Black Screen on External Display After Update to Catalina

[kitjv26]

After updating my iMac27 to Catalina, the screen on my external NEC display is black. I tried re-setting the SMC (system management controller), as well as rebooting the external display. Still the screen remains black. Can someone kindly offer some help. Thank you so much.

[chscag]

Not enough information.

Which iMac 27"? And how do you have the NEC attached, which port and adapter? NEC model?

Apple has enhanced security with the implementation of Catalina and many external devices are affected which may include your monitor.

[kitjv26]

iMac 27, Retina 5K, 2017. NEC display is attached via Thunderbolt. NEC model is MultiSync P221W.

Thank you.

[MacInWin]

Does it appear in System Preferences/Display/Arrangement?

[chscag]

Thanks for posting the additional info. I have the same model iMac as you have but I do not use an external monitor nor do I have one to test with.

I suspect you're using an adapter since your NEC only has DVI and VGA inputs. Are you able to try connecting it another way? Or perhaps try another adapter/cable?

[kitjv26]

Does it appear in System Preferences/Display/Arrangement?

No, it does not. The external NEC display definitely powers on, but the screen remains black.

[kitjv26]

Thanks for posting the additional info. I have the same model iMac as you have but I do not use an external monitor nor do I have one to test with.

I suspect you're using an adapter since your NEC only has DVI and VGA inputs. Are you able to try connecting it another way? Or perhaps try another adapter/cable?

The connection is a DVI to Thunderbolt. I have no reason to think that the connector is the issue since it worked flawlessly with both the High Sierra & Mojave OS. But when I first launched Catalina, the screen on the external display went black. Also, I am having other unrelated issues since installing Catalina.

[chscag]

Actually the connection is Thunderbolt to DVI not the other way around. I wasn't suggesting the connection is bad, what I was suggesting is to try connecting it another way (perhaps to the VGA input on the NEC) or substitute another cable. That would at least eliminate any cable or adapter problem. You could have had a coincidental failure when you installed Catalina.

Do you have any kind of backup that you can revert back to Mojave with? Time Machine? clone?

[kitjv26]

I will try the VGA input on the NEC. I will have to get a cable first. I do have a backup external drive which I tried using as my startup drive since Mojave is still mounted as the OS. I was surprised that the screen on my NEC display continued to be black. I rebooted both the NEC & the iMac to no avail.

[ferrarr]

Was the display connected when you did the upgrade? I usually disconnect devices when doing the upgrade.

I would try an SMC and NVRAm reset.

SMC, How to reset the System Management Controller (SMC) on your Mac - Apple Support
NVRAM, Reset NVRAM or PRAM on your Mac - Apple Support

[kitjv26]

Yes, the display was indeed connected to the iMac when I did the upgrade. I reset the SMC & NVRAM several times with no luck. . Thanks for the suggestion. I am currently working with a senior advisor at Apple to help resolve this issue.

[ferrarr]

When you did the resets, was the display cable disconnected from the iMac?

[kitjv26]

When you did the resets, was the display cable disconnected from the iMac?

No, the cable was still connected. Should I to the resets again with the cable disconnected & then reconnect?

[ferrarr]

No, the cable was still connected. Should I to the resets again with the cable disconnected & then reconnect?

It won't hurt to try it?

[kitjv26]

Sometimes the Mac gods work in mysterious ways. Having disconnected the external display from my iMac as I searched for a cable to connect it to my MacBook Pro, the display remained disconnected for about 45 minutes. Having no luck in locating a cable, I reconnected the display to my iMac. After about 2 minutes, the display was recognized & was working again. Out of the clear blue.

Thank you for your help! Much appreciated.