Disclosed embodiments of the invention include methods and systems for upgrading an existing irrigation system to increase its sensing and control capability without requiring extensive rewiring. A controller module is installed between an irrigation controller and a zone valve and physically proximate to the irrigation controller without disturbing most of the existing wiring between the irrigation controller and the zone valve. A field module is installed between the controller module and the zone valve without disturbing most of the existing wiring between the irrigation controller and the zone valve. The controller module and field module are the communicatively coupled primarily using the existing wiring. The controller module may encode commands transmitted to the field module and/or decode encoded data transmitted from the field module. The field module may encode data transmitted to the controller module and/or decode encoded commands transmitted from the controller module.