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I have a new Macbook 16" running 10.15.3 OSx with an AMD Radeon Pro 5500M 4 GB for the graphics card and I'm unable to get the desktop to extend using just Display Port chaining.

The monitors I'm trying to chain together are a Dell P2719HC 27-inch (1920 x 1080) and a Dell U2415 24-inch (1920 x 1200). The 27" is a new monitor that I received with the laptop however the 24" I've had for several years and was using in a chained model with my Lenovo laptop that I'm moving away from.

I have tried the following configurations without any success and the best that I've been able to achieve is that the 24" mirrors the 27" but will never extend to it's own desktop.

Chained

MBP USB-C -> 27" USB-C (MST Enabled) -> DP -> 24" DP (DP 1.2 Enabled) = "Mirror"

MBP USB-C -> 27" DP (MST Enabled) -> DP -> 24" DP (DP 1.2 Enabled) = "Mirror"

MBP USB-C -> 27" DP (MST Enabled) -> DP -> 24" DP (DP 1.2 Disabled) = "Mirror"

MBP USB-C -> 24" DP (DP 1.2 Enabled) -> DP -> 27" DP (MST Enabled) = "Out of Sync Error on 27"

MBP USB-C -> 24" DP (DP 1.2 Enabled) -> DP -> 27" DP (MST Disabled) = "Out of Sync Error"

UnChained (Using two cables to drive them at the same time)

MBP USB-C -> 27" USB-C (MST Enabled) = "Extended"

MBP USB-C -> 24" DP (DP 1.2 Enabled) = "Extended"

What is causing the chaining to only allow mirroring?

** UPDATE ** (3/22 @ 13:27 PST)

I've attempted to go back to a know good configuration using the original two monitors that I used with my Lenovo setup to try chaining those two monitors together removing the 27" from the equation and even with that configuration the two external monitors will only mirror each other. It seems like the USB-C ports on the new MacBooks are not true Display Port 1.2+ compatible and requires individual connections to each external monitor that you want to extend your desktop to. What gives apple?

  • What happens if you try to change to change from mirror to extend in System Settings? Does your Mac even recognize the second display when chaining? – idmean Mar 22 at 18:34
  • @idmean doesn’t see the second monitor at all when they are chained – Aaron Mar 22 at 18:38
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macOS does not support MST (Multi-Stream Transport) daisy chaining, so you cannot get such a setup working with extended desktop at all. You can get mirrored displays working (i.e. both monitors display the same picture), but this is obviously not what you want.

You can get extended desktop working in a MST daisy-chained setup as you have by running Windows or Linux on the MacBook Pro.

As you have figured out yourself, you could connect both displays with extended desktop by using two separate cables that both plugin to the MacBook Pro.

If you want to use a single cable to connect multiple monitors in extended desktop on macOS, you'll need to use either Thunderbolt monitors (those support daisy chaining), or a Thunderbolt dock such as the OWC Thunderbolt 3 Dock or similar.

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  • Thank you for the detailed answer. One follow up I have is MST interchangeable with DP1.2+? Or are they two different technologies? – Aaron Mar 22 at 21:35
  • MST is just one of the many functionalities made possible by the DisplayPort protocol. It was introduced with DisplayPort 1.2, and basically multiplexes multiple signals into one. – jksoegaard Mar 22 at 21:39

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