An apparatus for determining and/or monitoring at least one process variable of a medium. The apparatus includes: at least one sensor unit for registering the process variable, wherein the sensor unit produces measurement signals; at least one electronics unit for controlling the sensor unit, wherein the electronics unit has at least one microprocessor; and at least one memory unit, which is associated with the sensor unit, and in which control data are storable. The control data specifically relate to the sensor unit and can be read out by the electronics unit. The microprocessor of the electronics unit is embodied in such a manner, that the microprocessor executes in an inactive state only basic functionalities, and that the microprocessor in an active state controls the sensor unit. The microprocessor transfers from the inactive state into the active state transfers by loading control data from the memory unit.