A processor is connected by a bus to a plurality of station modules, each station module being connected to a corresponding valve and having a micro-controller. The processor executes a stored watering program and controls the station modules in accordance with the program. Messages are bi-directionally sent and received over the bus that are generated by the processor and the micro-controllers. Each message contains an identification field with an instruction that describes a meaning of the message. The processor and the micro-controllers each interpret the messages received thereby and perform predetermined functions in response thereto.