Build a PCI or PCI-E card that takes an external signal to power a machine. The card will use standby 3.3v power from the computer to feed the circuits. When triggered, the card will ground the power switch pin on the motherboard, connected via a 2-pin header on the card, to power it up. When powered, a switched 3.3v pin will be used to “disable” the logic and prevent the external controller from shutting it down if accidentally triggered.
Another part of the logic can be used to trigger ACPI shutdown functions in the OS when the machine is on, also controlled from the remote device.
Connections to the card will be, 2x Cat5e Jacks or RS232 connectors in parallel for daisy chaining, a PCI or PCI-E connection, and the 2-pin triggered connection. An optional feature, 4-pin usb connection for 5v standby power if the card was mounted off the board. The card will be mounted on the case to access the I/O ports.
Connections on the external ports will include, the trigger wire and ground (signal input ranging 4v to 15v), a power on feed to sending device, some other signals are possible. Optionally, the cable can be used for 2 channels on a single cable, possibly more on a RS232 cable and a jumper pins for toggling the channel it responds on.
The rear panel will have a power and activity led for their respective functions. Power OK, System On, Activity on trigger wire. Internally, there can be a header to breakout these led signals for the chassis.