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.