1.1 General
The fieldbus application layer (FAL) provides user programs with a means to access
the fieldbus communication environment. In this respect, the FAL can be viewed as
a “window between corresponding application programs.”
This part of IEC 61158 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 4 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 International Standard specifies interactions between remote applications and
defines the externally visible behavior provided by the Type 4 fieldbus application
layer in terms of
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a) the formal abstract syntax defining the application layer protocol data units conveyed
between communicating application entities;
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b) the transfer syntax defining encoding rules that are applied to the application layer
protocol data units;
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c) the application context state machine defining the application service behavior visible
between communicating application entities;
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d) the application relationship state machines defining the communication behavior visible
between communicating application entities.
The purpose of this document is to define the protocol provided to
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1) define the wire-representation of the service primitives defined in IEC 61158‑5‑4, and
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2) define the externally visible behavior associated with their transfer.
This document specifies the protocol of the Type 4 fieldbus application layer, in
conformance with the OSI Basic Reference Model (ISO/IEC 7498‑1) and the OSI application layer structure (ISO/IEC 9545).