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9368
April 20th, 2020 05:00
U2520D, MacBook Pro 2015, MST usage
Hi
I am trying to have MST with my MacBook and two U2520D displays
I have a Thunderbolt 2 cable connected from my Mac to Thunderbolt 2 female to Thunderbolt 3 male adapter from Apple going into the USB Type-C port at the back of the U2520D. I had a chat with Apple and they said it should work.
This configuration does not work even with a Windows laptop. It works fine with just using USB Type-C cable from the U2520D to Windows laptop.
How to connect my MacBook Pro with Thunderbolt 2 port with a MST?
thanks
jphughan
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April 20th, 2020 07:00
@_mac I'm surprised Apple Support said this will work -- maybe they thought this was a Thunderbolt display? -- but it definitely won't. First, I'm fairly sure that Thunderbolt 3 to Thunderbolt 2 adapters can only be used for actual Thunderbolt traffic rather than as a Mini-DisplayPort to USB-C adapter -- and the U2520D is not a Thunderbolt device, but rather just a regular USB-C device. And second, using the USB-C input on a display requires that the host system support DisplayPort Alt Mode, which is the ability of a USB-C port to run DisplayPort (and optionally USB 3.x simultaneously). A Thunderbolt 2 source port will not support that. Thunderbolt 2 (and TB3 for that matter) carries only DisplayPort and PCIe, so Thunderbolt peripherals that want to run USB need to incorporate a USB to PCIe bridge chip for the link between the system and peripheral -- but again, the U2520D is not a native Thunderbolt display. Consequently, at best you'll only get video output this way by using the Mac's TB2 port as a regular Mini-DisplayPort output, although here again I don't know if you can use a regular DisplayPort source port to send into a display's USB-C input. A source system's USB-C port that supports video output can definitely be used to send into a display's regular DisplayPort input using a USB-C to DP cable, but I'm not sure if the reverse is true. And again, at that point you'd only get video, not USB or system charging. You might just have to use a regular Mini-DisplayPort to DisplayPort cable to get any signal at all.
As to the additional question about MST, as far as I've been able to find, Mac OS still does not support DisplayPort MST at all. That's been the source of a lot of complaints on Apple's forums and elsewhere, and the reason DisplayPort MST hubs always say "Windows only; does not work with Mac OS". That said, I have seen one post from one user here who claims to have gotten a DisplayPort daisy chain running with his Mac -- but I have found no other sources indicating that success, never mind any official source indicating that Mac OS now supports DisplayPort MST or what OS version and/or hardware is required to achieve that.
_mac
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May 1st, 2020 03:00
Alright so I ended up using mDP to DP cable to my primary monitor and then MST to secondary monitor for Windows laptop and for my MacBook. I use the same mDP to DP to one monitor and for the other one I use HDMI. Unfortunately, the other monitor does not switch to HDMI automatically so I have to do it manually every time.
jphughan
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April 20th, 2020 07:00
@_mac just to clarify a point in the above post a bit, if you got a simple Mini-DisplayPort to DisplayPort cable to connect your Mac to your U2520D, you'll get an image on the display. At that point, IF your Mac has both hardware-level and OS/driver-level support for DisplayPort MST -- which again I am not remotely confident is available, since I've only seen one report by one user of MST working from a Mac -- then you'd also get MST because the display would be able to take the signal you're sending into its DisplayPort input and use that for MST to a second U2520D. DisplayPort MST didn't even become available until DisplayPort 1.2, and DisplayPort 1.2 has enough bandwidth to run dual QHD displays. I myself am running a pair of Dell QHD displays in a daisy chain using either a USB-C to DP cable or Mini-DP to DP cable to connect my laptop to the first display, depending on which laptop I'm using.
_mac
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April 21st, 2020 07:00
@jphughan Thanks for your response and the isight. It makes sense.
i spoke to apple again and they still saying it should work and that limitation is only applicable to apple usb c devices. This is so confusing.
If theres no option fir me to mst with my mac this is what i am thinking for my manageable mac and windows setup
mac:
mDP to DP cable to U2520D#1
hdmi to U2520D#2 (so i dont block the other DP port)
windows:
mDP to DP cable to U2520D#1
DP from U2520D#1 to U2520D#2
is this feasible or is there a better way to do this?
jphughan
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14K Posts
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April 21st, 2020 09:00
@_mac Apple is still saying WHAT should work, exactly? If they're still claiming that you should be able to use a Thunderbolt 2 source port plus a Thunderbolt 2 to Thunderbolt 3 adapter on the display end in order to send a regular DisplayPort signal into a regular USB-C (non-Thunderbolt 3) display input, then I don't know what to tell you other than that they're wrong. If the display had an actual Thunderbolt 3 input and was backward compatible with Thunderbolt 2, then yes that likely would work -- and maybe since Apple's own displays use Thunderbolt, they might simply be assuming that Dell's displays do as well, but as of this writing Dell has no Thunderbolt displays. Or maybe they just figure that a "raw" DisplayPort signal from a Thunderbolt 2 port running in "Mini-DisplayPort mode" can be sent into a USB-C input, but here again while I'm not 100% on this, I'm fairly sure that isn't the case. A USB-C source port on a laptop can be used to send a raw DisplayPort signal, but I don't think a USB-C input on a display can receive one; I believe when there's a USB-C input on the display side, it must perform an actual USB-C negotiation with the host in order to run "DisplayPort Alt Mode", and neither a Thunderbolt 2 port nor a Mini-DisplayPort port would perform that negotiation. The reason I suspect this is because I have a USB-C to DP cable myself, and unlike every other cable on my system, it is detected as an active device on my PC. It actually shows up in Device Manager as a USB-C Video Cable. That to me indicates that the system is performing a USB-C negotiation with an active chip in that cable to run DisplayPort Alt Mode from the USB-C port into that chip, and then that chip transmits a "raw" DisplayPort signal for the DisplayPort input on the display I use with that cable. But again, that involves some intelligence that seems to be lacking/impossible in your setup.
I also seriously question whether a Thunderbolt 2 to Thunderbolt 3 adapter can even be used as a Mini-DisplayPort to USB-C adapter, i.e. when there's no actual Thunderbolt signal involved, but I'm not sure about that.
Anyhow, yes the display setup you proposed would work, although it will mean you'll have to toggle the input on Display #2 back and forth.