This document of IEC 61158 describes basic packet communication services and models in an automation control industrial field environment. The Type 28 data-link layer provides time critical and non-time-critical communication services. Time-critical refers to the requirement to complete specified functions between devices in a defined time window in an industrial field environment. Failure to complete specified functions within the time window may lead to failure or harm in industrial production.
This document defines in an abstract way the externally visible service provided by the Type 28 fieldbus data-link layer in terms of
a) function description;
b) primitive actions and events with primitive sequence diagram;
c) the form of externally service interface and related parameters.
The purpose of this document is to define the services provided to:
- the Type 28 fieldbus application layer at the boundary between the application and data link layers of the fieldbus reference model;
- systems management at the boundary between the data-link layer and systems management of the fieldbus reference model.
Type 28 DL-service provides both a connected and a connectionless subset of those services provided by OSI data-link protocols as specified in ISO/IEC 8886.
The principal objective of this document is to specify the characteristics of conceptual data-link layer services suitable for time-critical communications and thus supplement the OSI Basic Reference Model in guiding the development of data-link protocols for time-sensitive communications. A secondary objective is to provide migration paths from previously-existing industrial communications protocols.
This specification may be used as the basis for formal DL-Programming-Interfaces. Nevertheless, it is not a formal programming interface, and any such interface will need to address implementation issues not covered by this specification, including:
a) the sizes and octet ordering of various multi-octet service parameters; and
b) the correlation of paired request and confirm, or indication and response primitives.
This document does not specify individual implementations or products, nor does it constrain the implementations of data-link entities within industrial automation systems.
There is no conformance of equipment to this data-link layer service definition standard. Instead, conformance is achieved through implementation of the corresponding data-link protocol that fulfills the Type 28 data-link layer services defined in this document.