Unknown serial device/protocol
Probably the PIC pulls down the voltage (which is pulled up by resistor to +12V [+13V as you measured it]) through an NPN transistor to make an open-collector shared serial bus.  The protocol could be anything - you'll need a logic analyzer to work out what it is.

It's probably something along the lines of:







simulate this circuit &ndash; Schematic created using CircuitLab