Field unit and a method for operating an automation system

The invention relates to a field unit comprising a bus node (4) which is designed for signal conversion between an external bus protocol and an internal bus protocol for the purpose of communicating with at least one functional module (5) in order for data packets to be transmitted sequentially along an internal bus system (5) using said internal bus protocol, and also comprising functional modules (5) which are designed to process data packets and transmit signals to at least one subordinate functional component (15 to 19). The invention is characterised in that the bus node (4) is designed to differentiate between data packets from the superordinate control device (2) such that, in said bus node (4), non-security-related data packets (27) are processed at the internal bus system (7) and security-related data packets (24) are forwarded to said internal bus system (7), and in that at least one functional module (5) connected to said internal bus system (7) is designed as a security module (9) in which security-related data packets (24, 30) can be processed.