Curious Cable Question

[krs]

....... the original will work in an hour or so after the external display is plugged back into both the power source & the iMac.

Can you explain exactly what happens step-by-step
It sounds as if the iMac takes an hour or so to detect the external display and then it works properly???

Did you try various combinations of the sequence when you plug the cable in vs powering up the iMac and/or powering up the monitor.
I have not used an external display on a Mac for a long time, but I remember years ago when I did, I had to power up and plug in the cable to the monitor in a specific sequence.

 

[MacInWin]

Just one comment for kitjv26. Just because it looks like "just a cable" does not mean there is no embedded chip in the connector. It could be that either there is no chip, and the system expects one, or there is a chip, but it's incompatible. More and more cables these days are "intelligent" cables rather than just a length of conductor.

 

[kitjv26]

Just one comment for kitjv26. Just because it looks like "just a cable" does not mean there is no embedded chip in the connector. It could be that either there is no chip, and the system expects one, or there is a chip, but it's incompatible. More and more cables these days are "intelligent" cables rather than just a length of conductor.

Yes, that is something that I have learned since dealing with this issue.

 

[kitjv26]

Can you explain exactly what happens step-by-step
It sounds as if the iMac takes an hour or so to detect the external display and then it works properly???

Did you try various combinations of the sequence when you plug the cable in vs powering up the iMac and/or powering up the monitor.
I have not used an external display on a Mac for a long time, but I remember years ago when I did, I had to power up and plug in the cable to the monitor in a specific sequence.

Here is a compendious run-down of this issue. I had used the NEC external display with my iMac (and the same cable) since 2017. Never a problem. Immediately after upgrading to Catalina, the iMac restarted & the external display went "black" as if it was not detected. I tried a variety of steps including: resetting the NVRAM & the SMC, power cycling the iMac, as well as the external display. Having no success bringing the display back to life, I went to bed that evening. The next morning I awoke the iMac from sleep & to my surprise the external display was up & running.

All was fine until I had to restart the iMac for some reason or other & the external display again went black. Some experimentation indicated that if I unplugged the external display from both the power source & the iMac and then re-plugged them, within an hour the display would be up & running again. As long as I do not restart the iMac or disconnect the display cable from the USB-C port on the iMac, the display works fine. If I simply unplug the display from the power source (but leave the cable connected to the iMac), the display will reboot; no problem.

As a side note, when the display is functioning perfectly & I navigate to Apple Menu > About This Mac > System Report > Thunderbolt, on both USB-C ports ("no device is detected".

That's pretty much it. I hope there is some glimmer of insight in my explanation. Thank you kindly!

 

[MacInWin]

What is in System Preferences/Dispays? When you get there, does a separate box open on both the iMac and the external? on the Arrangement tab, do you see both displays?

 

[krs]

..........when the display is functioning perfectly & I navigate to Apple Menu > About This Mac > System Report > Thunderbolt, on both USB-C ports ("no device is detected".

What does it show if you look under Apple Menu > About This Mac > System Report > Graphics/Displays?

 

[krs]

@kitjv26 - Can you post a link to the cable you are using now - the original one.

Looks like other people have pretty much the same problem that you have, for instance:

Has anyone gotten this cable to work with macos catalina?
Answer:This worked great on Mojave, when upgraded to Catalina it rarely detects the monitor when I first start up - but with no changes it will often 'appear' later in the day. Does not make any sense at all. Turning off/on restarting reset NVRAM/SMC etc does not seem to have any real effect. Don't think it's a cable issue, as when it's working it works great.

Sounds to me that the problem is related to a marginal digital signal level.
I don't think there is a "fix" with the components you have now.
Either live with the present situation, ie wait a long time after rebooting the iMac, or try to find a cable from another supplier with a different chip set and where reviews and comments indicate that the cable will work with a 2017 iMac and Catalina and (ideally) with your monitor.

PS: You could also try flipping the cable around at the USB-C connector end to see if that makes any difference.
The USB-C connector is supposed to be symmetrical but that is only true if it is wired correctly. Honestly - I doubt that will make any difference - the long delay that you encounter before the monitor shows an image indicates that the iMac and monitor can't communicate initially which points back to a digital signal level issue.

 

[chscag]

Sounds more like another Catalina bug that Apple needs to squash. Sorry, but the more I read about the bugs and problems folks are having with Catalina the more it reminds me of two infamous operating systems from Microsoft: WinME and Vista. Anyone remember those?

 

[MacInWin]

Sorry, but the more I read about the bugs and problems folks are having with Catalina the more it reminds me of two infamous operating systems from Microsoft: WinME and Vista. Anyone remember those?

Charlie, I don't think Catalina is anywhere near that bad. I'm running it just fine on a mid-2015 15" MBPr with a Thunderbolt external display, eleven external drives, two printers and a Thunderbolt hub. Zero issues. I think there are more complaints about Catalina because of the huge shift in how it is installed and the abandonment of 32-bit software. I'm also wondering if the 32-bit impacts older intelligent cables? Going forward, if the tech companies continue putting intelligence in cables it's going make us all more careful about what we buy and where we buy it.

 

[chscag]

Hey Jake:

There's always a success story in every endeavor. Must be all that good living.

 

[MacInWin]

Nah, dumb luck. I will say that I tend to stay close to the Apple Ecosystem--my TB dock is from Belkin, I use an Apple Thunderbolt Display for an external monitor. I don't use too much exotic software, mostly mainstream (Office, Photoshop, stuff from the App Store) or very reliable (OnyX, DetectX Swift, etc). And I was cautious about jumping to Catalina. I have one 32-bit app that now has in hand the 64-bit driver they needed from the hardware vendor to update their application for scanning slides and promises an update soon. In the meantime, I'm not scanning slides. Most of the "hard" issues I've seen with Catalina have involved hardware/software that is a little more rare than anything I do. And I am willing to spend a bit more for cables/adapters from companies whose name and quality I trust.

 

[krs]

From what I have read, this seems to be the best TB3 to DVI cable there is right now. Works with displays where others didn't - they are also the only company so far that actually specifies the chip set they use.USB-C to DVI Adapter Cable - 6.6 ft. / 2 m - USB-C Video Adapters | StarTech.com Canada

However, as they say, the proof is in the pudding.
If you want to try it, make sure you get it from a place where you can return it if it doesn't work.
Amazon.com has it and I think you can return it postage-free if it does not work.

 

[kitjv26]

What is in System Preferences/Dispays? When you get there, does a separate box open on both the iMac and the external? on the Arrangement tab, do you see both displays?

Yes, there is a separate box on the iMac & the external display. And on the Arrangement tab, both displays are indicated.

- - - Updated - - -

What does it show if you look under Apple Menu > About This Mac > System Report > Graphics/Displays?

Under Graphics/Displays, both displays are there.

 

[kitjv26]

I think that I concur with krs regarding finding a cable that is compatible with both my iMac & Catalina. The idea of rebooting the external display & waiting until it decides to return to life is only acceptable until I find a permanent solution.

Thank you all for your input. I truly appreciate it.

 

[kitjv26]

From what I have read, this seems to be the best TB3 to DVI cable there is right now. Works with displays where others didn't - they are also the only company so far that actually specifies the chip set they use.USB-C to DVI Adapter Cable - 6.6 ft. / 2 m - USB-C Video Adapters | StarTech.com Canada

However, as they say, the proof is in the pudding.
If you want to try it, make sure you get it from a place where you can return it if it doesn't work.
Amazon.com has it and I think you can return it postage-free if it does not work.

Thank you for the heads-up! Will follow through.

 

[krs]

Yes, there is a separate box on the iMac & the external display. And on the Arrangement tab, both displays are indicated.

Under Graphics/Displays, both displays are there.

I assume in both these cases the external display is actually working - or is it?

 

[kitjv26]

I assume in both these cases the external display is actually working - or is it?

Indeed it is!

 

[MacInWin]

Next time it ISN'T working, look at those System Preferences to see if the monitor is there in that case. Could be useful information for troubleshooting.