IEC 61158-6-27:2023 provides common elements for basic time-critical and non-time-critical messaging communications between application programs in an automation environment and material specific to Type 27 fieldbus. The term "time-critical" is used to represent the presence of a time-window, within which one or more specified actions are required to be completed with some defined level of certainty. Failure to complete specified actions within the time window risks failure of the applications requesting the actions, with attendant risk to equipment, plant and possibly human life.
This document defines in an abstract way the externally visible behavior provided by the Type 27 fieldbus application layer in terms of
the abstract syntax defining the application layer protocol data units conveyed between communicating application entities,
the transfer syntax defining the application layer protocol data units conveyed between communicating application entities,
the application context state machines defining the application service behavior visibly between communicating application entities, and
the application relationship state machines defining the communication behavior visibly between communicating application entities.