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 17 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 specifies interactions between remote applications and defines the externally
visible behavior provided by the Type 17 fieldbus application layer in terms of
a) the formal abstract syntax defining the application layer protocol data units conveyed
between communicating application entities;
b) the transfer syntax defining encoding rules that are applied to the application layer
protocol data units;
c) the application context state machine defining the application service behavior visible
between communicating application entities;
d) the application relationship state machines defining the communication behavior visible
between communicating application entities.
The purpose of this standard is to define the protocol provided to
1) define the wire-representation of the service primitives defined in IEC 61158-5-17, and
2) define the externally visible behavior associated with their transfer.
This standard specifies the protocol of the Type 17 fieldbus application layer, in conformance
with the OSI Basic Reference Model (ISO/IEC 7498) and the OSI application layer structure
(ISO/IEC 9545).