Unknown serial device/protocol

Question

My light switches at home are connected to a circuit board. The light switches themselves are push buttons connected to it. The circuit board is in turn connected with 3 wires (black, yellow, red) leading to another circuit board controlling an array of relays which switch the mains power to the lights. All circuit boards are connected to eachother with those 3 wires.

With a multimeter I found black to be GND, yellow around 13.3 VDC (it dips to 13 when push button is pressed), and red a steady 13V.

I couldn't find any serial protocols with 3 wires and those voltages. I thought I could search the correct adapter or circuit board needed, to ultimately hook up a raspberry pi / arduino to those 3 wires and send whatever serial data to control my lights.

I've looked into logic analyzers, osciloscopes, icsp, avr's, ... I need a little guidance in what the next step should be.

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Answer 1

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 – Schematic created using CircuitLab}