scope:

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." The FAL is an Application Layer Communication Standard designed to support the conveyance of time-critical and non-time-critical application requests and responses among devices in an automation environment. The term "time-critical" is used to represent the presence of an application 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 PAS complies with the structure and services of the IEC fieldbus Application Layer. It is specified 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. The AL services within this PAS specify interactions between remote applications in terms of:;an abstract model for defining application resources (objects) capable of being manipulated by users via the use of FAL Services,;the primitives (interactions between the FAL and the FAL user) associated with each FAL Service;;the parameters associated with each primitive;;the interrelationship between and the valid sequences of the primitives for each service. 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 PAS provide access to the FAL to control certain aspects of its operation. Isochronous Mode Application class specification containing IsoM Data, and IO Device to IO Device provider consumer relationship. The AL protocols within this PAS specify interactions between remote applications in terms of:;the encoding rules that are applied to all the Application Layer Protocol Data Units (APDUs);;the formal Abstract Syntax definitions of such APDUs;;the protocol state machine descriptions that handle the APDUs and the primitives in the correct sequences;;the mappings of the APDUs to and from the Data Link Layer services. The FAL encoding rules are designed assuming that both the encoder (sender) and the decoder (receiver) have the common knowledge of the abstract syntax. Wherever possible, data type identifiers are not encoded and transferred over the network.