1.1 General
This part of IEC 61158 provides common elements for basic 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 Type 2 fieldbus data-link layer in terms of:
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• the primitive actions and events of the service;
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• the parameters associated with each primitive action and event, and the form which they take; and
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• the interrelationship between these actions and events, and their valid sequences.
The purpose of this document is to define the services provided to:
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• the Type 2 fieldbus application layer at the boundary between the application and data-link layers of the fieldbus reference model;
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• systems management at the boundary between the data-link layer and systems management of the fieldbus reference model.
Type 2 DL-service provides both a connected and a connectionless subset of those services
specified in ISO/IEC 8886.