U2721DE / U2719D DisplayPort Out Version?

@eikeka  The MST support table in the documentation of the U2721DE suggests that the DP output may be only DP 1.2/HBR2.  The reason is that according to that table, if you have an HBR3 system connected via USB-C and have the USB-C Prioritization option set to High Data Speed mode (2 HBR lanes + USB 3.0), then you cannot run any "external" (read: daisy chained) displays.  If the DP MST output actually supported HBR3, then 2 HBR3 lanes from the source system would be enough bandwidth to run dual QHD displays.  Similarly, if the USB-C Prioritization option was set to High Resolution (4 HBR lanes + USB 2.0), then an HBR3 system would have enough bandwidth for 4x QHD displays (although limitations from the system GPU might limit you to 3), but the documentation indicates that you can only run a single daisy chained display in this scenario.

If the documentation is correct, this is pretty disappointing, because it essentially means there is no benefit to having HBR3 support on the display.  The ONLY scenario where HBR3 would have been useful would have been in providing extra bandwidth to run additional displays, but it appears that isn't possible.  The MST support tables show identical capabilities for HBR2 vs. HBR3 source systems.  And since the displays don't support HDR or DisplayPort DSC, there doesn't seem to be any other benefit of the DisplayPort 1.4 support advertised on these displays either.

@jphughanThat's the reason why I'm asking.

If that's true there is basically no WQHD monitor with DP1.4 Out available and it's impossible to use the benefit of 1.4 for a three WQHD setup...

Well maybe there is someone who could test daisy chaining these monitors 3-4 in a row. But I will probably stay away from daisy chaining for now, as it's too inavailable what is supported and which monitor got which capabilities.

@eikeka  I agree that the lack of information and real world testing is frustrating. And if it’s true that the MST output is 1.2, then there’s no real point advertising DP 1.4 support on the USB-C or even the DP inputs. And actually, now that I think about it, the fact that the documentation only indicates support for dual QHD even when using the DP input (not USB-C) suggests that the ENTIRE CHAIN might be downgraded to HBR2 when the MST output is in use. Otherwise, if the system could send HBR3 to the first display, there would still be enough bandwidth to fully saturate an HBR2 connection on the MST output. In that case it should be possible to run at least triple QHD, but the documentation gives no indication that that’s possible.

One possible workaround might be to use a DisplayPort MST hub that supported DP 1.4/HBR3. That device MIGHT be able to negotiate an HBR3 link with the source system and independently negotiate HBR2 links with other displays. But MST hubs that support DP 1.4 and 3+ outputs are still somewhat rare.

I just ordered a P2720DC and was hoping to determine whether I made the right choice vs. the U2721DE. I'm specifically aiming to use the DP MST functionality while having no need for the RJ45 Ethernet connection that differentiates the two but am particularly interested in the DP1.4 feature. 

According to the online manual (pg 13)  it suggests the maximum external monitor resolution would still be limited to 2560x1440 at 60 Hz but was wondering if this would be different with DP1.4. I'm wondering if the "secondary" display could be run at a full DP1.4 bandwidth (in this case 5120x1440 at 120 Hz, or even at 60 Hz). My current plan was to use the 49" display (Samsung CRG9) in a Picture-By-Picture mode (dual QHD inputs), to work within the limitations of DP1.2 (QHD only). 

My situation is below (typo on the P2720DC, not P2020DC) but it would awesome if I could actually use the full CRG9 display resolution vs. just the PBP limits.

The price difference between the U2721DE ($580) vs. P2720DC ($360) is also fairly significant right now

@Jedi2155  Even if the PC source, the input of the first display, and the MST output of the first display all supported DP 1.4, you'd still only have a single DP 1.4 link's worth of bandwidth available to the entire chain.  5120x1440 at 120 Hz would consume most of that, so it's unlikely that you'd have enough bandwidth to run that type of display as the second display in a chain.  The bandwidth required by the first display would likely not leave enough remaining to pass down to the second.  5120x1440 at 60 Hz would be a possibility if you had DP 1.4 in use along the entire chain, although that would still depend on the resolution of the first display in the chain.  But as you say, the documentation indicates that only a single QHD 60 Hz display can be attached that way.  This suggests that not only is the MST output limited to DP 1.2, but using it downgrades the entire daisy chain to DP 1.2, which is only enough for 2x QHD 60 Hz total.  If the MST output did NOT cause a full chain downgrade, then in theory if you were feeding the first display a DP 1.4 signal, you'd still have enough bandwidth left to fully saturate the DP 1.2 MST output to run two more QHD displays via daisy chaining -- but the documentation doesn't suggest that's possible.

So assuming the documentation is accurate, it means there is absolutely no benefit to the display supporting DP 1.4 on its inputs, because the ONLY reason to support DP 1.4 would have been to have more bandwidth available for daisy chaining.  The only other reason would have been to support HDR, but these displays don't anyway.

As already said: The CRG9 is limited by the P2720DC as the daisy chain does not support powering higher resolutions than the previous link.

And just my two cents: Your setup looks quite expensive and static. I would recommend a hub for connecting your notebook. Just hide it behind a monitor or under the desk and you can even have some more connectivity (good hubs support 3 display outputs, ethernet and multiple USB-A and USB-C inputs) while your setup is not depending on daisy-chainable monitors.

You also save yourself the hassle of asking for the specification and trying to figure out which DP-Out Version the monitor supports etc.

I had debated getting a Dell WD19TB dock ($280) as the hub several months back, but didn't see the need to if the display itself has all the hub capability built in already. Although at $200 difference between the U2721DE and P2720DC, then the hub is the obvious choice if I really need full resolution capability. Thanks for helping remind me to bring that alternative solution to light.

Interestingly, the only thing lacking in my setup is the new Dell monitor to make all my current displays have a 1440p vertical resolution. The 27" Dell is aimed to replacing an existing 32" 4K display decided to start failing due to me using an unshielded HDMI cable (word of warning here...) and would make all my displays have a 1440p vertical resolution. Really wish Dell made these displays 120 Hz capable as well but oh well...

Also need to still acquire a 55" OLED (looking at LG CX series with Freesync/G-Sync). I've been studying these displays for a few months now and had originally decided on a PX7 prime to replace an 8 year old QHD IPS 120 Hz display (Yamakasi Catleap 2B), instead of it, but my 2nd failed display gave the Dell another opportunity.

FYI to anyone wondering, I did get my P2720DC today and the USB-C MST did work to output to my Samsung CRG9 at 2560x1440 correctly (PBP) and it also worked up to 3840x1080 resolution. It could not do 5120x1440.

I just discovered I could actually run a 3840x1080 @100 Hz with my P2720DC QHD USB-C (DP1.2) daisy chain. It failed at 120 Hz though breaking the whole chain.

I tried to create a custom resolution of 5120x1440 at 30 Hz but have not been successful so far.

Wishing I had a U2719D or U2721DE to test...