How exactly should HDMI cable detection work?
I am using 2 the same monitors – Dell U2414H (all the same revision – A03) connected to 2 PCs by HDMI.
Each monitor is connected by HDMI to a different PC (and no other devices) with different GPUs. I turn off both of these PCs using full software shutdown from Windows (no hibernation or Fast Startup).
- After I turn off one PC on the monitor appears "No HDMI Cable. The display will go into Power Save Mode in 5 minutes" information.
- After I turn off the other PC on the monitor immediately appears "Entering Power Save Mode" information and it is immediately in Power Save Mode. It stays in this mode even after I turn off the PC's PSU. Only physical disconnection of HDMI cable (from GPU or monitor itself) causes the "No HDMI Cable. The display will go into Power Save Mode in 5 minutes" information.
Looks like after system shutdown in the first case the monitor "thinks" the HDMI cable was disconnected, while in the other case monitor "knows" the cable is still in.
- Should turning off PC be recognized by monitor as unplugging HDMI or not? Which behavior is correct, according to HDMI standards or other standards?
- How basically HDMI hot plug detection works electrically? Is monitor sending electrical signal to check if it is connected and the signal comes back to monitor? Or GPU is sending a signal to monitor to "inform" it it's connected?
After JRI answer I did further investigation. It was suggested that PC can send a command to the monitor to go in standby mode through Enhanced Display Data Channel (E-DDC). Dell U2414H manual says it uses DDC/CI standard (similar but older than E-DDC standard - according to wikipedia). It was enabled in both of my monitors, so I tried to disable it in OSD in both monitors to see if it will change monitor reaction on PC shutdown. It turned out that disabling DDC/CI didn't change anything.
Further investigation:
In both PCs there's setting in Control Panel > Power options > Edit Plan Settings > Turn off the display > 15 minutes. AFAIK this option kills video output from GPU after 15 minutes.
Each monitor reacts the same way as in full system shutdown situation – one is showing "No HDMI Cable. The display will go into Power Save Mode in 5 minutes" information, the other is showing "Entering Power Save Mode" information and is immediately going into Power Save Mode.
Even further investigation:
I tried turning on each monitor with the HDMI cable connected to PC, but the PCs were disconnected from electrical outlet. Even then results were the same – one monitor is showing "No HDMI Cable. The display will go into Power Save Mode in 5 minutes" information, the other is showing "Entering Power Save Mode" information and is immediately going into Power Save Mode.
So in the second monitor only physical disconnection of HDMI cable (from GPU or monitor itself) causes the "No HDMI Cable. The display will go into Power Save Mode in 5 minutes" information.
Conclusion is that monitor itself can recognize that HDMI cable is connected to the monitor and GPU on the other side of the cable, even without any signal coming from the GPU.
How is this working (taking aside that only one of my Dell U2414H monitors is recognizing HDMI cable this way)? I guess the monitor is sending an electrical signal that is traveling through HDMI cable (probably through Hot Plug Detect line) and recognizing that there is something connected both sides of the cable.