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November 25th, 2020 02:00

P2421DC, two, daisy chain, Macbook Air/Pro M1

Hello everybody,

I am looking into buying two P2421DC because of the USB Type-C connector and the daisy chain feature. But I'm not sure if the setup will work like I imagine it, so here are two questions:

- Is it possible to daisy chain from a single Thunderbolt 3/USB Type-C to two P2421DC monitors, first connected via the provided USB Type-C cable (with Power Delivery) and then connect the second monitor via a DP to DP cable (provided in the box?) from the first P2421 Monitor (connected to the Macbook)?

- Do the new Macbook with M1chip support DP1.2 via Thunderbolt 3/USB Type-C so I can connect two displays to this computer via daisy chain? It is reported, that the new MacBooks only support one display up to 6k resolution, will the daisy chain "act as" one or two displays?

Thanks in advance.

Best regards nbock

4 Operator


14K Posts

November 25th, 2020 10:00

@nbock  This would be possible if you were using a PC, but not from the current M1 systems, or any Mac system as of this writing.  There are two issues with your plan;

  • macOS does not support DisplayPort MST, which is required for DisplayPort daisy chaining.  This has been the subject of complaints for years on this forum, Apple's forum, and elsewhere on the Internet.  DisplayPort MST allows a single DisplayPort interface to be carved up to run separate displays, but macOS doesn't allow that.  The only displays that can be daisy chained on Macs are Thunderbolt displays, since Thunderbolt daisy chaining is a distinct entity.  Thunderbolt 3 can carry 2 full GPU interfaces, so in a Thunderbolt daisy chain scenario, each display still gets allocated its own GPU interface.  But even that limits you to 2 displays, whereas PCs can run 3+ displays from that type of setup as long as the setup falls within available bandwidth.  Ironically on Macs that run Boot Camp (which I realize isn't possible on the M1 systems, at least today), when those Macs are running Windows, DisplayPort daisy chaining works just fine, so it's not a hardware limitation.  It's just an OS limitation.
  • A daisy chain does not result in a single "spanned" display, which would create all sorts of inconveniences for applications that didn't realize there were two independent displays involved.  Picture trying to full screen an application on one physical display or a dialog box meant to appear in the middle of the display.  Instead, a daisy chain involves each display being treated completely independently, as if each one was connected directly to its own output.  The problem there is that the current M1-based Macs seem to be limited to 2 total displays.  The Mac Mini supports 2 external displays since it obviously has no built-in display, but the M1 Air and 13" MBP specifically say they only support one external display.  This is a downgrade from their predecessors that supported 3 total displays, i.e. the built-in display and 2 external displays, because Intel GPUs today can run up to 3 simultaneous independent displays.  (The 15-16" MBPs can run more because the Intel GPU has direct control of the built-in display and the discrete GPU, which supports 4 displays all on its own, has direct control of the TB3 outputs.  So it can run up to 5 total displays.)

6 Posts

November 25th, 2020 14:00



Thanks for the fast response.
Unluckily I can not work on a windows system and so i hoped that, as you correctly mentioned the M1 Macbooks only support on external display, that it could maybe possible via the daisy chain feature.

So thanks again, now I'm informed how my situation looks like, maybe I will look into one bigger display or wait for the hopefully upcoming Macbooks with M2 that support two external screens.

Best regards nbock

4 Operator


14K Posts

November 25th, 2020 15:00

@nbock  Happy to help, even if I didn't have better news for you.  But if you're considering a dual QHD setup, there actual "dual QHD" displays, like the U4919DW.  It's a super-ultrawide (32:9 aspect ratio rather than 21:9) with a resolution of 5120x1440.  It is literally two QHD displays fused together.  Of course having all of that in a single display gives you a bit less flexibility around placement.  You couldn't have one display right in front of you and the other display beside it angled a bit towards you, for example.  And some people prefer having separate displays to help "compartmentalize" different things, or to quickly full screen an application on one display while keeping another display for something else, although there might be applications that would help "carve up" a single large display into different setups to help with that.  Or you might be someone that prefers having a single canvas uninterrupted by bezels.

Full disclosure: I'm not certain that the U4919DW would work with the M1 Mac Mini.  It should, but it's also true that multiple Dell displays seem to have various interoperability issues with Macs, both in terms of running specific resolution and refresh rates and (in the case of laptops) sometimes around USB Power Delivery.  Unfortunately Dell's position seems to be, "We don't test or support Apple systems. Talk to Apple."  And Apple says to talk to Dell.  But there are other 5120x1440 displays out there too.  Good luck!

6 Posts

November 25th, 2020 15:00

Thanks for the further recommendation!


I will take a look into ultra wide screens, I just "ignored" them until now because I am heavily using Zoom right now (and looks like this will go on for a while). The problem with this is that you can choose between sharing a full desktop or just one application. The second options is a limitation because if you want to showcase something from another application it won't show up.

e.g. I'm in a keynote and then "tab out" into safari to show a video or something, this only works when you share the whole desktop.


Furthermore Keynote uses the second screen for moderation text, next-up slide and so on. With just one big screen this will then be displayed on the tiny macbooks screen. But this place is kind of reserved for the other participants in the meeting...

So I am looking into the same trouble as with my current single external screen setup.

-> both the participants view and the moderation screen are on the macbook and I can't see the moderations because the participants video are an overlay window. This is a very specialised problem, but I think there are a lot of educators out there that struggle in the same way!

But buying an "old" intel macbook for double the price with a third of the M1's power seems just stupid atm to me...

Hopefully they come up with the higher spec 13" Macbook Pro soon and integrate two external monitors.

As for the Dell and Mac compatibility I can refer to the P2418D and the U2720Q. Both work fine with several Macbooks.

First works fine with 2015 and older macbooks via thunderbolt to hdmi adapters (Air) or directly via HDMI (Pro) aswell as a current 2020 Intel MB Air. The second one works fine with the same 2020 MBA but the computer struggles with the 4k Resolution together with Zoom, Keynote and some other apps open But from Dell's side the connection via USB C with PD works fine!


Hope that this detailed experience report helps someone somehow

Mac Mini is no real opportunity for me unfortunately, as hopefully we can go back to university sometime

As I understood the new M1 Mac Mini does only support one screen via Thunderbolt (even if it has more ports - I think it has 2) and one via HDMI so I am not sure whether it could work with two "Thunderbolt" daisy Chained Displays or if you would still need to connect each by itself.

If they would only support two displays on the Macbooks as with the Mini, which should be technically possible with the M1, (but maybe there is a seperated display driver in the Mini for the Hdmi - who knows?!) I could deal with turning off the Macbooks screen (or just close the Macbook) and use two bigger external displays - it's a shame that this is even NOT possible (at the moment)

So I guess on my wishlist for Dell stands an affordable Thunderbolt Display with "Thunderbolt Chain" with either 24" wqhd or 27" 4k


Thanks again & best regards

4 Operator


14K Posts

November 25th, 2020 17:00

@nbock  Yep, your use cases are exactly why I also prefer having physically separate displays rather than one huge one.  But I agree that buying an Intel-based Mac Mini at this point wouldn't make sense except for some very specialized situations, perhaps those that might benefit from having 4x TB3 ports.

I have a feeling that at least the MacBook Pros will support multiple external displays at the next rev.  I'm actually amazed that Apple released the 13" MBP with this limitation, since "Pro" and "only one external display" just don't belong together, in my view.  I could sort of understand it with the Air, although even there it's a downgrade from the Intel version.  Maybe their market research indicated that a solid chunk of Air and 13" MBP users only used at most one external display?

As for the U2720Q, I really discourage using 27" 4K displays in general, but especially on Macs.  The reason is that in the Mac world, standard (non-Retina) displays have always targeted 100-110 ppi -- like the old 23" 1920x1200 display and 30" 2560x1600 displays.  When interfaces and art assets were being designed and sized, it was on the assumption that they would be shown on displays with that density, so something meant to be an inch wide would actually be a real-world inch wide on such a display.  (Windows targeted 96 ppi, if you're curious.)  Retina displays doubled density to 200-220 ppi, and Retina art assets are designed as "@2x", meaning twice as many pixels horizontally and vertically.  The problem with a 27" 4K display is that it gives you 163 ppi, which is right between the two setups that Apple optimizes for.  So if you use standard art assets, things will probably be too small.  If you use Retina assets, they'll be too large.  The only option at that point is to render using one or the other and then use post-render GPU scaling to upsize or downsize, but that introduces scaling artifacts.  And 163 ppi isn't quite enough density to give you a true Retina experience.  This is all exactly why Apple's own 27" displays in their iMacs use 5K resolution.  A 27" QHD display, on the other hand, is 109 ppi, which is about right for a standard display.  The P2418D is 23.8" QHD, which is 123 ppi.  That's pushing it a bit.  I personally think QHD on a display that size can be a bit tricky to read without any scaling enabled, at least at typical viewing distances.  I love 27" QHD though.

As for how to connect displays to the M1 Mac Mini, having checked the Tech Specs page for it again, it does seem to suggest that only one display can be USB-C/TB3, while the other has to be HDMI.  That's surprising if true, but it may well be.  And then yes that would suggest that even a Thunderbolt daisy chain wouldn't work.

As for Dell making Thunderbolt displays, Thunderbolt isn't nearly as common on the PC side as it is on the Apple side, and it was only recently that Intel developed a Thunderbolt chipset that was backward compatible with regular USB-C systems (at reduced functionality), so that's probably why Dell hasn't made Thunderbolt displays.  They're actually pretty rare in the market overall.

But there are other options for taking advantage of Thunderbolt multi-display output.  One obvious option for laptops is a Thunderbolt docking station that has multiple display outputs, like the CalDigit TS3+, which offers USB-C and DisplayPort outputs.  It can run dual 4K 60 Hz displays from Intel-based Macs just fine, no need for Thunderbolt displays and daisy chains.  The value proposition of a dock connected to a Mac Mini is admittedly not great, though.  For that, there are Thunderbolt 3 to Dual DisplayPort adapters.  They cost maybe $50-70, but they give you a lot more choices of display models.

As for the M1 supporting dual external displays even on the MacBooks, I think the built-in display always counts as a display even if the lid is closed.  The reason I say this is based on my experience on the PC side.  Intel GPUs today can run up to 3 simultaneous displays.  In Windows, if you choose to disable the built-in display, all 3 of those can be external.  But on Linux, it isn't possible to completely disable the built-in display, so the same system model that could handle 3 external displays when running Windows can only run 2 external displays when running Linux.  The documentation of Dell's docking stations calls this out when outlining the display setups that are supported.  macOS has Linux/Unix underpinnings, so that may be at play here.

6 Posts

November 26th, 2020 02:00


In general I am not looking into buying an Mac Mini, because it's not portable and I will have a need for this in normal non-pandemic life, as mentioned in my last post.

For now this would be the best solution (M1 Chip Mac and two displays supported), but I am not sure whether it's a good idea to invest into a desktop solution, when there could be the need of a mobile work solution again next year.

About the M1 Mac Mini:

I rechecked about the M1 Mac Mini and even the apple website states that it only supports one screen via Thunderbolt 3 / USB Type C:

  • One display with up to 6K resolution at 60Hz connected via Thunderbolt and one display with up to 4K resolution at 60Hz connected via HDMI 2.0

So no "Thunderbolt" Daisy Chaining here too...

About the MacBooks Air and Pro:

I fully agree with you that they should have bring two external display support to the 13" base Macbook PRO, as it says it's for professional users.

I think for the Air only one external display is acceptable, even if the latest intel Airs supported two, I have to say as I am coming from an 2015 Macbook Air with only one thunderbolt port (for video output, no HDMI) and I never had the need for more than one external monitor in 4 years, I fully accept the one external screen support on the Air.

But I think it's a must have on the Pro (if they couldn't do it technically, then please let the users turn off the internal screen to use two external ones), this would then justify the 300 USD price difference, as the most reviews tell just go for the base air, as performance is not a problem anymore (at least for the most use cases).

About PPIs and Apple's targets:

Thanks for the information on scale and Apple's PPI targets! Didn't know that.
So you would then recommend getting one 27" QHD screen with 109 ppi instead of a 24" QHD with 123 ppi?
As I have no experience in the 27" QHD configuration and liked the sharpness of QHD on 24" I'm curious what you and maybe others would suggest  

About the Thunderbolt Adapters:
As from what I read and your last answer, the Thunderbolt to two HDMI/Displayport Adapters also only work for Intel based Macs, right?

the other stuff:
Yes the new M1 MacBooks do NOT support two external screens even if the lid is closed. This is reported by multiple sources on YouTube, reddit and so on.

So until Apple releases the newer "Pro" Models with hopefully two supported Screens, the only thing to do is going for a single screen setup, is that correct?

It's interesting that Dell seems to just ignore all Apple and Mac things, but still many design agencies (Mac only territories) have Dell Screens from what I can tell here in Germany. So still with some downers, they seem to make good screens for this users ...


Thanks again for your detailed answers and the hint on Apple's PPI targets!

4 Operator


14K Posts

November 26th, 2020 08:00

@nbock Regarding display setups, if you're comfortable using QHD resolution on a 24" display, then going to 27" QHD in order to make a given amount of workspace larger at the cost of reduced pixel density might not be a better choice for you.  But 27" 4K would be a significant increase in pixel density over 24" QHD, to the point that it might not be a great choice either.  This is a handy website for calculating pixel densities if you want to experiment.

For TB3 to Dual DisplayPort adapters, yes my expectation would be that they would only be useful on Intel-based Macs at the moment.  I just mentioned them so that if you find an Apple Silicon-based Mac later that can run two displays over TB3, that would be a way to run both of them from a single port without having to find displays that supported Thunderbolt daisy chaining -- assuming that these adapters end up working with those future systems, of course.

If you really want an M1 system, there is another way to run an additional display, namely through a USB dongle that relies on "indirect display" technology called DisplayLink -- not to be confused with DisplayPort.  Plugable makes such dongles.  They will allow you to exceed GPU limits on display quantity because DisplayLink displays aren't driven directly by the GPU.  But that technology comes with some drawbacks that can be significant to certain use cases.  I wrote about those here.  Somebody else mentioned having found confirmation that DisplayLink drivers work on Big Sur and on M1, so if you're desperate to get an M1 system but need multiple displays, this might be feasible.  But if you can wait for a system that can run multiple external displays natively from the GPU, I personally would.

In terms of Dell displays and Apple systems being a popular setup, I agree it's quite common, partly because Dell is a major display vendor in the market overall.  And in fairness, the interoperability issues I've read about seem to be related mostly to ultrawide or 4K displays, especially when using USB-C as opposed to using a USB-C to DisplayPort cable or something.  I didn't mean to suggest that trying to use Dell displays with Macs was hopeless, but rather that if you're in the market, it might be worth doing a quick check to see if you can find real world confirmation from a Mac user that the display works as expected.  There are enough complaints that there's even a "sticky" thread in this Displays section about Mac issues here.  And even that doesn't list all of them.  A post in this thread mentioned a change made by the major display/TV review site  That site maintain a "5 Best Monitors for MacBook Pro" list here.  It's been updated recently, and it still says this: "Due to numerous complaints of compatibility issues and Dell's unwillingness to offer support to those affected, all Dell monitors have been removed from our list of recommendations for the time being and until the situation is resolved. At this time, we don't test for compatibility; if you run into any issues using a monitor when connected to a MacBook, please let us know in the discussions below."  It's unfortunate that Dell and Apple seem unable or unwilling to work together for the benefit of their mutual customers.

6 Posts

November 28th, 2020 02:00


Yeah as I wrote my bachelor thesis on 24" WQHD (P2416D) and later worked on the newer P2418D I feel comfortable with this combination. I think I will just stick to this as I look into a dual monitor setup anyways.

As you wrote before the combination of 27" and 4k is in the sub-optimal space for MacOS of around 160 ppi, so this doesn't seem to make sense either. We also have U2720Q at the university and the Macbook Air 2020 (Intel) just feels slow with this 4k display connected via USB Type C/ Thunderbolt 3, at least when you run a Zoom call and work simultaneous! Maybe this has to do with the whole scaling thing you mentioned before ...


Thanks for the recommendation of TB3 to DisplayPort Adapters, maybe I will have a look into them when there is support for it. At the moment I would be super happy with connection two USB C cables to my laptop to use two monitors

I read about the whole Displaylink thing in my research about the M1 MacBooks, but more or less the most people think it doesn't run well and if you have to opportunity better steer clear from it. Do you have in-person experience with this systems?

Wow thanks for clarification, didn't knew about these incompatibilities of Dell Displays with Macs. Sounds very bad to me!  

best regards

4 Operator


14K Posts

November 28th, 2020 07:00

@nbock I’ve only used DisplayLink on Windows, and my experience there is that if you’re displaying mostly static content like typical productivity apps, it works perfectly well. But if you’re displaying content where a large portion of the display changes often, such as watching full screen video, the experience can degrade pretty noticeably. However, that was admittedly a while ago, so maybe newer CPUs can handle the extra load better — at least as long as you’re not using that performance capacity for other things. And as it happens, an article about DispayLink on M1 was just posted recently, including a video. Take a look here. If you can wait a while, I’d still probably wait for a new version of the system that can run multiple external displays. But if you need a new laptop now, an M1 with DisplayLink may well be a better choice than an Intel Mac. 

6 Posts

November 29th, 2020 09:00

 Thanks for your experience report.

I have a mixed workload of remote design tools, Adobe CC, Keynote (combined with Zoom) and sometimes video or audio editing. 

Thanks for the link, looks more promising as I would have expected. 

I will try to stick around longer with my actual workhorse and hope for a Q1 release of the "bigger" Macbook Pros as suggested.

Else I may have a detailed look into Display link next year.

Thanks again and best regards!


4 Operator


14K Posts

December 1st, 2020 14:00

@SteveRKO  It's certainly worth calling out as an option since it may work for some people, but I'm not sure that a workaround that involves a device that costs at least $150, limits you to 1080p resolution, and has lag that can be problematic in at least some people's use cases would qualify as a decent workaround.  Unless you already happened to have a spare Apple TV lying around, a DisplayLink adapter would likely be a better solution overall (unless it's a system where you don't have access to install the necessary software), since that will support higher resolution, aspect ratios other than 16:9, and should perform somewhat better.  But even that isn't a substitute for a true GPU output.

1 Message

December 1st, 2020 14:00

The M1 MacBook Air can support two external displays, one wired normally and one via AirPlay. You need Apple TV or similar to connect to the second external monitor. Select "Use as separate display" and all three are independent and can be arranged in the Displays app. Seems a decent workaround.

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