Retrofitting module for a field device and field device with a modular design

A retrofitting module for a process automation field device comprising field device electronics with at least one communication interface, wherein the retrofitting module has a security module, wherein the security module cooperates with the field device electronics such that a predetermined IT security level (SL) is achieved, wherein the retrofitting module has a first electrical interface for connecting the field device electronics of the field device and a communication module for connecting to a superordinate unit, and wherein a connection of the first electrical interface to the communication interface irreversibly interrupts a connection of the field device electronics to the superordinate unit.