iMac 2020 "forgets" external monitor episodically

[Eric Brody]

I have an iMac 2020 with a 4GB SSD, 64GB RAM and an AMD Radeon Pro 5500 XT 8GB graphics card.. I have used an NEC 271W since 2013 with my former Mac Pro 2013.  It is connected to the iMac via the OWC Thunderbolt dock via the Display Port on the dock. Most of the time when the iMac sleeps, both monitors awaken. However occasionally the NEC stays dark. The only solution is an annoying need to reboot. When I go to "settings," "displays" both monitors usually appear but when the NEC stays dark, the iMac fails to show the NEC display; it's just not there. I read Apple's suggestions about holding the option key with "gather windows," to get to "detect displays," but nothing happens. Variations of this problem have been on various fora since at least 2015.

Is anyone else suffering from this first world problem or am I alone in my frustration?

Thanks.
Eric

[digitaldog]

Tried resetting SMC?
https://www.howtogeek.com/312086/how-and-when-to-reset-the-smc-on-your-mac/

[Eric Brody]

Thanks Andrew, tried it. Yesterday all was well, today, not so much. Appreciate your thoughts.

[sf]

I've seen this too. Turning the "missing" monitor off and on before opening the Displays panel of System Preferences sometimes helps.

S

[Eric Brody]

Thanks sf, tried it and it was not successful. It was doing fine for about three days and then... hit a key, the iMac screen came on but not the NEC.
I suspect it may have something to do with my backup system with Carbon Copy Cloner and the machine awakening to do backups but really have no clue.

[degrub]

My wife’s 2019 MBpro has the same issue with the Caldigit TB3 dock and a Dell screen. Apple replaced the interface board for the LHS TB ports once. Seemed to help for a while, at least a more consistent set of actions to get the sync to happen.  However, after a few OS updates, back to the original randomness. Funny thing was a cheaper dock was more reliable in using the power off the dock and screen technique and then powering on in specific order. Sometimes saw a difference if using hdmi instead of DP.

My guess is there is a hardware timing issue between the three devices. Of course none of the vendors feels it is worth a lot of engineering time to figure it out since it is not always consistent failure and each’s hardware/firmware meets the interface standard specs 😉. Guess we have to buy gear from only one vendor, right ?