Irrigation controllers come in many different versions. The smallest type of irrigation controller is used in gardens and has perhaps 4-9 outputs to individual solenoid valves
The next size irrigation controller is known as ‘light commercial’. This type of irrigation controller has 15-48 outputs. Currently in the USA, most of these are what is called ‘multi-wire’; that is, an individual wire goes to each solenoid together with a common. When the irrigation controller wants to turn on a valve, it energizes that wire with 24VAC.
In Europe and for that matter, most of the rest of the world, instead of individual wires to each solenoid, a common 2wire cable passes each valve. At the valve, an electronic circuit called a decoder, or 2wire decoder, is connected to the 2wire cable with its outputs connected to the solenoid. The 2wire decoder has an address in it, each being unique. The irrigation controller, as well as supplying 24VAC, also encodes on and off commands for a particular address onto the same 2wire. All the 2wire decoders hear the same commands, but only the 2wire decoder with the matching address responds by turning its solenoid output accordingly.
This type of irrigation controller is called a decoder irrigation controller or decoder irrigation control system.
In the early days of 2wire decoders, about 30 years ago, they were very prone to lightning damage and water penetration. Using regular lightning grounding rods and modules along the 2wire path, most manufacturers have circumvented this. TWL Irrigation has for 20 years used an alternative lightning protection scheme that does not require frequent grounding. This saves a lot of money and labour.
The latest product from TWL Irrigation, is a decoder, so small, it is encapsulated into the top of a solenoid. This is called a “Decoder In A Solenoid” or DIAS.