统一架构 第8部分：数据访问

[中文适用范围]： IEC 62541-8:2026 RLV 包含官方的 IEC 国际标准及其红线版本。红线版本仅提供英文版，便于用户快速、直观地比较官方 IEC 标准与其前一版之间的所有修改内容。IEC 62541-8:2025 定义了与数据访问（DA）相关的信息模型，特别包括用于数据访问的额外 VariableTypes、对 NodeClasses 和 Attributes 的补充描述、额外的 Properties，以及其他信息和行为。完整地址空间模型（包括所有 NodeClasses 和 Attributes）在 IEC 62541-3 中进行了规定。用于检测和访问数据的服务在 IEC 62541-4 中进行了规定。附录 A 规定了如何将来自 OPC COM 数据访问（DA）服务器的信息映射到数据访问模型。本版第四版取消并取代了 2020 年发布的第三版。本版为技术修订版。与前一版相比，本版包含以下重大技术变更：a) 新增“数量模型”，该模型可以从 EngineeringUnit Properties 中引用。该模型定义了数量及其分配的单位，同时提供了替代单位及其转换规则。b) 新增关于 ValuePrecision 属性的规则：- 还可用于其他子类型，如持续时间和小数。- 当 ValuePrecision 具有负值时，新增了相关规则。 [外文原描述]： IEC 62541-8:2026 RLV contains both the official IEC International Standard and its Redline version. The Redline version is available in English only and provides you with a quick and easy way to compare all the changes between the official IEC Standard and its previous edition. IEC 62541-8:2025 defines the information model associated with Data Access (DA). It particularly includes additional VariableTypes and complementary descriptions of the NodeClasses and Attributes needed for Data Access, additional Properties, and other information and behaviour. The complete address space model, including all NodeClasses and Attributes is specified in IEC 62541‑3. The services to detect and access data are specified in IEC 62541‑4. Annex A specifies how the information received from OPC COM Data Access (DA) Servers is mapped to the Data Access model. This fourth edition cancels and replaces the third edition published in 2020. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: a) addition of a "Quantity Model" which can be referenced from EngineeringUnit Properties. The model defines quantities and assigned units. In addition it provides alternative units and the conversion to them. b) addition of rules for ValuePrecision Property: - can also be used for other subtypes like Duration and Decimal. - rules have been added when ValuePrecision has negative values.