The present invention proposes assigning a common system identification code to each of a control unit and a plurality of controllable slave units, storing that system identification code in a user-inaccessible memory of those units, and allowing pairing between the control unit and each of the slave units only if the system identification code of the control unit matches that of the respective slave unit. Thus a slave unit can only act upon a control signal if the control signal comes from the control master unit which shares the system identification code, i.e. is verifiably from the same family. This ensures that a slave unit only acts on instructions which come from a specified control unit, so providing security to the user. It is therefore impossible for a slave unit to act on instructions from any device other than the control master unit with which it shares a system identification code, which may be alternatively referred to as a family identification code or family ID.