fact I'm pretty sure he's posted in this very thread about it. The setup that seemed to work most reliably for him was to use downstream TB3 and the USB-C port near the HDMI output. That specific combination shouldn't be necessary -- only using downstream TB3 should be a requirement -- but he said that it seemed to work most reliably.
mafj2
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April 12th, 2021 13:00
I have tried following the guidance for two displays 2x UHD@60 with USB-C and DP connections.
My laptop is i7-8750H with Intel UHD630 (and RTX2060), connected to WD19TB with TB3 HBR2, which
got in turn two monitors connected. My UHD monitor (p32p-20) is connected with USB-C/DP,
other monitor (actually QHD) is connected with DP.
When connected this way my options seem to be exactly as with other connection types (DP+DP, or HDMI+DP),
that is UHD@60+FHD@60, UHD@30+QHD@60, or QHD@60+QHD@60
I have updated WD19TB firmware to the latest version 60 to no avail.
Had anyone successfully got two UHD@60 displays working in any configuration with HBR2, specifically
USB-C+DP/HDMI combo?
mafj2
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April 12th, 2021 14:00
Thanks for super quick and helpful reply.
I can confirm it works now!
I had given up on this port as I believe it simply did not seem to work at all with previous firmware (version 40).
Just to clarify:
* UHD@60 with USB-C off the USB-C marked with lightning symbol
* QHD@60 off the DP next to HDMI