Monitor not waking from sleep after 12.3 upgrade

Answer: Change your cable from "standard" to "high speed" - see below for explanation.

There are loads of articles on the net saying that their external monitor stopped being recognised after a software upgrade ("No Signal" error showing on the monitor).

There are so many upgrades to the operating system these days, it seems like anything which goes wrong can be blamed on the latest OS upgrade. Probably a fair chunk of new problems are indeed due to upgrades. Some might be other causes.

In my case I think it was a combination of two causes.

1) The software upgrade, AND

2) The cable that I was using to connect the Mac (an iMac Pro) to the external monitor (a Samsung S27D590).

The monitor has a maximum resolution of 1080p.

I tried a lot of things, especially fiddling around with the settings in Apple logo>>System Settings, Displays, and in Desktop & Screensaver. Restarting the Mac sometimes worked.

After a lot of fiddling around, I came to the theory that the problem might have something to do with the cable connecting the the two devices (USB-C to HDMI).

Initially I found the one way to resolve the problem was to disconnect the USB-C to HDMI cable and then re-connect it. - Very irritating fix! Finally I changed the USB-C to HDMI cable for a new one. The problem has not occurred since I did that.

In conclusion, I think the problem was that the cable connecting the monitor to the Mac wasn't working with the latest software upgrade on startup.

Why did changing the cable work?

Answer 1: Cable Quality: There are loads of "inferior quality" cables, particularly ones bought cheaply from China. Such cheaper cables are more likely to suffer from a poor connection issue (e.g. a loose or broken strand of wire inside the cable, or the connecting ends). Also the wires may not be pure copper. To save money, the manufacturer may have used an amalgam of copper and aluminium, resulting in signal loss - especially for longer cables.

Answer 2: If you bought a cheaper cable, or if you bought it a long time ago, it is unlikely to be up to the latest standards.

Cables are grouped into one of four categories, standard, high-speed, premium high-speed, and ultra-high-speed. Standard is the slowest of all the cable types at around 4.9 Gbps. Normally this should suffice to handle 1080p but not much else. Given that newish Macs can output signals at much higher resolution than 1080p, it is possible that the latest software attempts to start out in the the higher resolution mode. A "standard" cable can't handle that, and therefore can't report back that the monitor is only 1080p.

Nowadays, most shops only sell "High-Speed" cables or higher. These HDMI cables will support at least double the bandwidth of a standard cable and can handle 4K video with ease. If you buy a new cable from a reputable shop, and check the specification is "high-speed", or "ultra-high-speed", you should be fine. In other words, do not buy the cheapest cable.