OPC UA

Terms and definitions（术语和定义）

Event、Notification、Alarm

• Event: generic term used to describe an occurrence of some significance within a system or system component.
• Condition : generic term that is an extension to an Event.
• Alarm ： type of Event associated with a state condition that typically requires acknowledgement.
• Notification: generic term for data that announces the detection of an Event or of a changed Attribute value;Notifications are sent in NotificationMessages.
• EventNotifier: special Attribute of a Node that signifies that a Client may subscribe to that particular Node to receive Notifications of Event occurrences.
• MonitoredItem: Client-defined entity in the Server used to monitor Attributes or EventNotifiers for new values or Event occurrences and that generates Notifications for them.

OPC UA的监控项、订阅、和通知

OPC UA Services provide two capabilities to Clients. They allow Clients to issue requests to Servers and receive responses from them. They also allow Clients to subscribe to Servers for Notifications. Notifications are used by the Server to report occurrences such as Alarms, data value changes, Events, and Program execution results.

Object、Object Instance、ObjectType

• Object: Node that represents a physical or abstract element of a system.Objects are modelled using the OPC UA Object Model. Systems, subsystems and devices are examples of Objects. An Object may be defined as an instance of an ObjectType.
• Object Instance: synonym for Object. Not all Objects are defined by ObjectTypes.
• ObjectType: Node that represents the type definition for an Object.
• Complex Data: data that is composed of elements of more than one primitive data type, such as a structure.

Attribute与Variable

• Attribute ：primitive characteristic of a Node. All Attributes are defined by OPC UA, and may not be defined by Clients or Servers. Attributes are the only elements in the AddressSpace permitted to have data values.是AddressSpace中唯一允许具有数据值的元素。
• Variable: Node that contains a value.

ObjectNode与VariableNode是Compose关系，ObjectNode拥有一个haseComponents引用指向VariableNode。VariableNode的hasTypeDefinition指向VariableTypeNode。
说明：

• Node的属性字段叫Attribute，Node除了有Attribute还有Reference。而Object的属性字段叫Variable。
• Variable有另种类型Property与DataVariable，Property好像指的是单个值类似C语言中的int、float等类型，DataVariable指的是符合数据类似C语言中是struct
• DataVariable与Object的区别是，前者指的是数据类型，后者指的是设备或者模块类型。

Method与Service

• Method: callable software function that is a component of an Object.
• Service: Client-callable operation in an OPC UA Server. Services are defined in Part 4. A Service is similar to a method call in a programming language or an operation in a Web services WSDL contract.