Abstract: A security or automation system for domestic or business premises has plural units connected into a network by a communications bus, the units including one master unit and at least one user interface unit remote from the master unit that interprets and responds to forms, containing information for display, data capture, annunciation and timeout specification, as stored in and transmitted from the master unit. The system may be expanded by one or more application units, which have their own forms that are transmitted to the user interface unit, in order to communicate with the user and using a common format specified for the system and interpreted by the user interface, in order for the application unit to display data on and capture data from the user interface so that it may enhance its own operation and that of master unit through communications provided by a communications protocol managed by the master unit.