The serial port remote control allows you to receive raw remote control commands over a serial port.
First, open the config app, and go to Misc. → Serial Ports to set up the parameters for the serial port (baud rate etc.).
Then go to the Remotes page, and add a new Serial Port remote, selecting the desired serial port.
Commands - e.g. PLAYER 1-1 START - must be sent as plain ASCII text, terminated with a CR character (ASCII 13) each.