CEI EN 61158-6-18

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
18 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 18
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

a) define the wire-representation of the service primitives
defined in IEC 61158-5-18, and

b) define the externally visible behavior associated with their
transfer.

This standard specifies the protocol of the Type 18 fieldbus
application layer, in conformance with the OSI Basic Reference
Model (ISO/IEC 7498-1) and the OSI application layer structure
(ISO/IEC 9545).