50.02.3
ANSI/ISA-50.02, Part 3-1997 - Fieldbus Standard for Use in Industrial Control Systems Part 3: Data Link Service Definition
General reference, softbound, 152pp.
$155.00

Specifies the characteristics of a Data Link Service suitable for time-critical communications, and supplements the OSI Basic Reference Model in specification of Data Link protocols for time-critical communications. The term time-critical represents 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.

Scope:
The Fieldbus Data Link service definition is a Data Link layer Standard designed to support time-critical messaging communications between devices in an automation environment. 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 document defines in an abstract way the externally visible service provided by the Fieldbus Data Link Layer in terms of

• the primitive actions and events of the service;
• the parameters associated with each primitive action and event, and the form that they take; and
• the interrelationship between, and the valid sequences of, these actions and events.

The services defined in document are a superset of those provided by OSI Data Link Protocols as specified in ISO 8886. The services defined in this document may be used by any OSI Network Protocol or Fieldbus Application Protocol.
The principal objective of this document is to specify the characteristics of a conceptual Data Link Service suitable for time-critical communications, and thus supplement the OSI Basic Reference Model in guiding the development of Data Link protocols for time-critical communications.

This document does not specify individual implementations or products, nor does it constrain the implementations of Data Link entities within industrial automation systems. However, its annexes may recommend exposed programming language interfaces within such systems.

There is no conformance of equipment to this Data Link Service Definition standard. Instead, conformance is achieved through implementation of conforming Data Link protocols that fulfill the Data Link Service defined in this document. Conformance may also be achieved through implementation of exposed programming language interfaces as specified in annexes to this document.

Purpose:
The purpose of this document is to define the service provided to the Fieldbus Application Layer at the boundary between the Application and Data Link Layers of the Fieldbus Reference Model, and to the OSI Network Layer at the boundary between the Network and Data Link Layers of the OSI Basic Reference Model.

The Data Link Service is provided by the Data Link Protocol making use of the services available from the Physical Layer. This document also defines the Data Link Service characteristics that the immediately higher-level protocol may exploit. The relationship between the Standards for Fieldbus Data Link Service, Fieldbus Data Link Protocol, Fieldbus Physical Service, and OSI Network or Fieldbus Application Protocol is illustrated in Figure 1.

Throughout the set of Fieldbus standards, the term “service” refers to the abstract capability provided by one layer of the OSI or Fieldbus Basic Reference Model to the layer immediately above. Thus, the Data Link Service defined in this document is a conceptual architectural service, independent of administrative divisions.