On Sun, 29 Jul 2018 21:22:01 +0200, Axel Berger wrote:

>> until the sending end has had a chat with the far end
>
> How does an unpowered peripheral talk or listen?

I wondered if anyone would spot that Catch 22.  B-)

IIRC every few seconds the sending end applies a low voltage and
current limited pulse of power, not enough to damage anything but
enough for the far end to say "hi I'm here". This might just be by
changing some chracteristic of the line rather than an ASCII encoded
data stream. When the sender gets the correct response it starts to
supply the minimum and a real negociation is initiated. Once up and
running the load has to ensure it takes a minimum about of power,
otherwise the sender will shutdown.

--
Cheers
Dave.

--- SoupGate-Win32 v1.05