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 standard 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 9 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 standard defines in an abstract way the externally visible behavior provided
by the Type 9 fieldbus Application Layer in terms of
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a) the abstract syntax defining the application layer protocol data units conveyed between
communicating application entities,
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b) the transfer syntax defining the application layer protocol data units conveyed between
communicating application entities,
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c) the application context state machine defining the application service behavior visible
between communicating application entities; and
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d) the application relationship state machines defining the communication behavior visible
between communicating application entities; and.
The purpose of this standard is to define the protocol provided to
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1) define the wire-representation of the service primitives defined in IEC 61158‑5‑5, and
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2) define the externally visible behavior associated with their transfer.
This standard specifies the protocol of the Type 9 IEC fieldbus application layer, in conformance with the OSI Basic Reference Model (ISO/IEC 7498) and the OSI Application Layer Structure (ISO/IEC 9545).