An irrigation control system and a related method for controlling a large number of valves in accordance with a plurality of schedules of instructions provided by the user of the system. Schedules of instructions may be created or modified during normal operation of the system by utilizing a variety of command sequences on a keyboard and display device, and schedules may also be reviewed or monitored during operation. The control system includes apparatus for processing a plurality of digital inputs by means of which the conditions of sensing devices located in the field are monitored and can be used to condition execution of the schedules of instructions. The control system is disclosed for use with a two-wire irrigation system in which on/off control signals are encoded into a power signal for transmission along a pair of wires to which decoding units are connected. The control system includes an input/output service module which transmits on/off control signals to the decoding units repeatedly, as rapidly as the two-wire system will permit, to ensure that the valves remain switched in the manner intended.