Here is a new little project I want to share with you. I'm now combining a Gardena Garden Sprinkling computer with a Xanura SAX15.

http://www.gardena.com/servlet/ProductD ... &langId=15
Handy about the Gardena Sprinkling computer is that it has a pulse input, once you close the input on the sprinkler it won't activated the scheduled program. I program the sprinkler every day to give water for lets say 20 minutes. Also I gather every day information from my domotica system like:
Rainfall
Temperature
humidity
etc
Depending on these results you can decide to water the plants or not.
If I don't want to water the plants I Send a A10 / X10 signal to the XANURA SAX15 module which closes the relays and deactivates the sprinkler.
This is a very robust solution because the Gardena Solar Sprinkler computer has a failsafe in it as well. Lets say the XANURA Command to close the relay is not received by the SAX15. The Gardena will start sprinkling and stops after the time you set it. The only inconvenience you have is that your garden is sprinkled. Of course if your powerline is alright this won't happen.

The Xanura SAX15 has two galvanically separated potential free inputs for transposing contact outputs on external devices into Xanura powerline commands. It also has one galvanically separated potential free relay contact output.
www.xanura.nl
Pieter Knuvers
www.bwired.nl Online House in the netherlands. Domotica, Home Automation.