On 6 Jan 2021 10:44:48 GMT
Tim Streater  wrote:


>
> OK. So what device is providing the power to the HDMI cables, and
> why? In my setup I have a Dell display connected via its DVI (?)
> socket and an adapter to a small HDMI switcher that can route the
> display either to my Mac Mini or to the Pi.
>
> I did notice that, after disconnecting the HDMI from the Pi, and
> reconnecting it (all this with the Pi's power adapter unplugged from
> the mains), the Pi did not power up again. So whatever is on  teh
> HDMI is not a strong power source.
>

HDMI is Smart, God help us. You cannot generally power up an HDMI
source, then connect it to an HDMI sink, because the source will not
generally supply signals to an open cable. It wants to see something
alive on the other end before it will power up its output drivers. It's
all about energy saving.

Sadly, some HDMI sinks do the same. I found that my laptop could not
supply an HDMI signal to a particular Grundig monitor, because neither
end was willing to accept that the other existed. I tried many attempts
at powering them both up with varying delays between them, but could
not find a window in which both could see the other at the same time.
Both devices worked correctly with other HDMI devices.

The same will be true of HDMI 5 Volts. Not all devices do it at all,
and those which do will generally want to see something already
connected to the cable before they turn it on. Some cables do not have
the 5V line. There are several HDMI standards, and many cheap cables.
Only the most recent, for example, have an Ethernet connection.

--
Joe

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