P2720DC, Daisy chaining three?

Question

Hi, Is it possible to daisy chain three P2720DC to my computer? If not so, which Dell monitor is able to do this? Thanks!

jphughan · Accepted Answer

@AteB See, this is why it helps to specify your system model. Macs (technically macOS) do not support DisplayPort MST, which is the ability to split the bandwidth of a GPU interface across separate displays. Mac users have been complaining about this limitation on Apple's forums, here, and elsewhere for years. However, you're using a Thunderbolt 3 dock, which can tap into two independent GPU interfaces over that single Thunderbolt 3 link. So in that case you can run two displays from that dock because in that case, macOS can allocate one GPU interface to each display. But you can still only run two displays through the dock in that setup, even though there's technically enough total bandwidth to run 4x QHD displays. With a PC, where support for DisplayPort MST is common now, you could potentially run 3 or 4 through a Thunderbolt 3 dock, depending on the displays and system involved.

If you want to run triple QHD displays, connect two of them to the CalDigit dock and connect the third directly to another port on your MacBook. Note that for the CalDigit dock, there was an issue a while ago where using a regular USB-C cable to connect the CalDigit's USB-C port directly to a USB-C input on a display resulted in no video. CalDigit recently released a firmware update that's supposed to fix that, but I don't know how effective it is. When I helped a friend set up a CalDigit dock before that firmware update came out, he worked around it by using a USB-C to DisplayPort cable instead of a regular USB-C cable, and that worked fine.

jphughan · Answer

@AteB Fyi it doesn't make a lot of sense to ask if you can do something with "my computer" without providing any details about your computer. But in order to run 3x QHD from a single source port, you'd need a port that supported DisplayPort 1.3/HBR3. The much more common DisplayPort 1.2/HBR2 only has enough bandwidth to run 2x QHD. And in order to run 3x QHD specifically by daisy chaining the displays rather than using an MST hub on your output and connecting each display directly to that hub, then the displays themselves would need to support DP 1.3/HBR3. The P2720DC specs in the manual available on support.dell.com indicate that the display only supports DP 1.2. However, the MST page of that manual says that with a QHD source display, you can run 2 more QHD displays. In the context of that table, "external displays" means "additional displays beyond the MST source display" based on documentation of other Dell displays, because some other displays have tables like that where certain scenarios show a value of "0". So something seems to be incorrect. Either:

The displays support DP 1.3/HBR3 and a triple QHD daisy chain is possible, or
The displays only support DP 1.2/HBR2 and you can only run a dual QHD daisy chain, in which case the MST table is a typo or "external displays" has a different meaning here than in the manuals of other displays.

But again, without knowing anything about the PC you'd be trying to do this with, it's impossible to say anything for certain.

AteB · Answer

Thank you so much for your answer. This made it much clearer for me.

I'm using a 2019 16 inch Macbook Pro with a AMD Radeon Pro 5500M 4 GB by the way. I'm connecting my monitors through a Caldigit TS3 Plus docking station.

Maybe I can connect 2 QHD monitors to the DP 1.2 port on the Caldigit and another QHD to the USB-C Thunderbolt connector?

Should that do it?

AteB · Answer

Thank you so much for the information.

I will look in to it but maybe an external GPU with three or four native DP ports is a better way to go. That way I still only need one USB-c cable to be connected to my MBP.

Down part are the bigger expenses...

Thanks again, things are so much clearer now!

jphughan · Answer

@AteB  Happy to help. I’ve never looked into eGPU usage from Macs, but even if there are options there that work reliably, that would be a pretty drastic solution just to keep achieve single cable connectivity. An eGPU is relatively expensive, makes noise, takes up desk space, and consumes a fair amount of power.

Monitors

P2720DC, Daisy chaining three?

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