工业通信网络现场总线规范第6-21部分:应用层协议规范21型元件

[中文适用范围]： 本部分 IEC 61158 是一系列旨在促进自动化系统组件互连的文档之一。它与 IEC 61158-1 中描述的三层现场总线参考模型定义的相关标准组有关。本国际标准包含与 Type 21 通信协议特定的材料。 [外文原描述]： IEC 61158-6-21:2019 provides common elements for basic time-critical and non-time-critical messaging communications between application programs in an automation environment and material specific to Type 2 fieldbus. 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 International Standard specifies interactions between remote applications and defines the externally visible behavior provided by the Type 2 fieldbus application layer. The purpose of this document is to define the protocol provided to a) define the wire-representation of the service primitives defined in this document, and b) define the externally visible behavior associated with their transfer. This document specifies the protocol of the Type 2 fieldbus application layer, in conformance with the OSI Basic Reference Model (ISO/IEC 7498-1) and the OSI application layer structure (ISO/IEC 9545). This second edition includes the following significant technical changes with respect to the previous edition: - added WriteAndRead service; - miscellaneous editorial corrections.