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 23 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 defines in an abstract way the externally visible behavior
provided by the different Types of the fieldbus Application Layer in terms of:
-
a) the abstract syntax defining the application layer protocol data units conveyed between
communicating application entities,
-
b) the transfer syntax defining the application layer protocol data units conveyed between
communicating application entities,
-
c) the application context state machine defining the application service behavior visible
between communicating application entities; and
-
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:
-
a) define the wire-representation of the service primitives defined in IEC 61158‑5‑23, and
-
b) define the externally visible behavior associated with their transfer.
This document specifies the protocol of the IEC fieldbus Application Layer, in conformance
with the OSI Basic Reference Model (ISO/IEC 7498) and the OSI Application Layer Structure (ISO/IEC 9545).
FAL services and protocols are provided by FAL application-entities (AE) contained
within the application processes. The FALAE is composed of a set of object-oriented
Application Service Elements (ASEs) and a Layer Management Entity (LME) that manages
the AE. The ASEs provide communication services that operate on a set of related application
process object (APO) classes. One of the FAL ASEs is a management ASE that provides
a common set of services for the management of the instances of FAL classes.
Although these services specify, from the perspective of applications, how request
and responses are issued and delivered, they do not include a specification of what
the requesting and responding applications are to do with them. That is, the behavioral
aspects of the applications are not specified; only a definition of what requests
and responses they can send/receive is specified. This permits greater flexibility
to the FAL users in standardizing such object behavior. In addition to these services,
some supporting services are also defined in this document to provide access to the
FAL to control certain aspects of its operation.