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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.

  1. Should turning off PC be recognized by monitor as unplugging HDMI or not? Which behavior is correct, according to HDMI standards or other standards?
  2. 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.

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  • The difference is probably that one PC does not recognize the monitor. See link.
    – harrymc
    Commented Jun 13, 2019 at 20:27
  • @harrymc I don't think so – in both cases it is recognized by GPU drivers as DELL U2414H.
    – inrobert
    Commented Jun 14, 2019 at 20:43
  • I can only guess about the difference, but have you installed this driver?
    – harrymc
    Commented Jun 14, 2019 at 20:48
  • I doubt it has anything to do with drivers because the difference is after the system is fully shut down, so software won't make any difference. AFAIK monitor driver is only inf file which tells the monitor name to Windows device manager. I have those names correct in both cases – Windows detected it, I can't remember installing drivers. As the difference is in monitor reaction after system shutdown I guess it is electrical difference in communication with different GPUs. I wonder which reaction is correct and why.
    – inrobert
    Commented Jun 15, 2019 at 20:51
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    @Christopher Hostage In no case the monitor itself was off. It's about two Dell U2414H monitors different reactions to PC full system shutdown (or killing video output from GPU). In last 4 paragraphs I did analyze the situation when PCs (not monitors) were disconnected from electrical outlet. Even then one of the monitors "knew" the HDMI cable is connected to other device, even though this other device was disconnected from electrical outlet. Only physical disconnection of HDMI cable caused the "No HDMI Cable (...)" information. And I wonder how this works, mainly from electrical stand point.
    – inrobert
    Commented Jul 24, 2019 at 22:08

2 Answers 2

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HDMI connectors do include a hardware Hot Plug Detect pin (pin 19) that is used to electrically signal a connection. However, according to the HDMI specification v1.3a, it only tells the HDMI Source (PC) whether the Sink (monitor) is connected and able to provide its identity information, not the other way around. Interestingly though, it can potentially work even when the monitor is powered off, using power provided over the connection from the PC.

The PC does provide 5 volts to the monitor on connector pin 18, which the monitor can use to detect that the PC is present. However, it is only required to be present when the computer is signalling, so an absence of the +5V does not necessarily mean that the cable is unplugged, although it is probably a good indicator. I would expect that the loss of the +5V input is probably what causes the monitor to say that the cable is unplugged.

If your monitors are identical, then the different behaviour is either caused by your GPUs or their Windows drivers. It is likely that the monitor that goes straight into power saving mode is sent a command telling it to do so as part of the Windows (or GPU) shutdown sequence. The HDMI spec also includes Enhanced Display Data Channel, a signalling system that allows the computer to send Monitor Control Command Set (MCCS) commands to the monitor. The MCCS standard includes commands to switch off the monitor or put it into standby or suspend mode.

To my mind, going directly into Power Saving mode is the better response, because it saves more energy. The message about "No HDMI cable" is technically incorrect, if you haven't physically removed the cable. However, the HDMI specification itself doesn't obviously cover this behaviour. The Energy Star Displays specification v8 does though. It says that compliant monitors "shall automatically enter Sleep Mode or Off Mode within 5 minutes of being disconnected from a host computer". I guess that means that either behaviour is acceptable.

If you want to check whether it is the physical connection or the shutdown sequence that drives the behaviour, try physically unplugging the HDMI cable, rather than shutting down the PC. You could also try swapping the monitors between the PCs, to check whether they really are identical.

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  • Very informative.Unfortunately it's hard to swap my monitors as they are in different locations. I did further investigation with DDC/CI option and edited my question providing results. Is it possible that even with DDC/CI disabled in monitor the GPU is sending "go to standby" command to the monitor?
    – inrobert
    Commented Jun 20, 2019 at 19:33
  • @inrobert Did you try isolating the cables as being different?
    – T.J.L.
    Commented Oct 17, 2019 at 17:02
  • @ T.J.L. I did test cables in different scenario: one DELL U2414H monitor connected through HDMI to DVB-C decoder. I did verify what monitor shows when I am turning off decoder. With one cable (no name) – "Entering Power Save Mode" information, with two other cables (no name and Arkas PHQ-15) – "No HDMI Cable. The display will go into Power Save Mode in 5 minutes" information.
    – inrobert
    Commented Oct 20, 2019 at 13:29
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Many years lapsed but this problem remained.

The root of annoyance is DELL itself, since they economize on cables to be attached.

Just bought a new DELL monitor to replace the previous (still working without any issue but it is already 9 years old...) and ran into this bloody power save issue. Spent hours on internet wisdom, reset everything, refreshed drivers, etc.

The final remedy was purchase of a new HDMI cable (nothing special, cost: only 7 bucks). Apparently the one I have found in my drawer is designed for one of the previous HDMI versions, while the new monitor has a socket for HDMI 1.4.

From now on, there is no more waiting time before monitor can enter power save mode.

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  • In my case both of my cables attached to monitors are HDMI 1.4.
    – inrobert
    Commented Oct 20, 2019 at 13:36
  • Newer/different cable worked for me on a Dell UltraSharp U2415 paired with an Inspiron 3670. "Bad" cable was advertised as "High Speed" on the packaging, FWIW.
    – genpfault
    Commented Jan 19, 2020 at 16:35

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