Read it again. How can a USB keyboard power up the system that supplies it
power in the first place?
I don't know about laptops, but many desktops can do this. There is
usually a setting in the BIOS to wake on keyboard. PS/2 and USB keyboards
will work.[/QUOTE]
And, many systems have Ethernet cards which support a "wake on LAN"
feature. Send them the correct "magic packet" and they'll initiate a
power-on and boot the whole system.
In all of these cases, the system's power supply has some sort of
low-power "standby" capability - usually either a trickle feed from
the battery (in the case of a laptop) or a separate low-current
switching-mode supply (in the case of a desktop or server system).
The standby supply keeps a small portion of the hardware powered up
even when the system is "off".
The Macintosh II was the first consumer-type computer I know of which
had this feature - the onboard battery energized a line in the ADB
(keyboard/mouse) bus cable, and the keyboard had a pushswitch to
trigger the main power-on feature.
I had a Mac II which was "haunted" - on occasion it would power itself
up when there was no one in the room. The problem turned out to be
due to some flux residue left on the PC board inside the keyboard,
plus the fact that the Mac was left in a cold room overnight. When
we opened up the room the next morning, and warm/moist air from the
rest of the house flowed into the room, enough condensation would form
inside the keyboard to create a current path through the flux
residue... and the Mac motherboard detected this tiny trickle of
current flow and interpreted it as a push on the "power on" button.
Cleaning the PC-board surface with a defluxer spray, and then coating
the traces with some clear nail polish, was sufficient to exorcise the
"ghost in the machine".