Large commercial irrigation systems such as those used on golf courses or croplands use sprinklers, sensors or other components that are normally powered from 24 V AC power lines that can be several miles long and can serve many hundreds of components. Various systems have been proposed for powering and controlling the components of such a system with just two wires. For example, U.S. Pat. No. 3,521,130 to Davis et al., U.S. Pat. No. 3,723,827 to Griswold et al., and U.S. Pat. No. 4,241,375 to Ruggles disclose systems in which sprinkler valves along a cable are turned on in sequence by momentarily interrupting the power or transmitting an advance signal from time to time.
A problem with this approach is that it does not allow the operator to freely turn on or off any selected sprinkler or set of sprinklers at different times. This problem is typically resolved by providing separate controllers in the field to operate groups of sprinklers in accordance with a program stored in them, or transmitted to them by radio or other means. Alternatively, it has been proposed, as for example in U.S. Pat. No. 3,578,245 to Brock, to operate individual sprinkler sets from a central location by superimposing a frequency-modulated signal or DC pulses onto the 24 V AC power line.
All of these approaches are expensive. For example, a system with hundreds of sprinklers requires miles of expensive, heavy wiring to accommodate the current drawn by a large number of valves that may be watering simultaneously. Also, heavy use of D.C. current may cause electrolysis issues with electrical components.
One alternative to these traditional irrigation systems are two-wire power and communications systems, such as the system shown in U.S. Pat. No. 7,358,626, the contents of which are incorporated by reference. In such systems, two wires supply both A.C. power and digital control communications from a controller to a plurality of decoders.
While these A.C. power and digital communication systems generally work well, they also have several disadvantages. First, decoder circuitry that listens for communications are sensitive to power surges by lightning. Hence, expensive, external surge devices with ground rods must be installed at short intervals along the power and communication wires.
Second, some two wire power and communication systems use bursts of current by valve or sensor decoders to acknowledge or otherwise communicate with a central controller. These decoders relied on fixed resistor circuits to switch to a circuit path with a resistor between the common current and ground on the decoder. Since these resistor circuits resist at a fixed value, any reduction in the voltage causes a proportional reduction in the current pulse (e.g., due to Ohm's Law). Therefore if the current burst of the acknowledgement pulse is reduced sufficiently, the gateway 16 will not recognize it and therefore will falsely determine that the decoder did not receive a command. As decoders are installed further away from a gateway or central controller, the resistance of the power and communication wires becomes significant and reduces the voltage available across the decoder. Additionally, large numbers of decoders may also further reduce voltage and increase resistance on the power and communication wires. Hence, these systems effectively become limited in size and in the number of decoders.