External displays problem

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Level 1

10 points

External displays problem

Hello, the connection to my two external 27" displays from my iMac (Retina 5K, 27-inch, 2017) seems to have failed. The built-in display is A-OK. My two external 27" displays, (Asus PB278Q), are connected by cable from the iMac's Thunderbolt ports to the monitor's display ports. The displays have been functioning normally for two years. Now neither display will show any signs of being connected to the iMac, and they don't show up in the iMac's Displays control panel. I updated to Mojave 10.14.6, reset the pram, and reseated the cables but unfortunately no joy. Any ideas are welcome.

Thank you.

Posted on Sep 4, 2019 10:32 AM

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Answer 1

Best reply

den.thed

Level 9

77,248 points

Sep 4, 2019 12:56 PM in response to Mac2theCore

Perhaps the Thunderbolt port protection has tripped off.

Disconnect both external displays from the iMac and reset the SMC,

as per > How to reset the System Management Controller (SMC) on your Mac - Apple Support

only wait in step #3 for 60+ seconds instead of 15 seconds

Choose Apple menu > Shut Down and wait for your Mac to shut down.
Unplug the power cord.
Wait 60+ seconds instead 15 seconds
Plug the power cord back in.
Wait 5 seconds, then press the power button again to turn on your Mac.

at that point do another NVRAM reset and then reconnect the displays one at a time.

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Answer 2

Mac2theCore Author

Level 1

10 points

Sep 6, 2019 1:47 PM in response to Mac2theCore

I had my two external monitors connected to my iMac's USB-C (Thunderbolt) ports, where the monitor ends of the cables were "Display Port" type connectors. After a Mojave system update and an Apple firmware update on my 27" iMac, I consequently lost connection to the two external monitors.

The Fix: I had a USB-C (Thunderbolt) to HDMI cable stashed in my cable cabinet. Replacing the Display Port cables with those HDMI type cables restored comm from my iMac to my two 27" monitors.

The video cables I used are: Monoprice USB Type C to HDMI 3.1 Cable - 5Gbps, 4K@30Hz, Black, 6ft.

Monoprice Part# 24441

$22.24 ea.

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Answer 3

ProbsRus

Level 4

1,126 points

Sep 4, 2019 12:56 PM in response to Mac2theCore

Your post indicates that the displays previously worked fine. Did you change anything with your iMac prior to the issue occurring? Did you install any software updates? (which ones?) Did the update to Mojave 10.14.6 occur just prior to the issue, or did you install the upgrade after the issue occurred? Did you move the iMac and/or displays, or change out any of the cables? Did you make any changes to the Asus display OSD settings?

Which adapters/cables are you using to connect the displays? Are you connecting to the HDMI, VGA, DisplayPort, or DVI connector on the displays?

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Answer 4

den.thed

Level 9

77,248 points

Sep 4, 2019 1:49 PM in response to Mac2theCore

Do you have another Thunderbolt drive or device that you can test?

If not and/or other TB devices are not working as well, then you need to make an appointment at your local Apple Store or Apple authorized Service Provider and have it checked.

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Answer 5

Mac2theCore Author

Level 1

10 points

Sep 4, 2019 3:51 PM in response to den.thed

I just reinstalled Mojave too but no joy. I have an authorized Apple repair shop about 30 mins drive from here. I'll likely pack up the lot and hull the stuff down there. I still have the use of my iMac, just not the additional screen Real Estate. Thanks for your help folks.

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