OpenSprinkler › Forums › Hardware Questions › 2-Wire › Reply To: 2-Wire
I too am very interested in 2-wire system since I have a lot of valves spread over a lot of area and don’t want to run lots of wires or multiple controllers. I have lots of trees that need irrigation, and in practical terms this means one valve per tree. The number of valves add up quickly.
First, a small amount of background on 2-wire:
2-wire is a generic term for systems that employ a single pair of wires to drive all valves in the irrigation system. The two wires carry 60 Hz 24 VAC power. Modulated onto the 24 VAC power is signal that implements a communications protocol designed to talk to a solenoid driver located at each valve station. Some implementations clip a portion of the AC waveform to indicate a digital ‘1’ or ‘0’. For example, clip one positive cycle of the 24 VAC waveform at half amplitude for a ‘1’, clip the negative half of the 24 VAC waveform for a ‘0’. The electronics in the solenoid driver demodulate the data riding on the 24 VAC power and provide switched power to the valve when that particular solenoid driver is addressed by the system. So the solenoid driver extracts a stream of command data and 24 VAC power. Each solenoid driver has to have a unique “station address” to enable each solenoid driver to be uniquely addressed. Some method must be provided to program the station address into the solenoid controller. Commercial systems uses a dedicated (expensive) programmer for this purpose. Typical systems can address hundreds of valves. Commercial systems cost many thousands of dollars.
I think the main problem with commercially available 2-wire systems is the high cost of components. It is ridiculous that a 2-wire valve adapter is more than $60 per valve (and often much more). Looking online, I see 4 valve solenoid driver for $240. That is nuts. The controllers themselves are over $1000. The solenoid driver programmer is $600.
Seems to me that if OS used an existing 2-wire protocol that would work BUT adapting an exiting serial protocol to OS plus a simple/cheap design for the valve solenoid driver would be straightforward – and avoid potential patent infringement For example:
1. Add a serial-to-modulated power interface to the OS controller hardware. Use whatever serial protocol makes sense – async serial as used in the RS-232 standard would work fine. Maybe this could simply be a serial-to-2-wire converter that could be added to the OD hardware as an option.
2. Design a solenoid valve driver. Analog electronics to demodulate the signal from the AC carrier on the 2-wire cable that runs to all valve drivers plus digital electronics to interpret valve commands. A small/cheap microcontroller with some non-volatile, reprogrammable memory plus a few analog components. Make it small, cheap, easy to assemble. Include LEDs for: 2-wire power detected, command detected, command-to-me detected, valve on/off. Users could then assemble and pot the board in a small plastic cup (or medicine bottle, or whatever). Low cost at the solenoid is key, i.e. low cost-per-valve-station.
3. Need to engineer the system for lighting protection. A 2-wire system could span many thousands of feet.
4. Add support to OS firmware.
Each valve driver would need to be programmed with a unique valve address. I think a good way to do this is to use a custom command for the driver that sets the address and have the OS firmware include a programming mode in which only a single solenoid driver is connected to the 2-wire interface. Also include a test function to check that the address was set.
I’d love to see this feature as it really makes sense for large systems spread across a lot of area.