A method for operating an automation device comprising at least one master unit and at least one slave unit that is connected by a first bus, wherein messages are transmitted over the first bus while controlling a technical process. The messages comprise a process image data area for planned field devices, which are connected to the at least one slave unit by a second bus, and a planned reserved process image data area that is intended for possible expansions of the automation device with further field devices is connectable to the at least one slave unit. In accordance with the invention, the method is used to expand the automation device with field devices, i.e., field devices that comply with the Fieldbus Foundation specification, during control operation (RUN operation).