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 11 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 part of IEC 61158 defines in an abstract way the externally visible service provided by
the different Types of fieldbus Application Layer in terms of
a) an abstract model for defining application resources (objects) capable of being
manipulated by users via the use of the FAL service,
b) the primitive actions and events of the service;
c) the parameters associated with each primitive action and event, and the form which they
take; and
d) the interrelationship between these actions and events, and their valid sequences.
The purpose of this part of IEC 61158 is to define the services provided to
1) the FAL user at the boundary between the user and the Application Layer of the Fieldbus
Reference Model, and
2) Systems Management at the boundary between the Application Layer and Systems
Management of the Fieldbus Reference Model.
This part of IEC 61158 specifies the structure and services 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 FAL AE 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 standard to provide access to the FAL to control certain aspects of its operation.