A modularly constructed field device of process automation technology having a basic card with an executable, basic program, wherein the basic card is expandable by at least one expansion card containing at least one, executable, expansion program. For executing the basic program, the basic card comprises at least one computing unit and a first memory unit matched to memory requirement of the executable, basic program, characterized in that the expansion card comprises at least a second memory unit designed for memory requirement of the expansion program, an automatic detecting of the connected expansion card is provided by the basic card, and, for expanding the basic program by the expansion program, a partial and/or intermittent accessing of the second memory unit by the computing unit is provided.