The invention relates to a rail-mounted building automation device adaptable to space and communication requirements, comprising a communications module having a communications bus, an application module that implements the functionality. A bus connector supplies power and transferring data. The communications module includes a first casing having ten bus connector pins, a lower body with the communications bus, having a wall further forward than another in relation to a user. The application module includes a second casing with a pin-receiving connector having three terminals, a first projection separated from a second projection at approximately the width of the rail, in use when the ten pins are introduced into the receiving connector and an upper body of the first casing is introduced into a third recess of the second casing, with both being joined by a securing apparatus.