Dell U2415 on Mac Pro (late 2013) Blinks then Black

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Level 1

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Dell U2415 on Mac Pro (late 2013) Blinks then Black

I've added a Dell U2415 monitor to my late 2013 Mac Pro as a second monitor. The primary monitor is an NEC 2070NX connected using a Thunderbolt to DVI adapter. Resolution of the NEC is set to the default (1600 x 1200 @ 60 Hz). The Dell is connected using its included mDP to DP cable. Its resolution is also set to the default (1920 x 1200 @ 60 Hz). Both monitors are on the same Thunderbolt bus. Everything was fine for several hours until I put the Mac to sleep. Shortly after waking the Dell's screen began flashing and then went black. Performing a hardware reset of the Dell took care of the issue for a while, but it recurred after the next sleep-wake cycle.

Dell support says that since they haven't tested this particular monitor with Macs they can't supply any suggestions. Interestingly, however, there are reports of similar behavior with Windows machines. Searching the web reveals people successfully using this monitor with this model Mac. I'm looking for suggestions from anyone who may have encountered the same issue and resolved it.

Mac Pro (Late 2013), OS X El Capitan (10.11.4)

Posted on Apr 18, 2016 12:17 PM

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Answer 1

lllaass

Level 10

210,411 points

Apr 18, 2016 12:22 PM in response to Warsaw Jack

Have you tried connecting the two monitors to two different buses as recommended by Apple?

Attach displays to different Thunderbolt busses when possible (see the figure below). Don't attach more than two displays to any bus. This means that if you use the HDMI port, be sure to then only use one of the bottom two Thunderbolt ports (Bus 0).

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Answer 2

Warsaw Jack Author

Level 1

12 points

Apr 18, 2016 12:30 PM in response to lllaass

Actually, I have tried that. As a matter of fact, that was the original configuration when the problem was first experienced.

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Answer 3

Warsaw Jack Author

Level 1

12 points

Apr 20, 2016 7:13 AM in response to Warsaw Jack

I seem to have resolved this problem and share this information as possible help to others as there are quite a few posts on the web concerning it with various Macs.

The Dell U2415 ships from the factory with DP 1.2 support disabled. Given Thunderbolt's backwards compatibility I wouldn't expect this to pose a problem. Nonetheless, I changed this setting to enabled and that seems to have addressed the issue. I've not experienced the blinking then black problem since. There's one other thing that I noticed that may be related. Prior to changing this setting although the Dell would go dark when the Mac entered sleep, it never entered power saving mode (as evidenced by no message indicating this and the power indicator remaining a steady "on"). Since changing the setting, when the Mac enters sleep mode the display goes dark, pops its sleep mode dialog and the power indicator begins a slow pulse as documented in the manual.

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