U2422HE, U2518D, daisy chain

@jphughan Thanks for your fast and very descriptive reply.

I must say that I have previously used this U2518D along with an old U2417H before (U2417H was connected via USB-C hub that has HDMI output) but it was lent to someone else hence I need a new second monitor and I completely agree with you that I had some issues with scaling like you described. I choose to accept this issue as Windows apps are improving.

I apologize for using the term "USB-C DP", what I meant was the USB-C to USB-C cable that would come with the monitor. 

The U2722DE is a very nice monitor but it is a bit out of my budget. So please clarify once more: will daisy-chaining from U2422HE to U2518D work? If not, I would just buy a USB-C to DisplayPort cable and connect it to U2518D then daisy-chain it to a cheaper monitor that supports DisplayPort input. The idea of this thread is that I thought I could get rid of extra HDMI cable while having benefit of Power Delivery to my laptop on a single USB-C cable then I would spend a bit more for the U2422HE.

I hope my reply help clarify what my intention is.

Thank you.

@BlackMiracle  A full DP 1.2 interface can run dual 1440p, but the U2422HE sets up the USB-C link to carry both video and USB 3.x, and carrying the latter cuts video bandwidth in half. Some Dell displays with USB-C inputs and native resolutions of 1440p and 4K have a USB-C Prioritization setting allowing you to choose whether you want that mode or an alternate mode that allows full video bandwidth by limiting USB data speeds to USB 2.0 — which is useful for daisy chain setups or running 4K displays from DP 1.2/ HBR2 systems — but Dell’s 1080p displays don’t seem to have that option. I guess they assume if you daisy chain anything at all, it will be another 1080p display, and even an HBR2 system can run dual 1080p and USB 3.x simultaneously.

All that said, I really wouldn’t mix a 24” 1080p display and a 25” 1440p display anyway. You’ll very likely need to use different scaling settings on each display due to the significantly different pixel density, and Windows does not handle multiple simultaneous scale factors well. Essentially, whichever display is non-primary will still be rendered at the primary scale factor and THEN have its post-render content resized by GPU scaling, which will not look as good as rendering for its target scale factor in the first place. If you want to run 1440p alongside 1080p, I’d strongly recommend that you get a 27” 1440p display — and then you’ll also get the USB-C Prioritization option you’ll need. Look at the U2722DE.

@BlackMiracle  Couple of thoughts in addition to my post above. The description of your proposed setup is a bit unclear since you say you plan to connect the Yoga to the U2422HE via “USB-C DP”. The U2422HE has a USB-C input, so my answer above assumes you plan to just use a USB-C to USB-C cable. If your “USB-C DP” description meant “USB-C to DisplayPort cable”, then your proposed daisy chain will work because in that case you’ll have a full DP 1.2 interface for video — but at that point you lose the ability to receive power and exchange USB data with the display (and use the display’s built-in Ethernet interface), which is the point of the USB-C input on the display.

Also, if you really want that combination of displays, you could of course keep the U2518D display connected via HDMI. You’ll have two cables to deal with, but both displays will work at that point since the USB-C output will only have to deal with the U2422HE, and you’ll get the benefits of a USB-C display. But I’d think about that scaling. If you’re comfortably using the U2518D at the default 100% scaling, then a U2422HE would work (and also, congratulations on having particularly keen eyesight!). But if you’re using the U2518D at something like 125-150% scaling, then the U2422HE would either look not quite perfect if you try to run it at 100% simultaneously, or you’d have to use it at the same scale factor as your U2518D and deal with loss of effective workspace due to things being larger than necessary.

@BlackMiracle  Happy to help. If you want to use a USB-C to USB-C connection to a U2422HE, then it will only set up the USB-C link to run USB 3.x and video. It can’t be configured to double available video bandwidth by limiting USB data speeds to USB 2.0 the way Dell offers on its 1440p and 4K USB-C displays. That said, there may be some hope here. I’ve seen threads from people who use a Dell USB-C docking station that configures the link the same way their USB-C displays do, i.e. half video bandwidth plus USB 3.x data. They try to run a dual 1440p display setup through that dock from a DP 1.2/HBR2 system, which doesn’t work…but they say that the result is that one display runs at 1440p and the other runs at 1080p.

So with the caveat that I have not personally tested the exact setup you’re considering, and not everything that SHOULD work a certain way in the tech world always does, I will say that based on reports I’ve seen involving what should functionally identical setups, there is a chance that you’d be able to run your setup as desired. (I went back and looked through previous threads while writing this reply, which I hadn’t done during my initial reply.) But if it doesn’t work, then you’d still have the benefit of Power Delivery and USB data to the U2422HE and you could keep the U2518D on HDMI. That to me seems like a better overall solution than your fallback of a USB-C to DP cable for a daisy chain, because yes in that case you’ve only got one display cable, but you’ll still have to keep separate power connected. And if you want to use the USB ports on either display, you’d have to run yet another cable for that. But even if you don’t care about USB data, you’re looking at two cables either way. In terms of the cost difference, bear in mind that another perk of having a USB-C display is that you have a dedicated power source always there, which frees up your laptop’s normal adapter to be parked elsewhere or just kept as a dedicated on-the-go charger rather than you having to unpack it and pack it back up every time you come and go from your desk. So factor that convenience into your valuation.

@jphughan Good point on the second paragraph.

I will go with U2422HE and will update this thread with information of how it goes with my configurations to be your future reference and for others in this situation.

Thanks again for all the information shared. Really appreciate it.

@BlackMiracle  You’re very welcome. If you decide to try the setup with the U2422HE, please report back how it went!

Hi @jphughan 

My U2422HE has arrived. Bad news, daisy-chaining it to U2518D does not allow 1440p resolution.

I used the included USB-C to C cable, plug it to one of laptop's USB-C ports (both support DP 1.2, one of them supports Thunderbolt but I tried both port, got same result) Then plug the included DP cable from U2422HE to U2518D and MST turned on automatically but I'm stuck at 1920x1080 resolution on both monitors. Note that I disabled the built-in laptop monitor (it's a 1080p display)

In Intel Graphics Control Panel, maximum resolution for U2518D is 2048x1080, not 2560x1440 native resolution.

I then tried adding custom resolution at 2560x1440 59Hz and it says "custom resolution exceeds the maximum bandwidth capacity"

Is this the end of the road for me? Anything I can config or check further? Let me know if you need more information.

Thanks.

@BlackMiracle  EDIT: Bad answer. See below.

@BlackMiracle  Sorry, I wrote that too early in the morning. The U2422HE doesn’t have that. If you can’t get 2560x1440 on your secondary display, then I don’t think there’s anything else you can do, short of maybe sacrificing refresh rate, but that’s certainly not ideal. I’m a bit surprised given that Dell’s USB-C docks seem to be able to run QHD+FHD even from DP 1.2 systems, and they use the same video + USB 3.x mode for USB-C links, so I’m not sure why you can’t achieve the same thing here, but if you can’t even after updating drivers and firmware and even trying custom resolutions, I don’t have any other suggestions at the moment that would retain the use of USB-C connectivity to the display.

Hi @jphughan 

Thanks for your reply. I have a bit of information to share.

Yesterday I tried unplugging the HDMI cable from U2518D and daisy-chained the two monitors. Yes, both monitors run at 1080p like I said before. But then I tried turning on my laptop display on Intel Graphics Control Panel and for some reason, my U2518D suddenly runs at 1440p while the U2422HE was lowered to 720p (all 3 displays run at 60Hz) Though I still can't get both monitors to run at native resolution when turning off laptop's monitor and lower the refresh rate didn't help.

Of course this is still not preferable, but at least can we assume that daisy-chaining from a lower resolution monitor (U2422HE) to a higher resolution monitor (U2518D) is possible? If so, if I happen to have a laptop that supports HBR3, is there a chance I can run both monitors at their native resolution?

Thanks.

@BlackMiracle  Yes, daisy chaining from a lower resolution display to a higher resolution display is possible, or at least it should be.  The possible catch is that I believe there are some displays that support HBR3 on their DP/USB-C inputs but only support HBR2 on their outputs.  I think I remember learning that the U2719DC works like this, but I'm not sure.  And unfortunately in that scenario I'm not sure if that only limits the amount of bandwidth that can be passed down the chain, or if that forces the entire daisy chain down to HBR2.  I just don't have the equipment to experiment with, and in many cases figuring out whether you're running HBR3 or HBR2 can be a challenge on its own.

But in theory, if you had an HBR3 system and the whole chain could run HBR3, then you'd be able to run dual displays up to 2560x1440 each even over a USB-C link that was also running USB 3.x data simultaneously.

If you can get the displays to both run at 1080p while your built-in display is off, then you should be able to maintain that setup when the built-in display is on.  It's possible that the change occurs because Windows can remember display setups to simplify scenarios such as moving from using your laptop standalone to using a docking station at work to using a different dual display setup at home.  Whenever it sees a given combination of displays, it's supposed to remember how they're positioned relative to each other, what resolution and scaling settings to use for each, etc.  So lighting up your built-in display is technically a different display environment profile.