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The data-link layer provides basic time-critical messaging communications between devices in an automation environment.
This protocol provides communication opportunities to all participating data-link entities, ordered and in a synchronous manner. Scheduled execution controls the time-critical transmissions which is combined with on demand access.
Scheduled execution can be provided at cyclic intervals.
Controller to device interactions may be added or removed during normal operation.
The protocol provides means to maintain clock synchronization across a subsection with a precision better than 25 ns.
The reliability of a system of connected redundant structures is enhanced by seamless redundancy.
This protocol optimizes each access opportunity by concatenating multiple DLSDUs into a single DLPDU, thereby improving data transfer efficiency.
The maximum system size is an unlimited number of linked nodes ordered in a hierarchical structure of subsections.
This document specifies:
a) procedures for the timely transfer of data and control information from one data-link user entity to a peer user entity, and among the data-link entities forming the distributed datalink service provider;
b) the structure of the fieldbus DLPDUs used for the transfer of data and control information by the protocol of this document, and their representation as physical interface data units.
The procedures are defined in terms of:
a) the interactions between peer-DL-entities through the exchange of fieldbus DLPDUs;
b) the interactions between a DL-service (DLS) provider and a DLS-user in the same system through the exchange of DLS primitives;
c) the interactions between a DLS-provider and a Ph-service provider in the same system through the exchange of Ph-service primitives.
These procedures are applicable to instances of communication between systems which support time-critical communications services within the data-link layer of the OSI or fieldbus reference models, and which require the ability to interconnect in an open systems interconnection environment.
Profiles provide a simple multi-attribute means of summarizing an implementation's capabilities, and thus its applicability to various time-critical communications needs.
This document also specifies conformance requirements for systems implementing these procedures. This document does not contain tests to demonstrate compliance with such requirements.
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