Data over power for Arduino

I would need a circuit that can be added to an Arduino board to allow it to send and receive data over its own power supply wires. You can see a typical example in the attached file. An independent power supply provides power to several arduino boards and shields. I need a circuit that can be added to these boards to allow data to be sent between them over the power connections. It is not possible to add extra wiring between the boards, The key points are: 1. The supply is 12v d.c. 2. The connection at the power supply cannot be accessed so you cannot modulate the supply directly at source. 3. The data rate does not need to be high speed. Usual UART speeds are fine. 4. The data protocol used is not important, you can choose what is best. 5. The data communication must be able to send and receive but there is only one master in the system and that will initiate all requests to the other boards and all communication is to or from the master. 6. You can use existing shields, components etc if something already exists to help do this. What are the deliverables: 1. Proposal of the design (brief description about how it will work) 2. Schematic of the design in Eagle CAD or another agreed tool (a board layout is not needed) 3. A demonstration that your design works. You will need to build a prototype of your circuit and show it working between two Arduino boards.