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5.3 Design goals ToC Previous Next

OPC UA provides a consistent, integrated AddressSpace and service model. This allows a single Server to integrate data, Alarms and Events, and history into its AddressSpace, and to provide access to them using an integrated set of Services. These Services also include an integrated security model.

OPC UA also allows Servers to provide Clients with type definitions for the Objects accessed from the AddressSpace. This allows information models to be used to describe the contents of the AddressSpace. OPC UA allows data to be exposed in many different formats, including binary structures and XML or JSON documents. The format of the data may be defined by OPC, other standard organizations or vendors. Through the AddressSpace, Clients can query the Server for the metadata that describes the format for the data. In many cases, Clients with no pre-programmed knowledge of the data formats will be able to determine the formats at runtime and properly utilize the data.

OPC UA adds support for many relationships between Nodes instead of being limited to just a single hierarchy. In this way, a Server may present data in a variety of hierarchies tailored to the way a set of Clients would typically like to view the data. This flexibility, combined with support for type definitions, makes OPC UA applicable to a wide array of problem domains. As illustrated in Figure 2, OPC UA is not targeted at just the SCADA, PLC and DCS interface, but also as a way to provide greater interoperability between higher level functions.

readme_files/image004.png Figure 2 – OPC UA Target applications

OPC UA is designed to provide robustness of published data. A major feature of all OPC servers is the ability to publish data and Event Notifications. OPC UA provides mechanisms for Clients to quickly detect and recover from communication failures associated with these transfers without having to wait for long timeouts provided by the underlying protocols.

OPC UA is designed to support a wide range of Servers, from plant floor PLCs to enterprise Servers. These Servers are characterized by a broad scope of size, performance, execution platforms and functional capabilities. Therefore, OPC UA defines a comprehensive set of capabilities, and Servers may implement a subset of these capabilities. To promote interoperability, OPC UA defines subsets, referred to as Profiles, to which Servers may claim conformance. Clients can then discover the Profiles of a Server, and tailor their interactions with that Server based on the Profiles. Profiles are defined in OPC 10000-7.

The OPC UA specifications are layered to isolate the core design from the underlying computing technology and network transport. This allows OPC UA to be mapped to future technologies as necessary, without negating the basic design. Mappings and data encodings are described in OPC 10000-6. Three data encodings are defined:

  • XML/text
  • UA Binary
  • JSON In addition, several protocols are defined:

  • WebSockets OPC UA Applications that support multiple transports and encodings will allow the end users to make decisions about trade-offs between performance and compatibility at the time of deployment, rather than having these trade-offs determined by the OPC vendor at the time of product definition.

OPC UA is designed as the migration path for OPC clients and servers that are based on Microsoft COM technology. Care has been taken in the design of OPC UA so that existing data exposed by OPC COM servers (DA, HDA and A&E) can easily be mapped and exposed via OPC UA. Vendors may choose migrating their products natively to OPC UA or use external wrappers to convert from OPC COM to OPC UA and vice-versa. Each of the previous OPC specifications defined its own address space model and its own set of Services. OPC UA unifies the previous models into a single integrated address space with a single set of Services.

OPC UA PubSub opens new application fields for OPC UA. The following are some example uses for PubSub:

  • Configurable peer to peer communication between controllers and between controllers and HMIs. The peers are not directly connected and do not even need to know about the existence of each other. The data exchange often requires a fixed time-window; it may be point-to-point or a multi-point connection.
  • Asynchronous workflows. For example, an order processing application can place an order on a message queue or an enterprise service bus. From there it can be processed by one or more workers.
  • Logging to multiple systems. For example, sensors or actuators can write logs to a monitoring system, an HMI, an archive application for later querying, and so on.
  • Servers representing services or devices can stream data to applications hosted in the cloud. For example, backend servers, big data analytics for system optimization and predictive maintenance. PubSub is not bound to a particular messaging system. Rather it can be mapped to various different systems as illustrated with two examples:

  • PubSub with UDP may be well-suited in production environments for frequent transmissions of small amounts of data. It also allows data exchange in one-to-one and one-to-many configurations.
  • The use of established messaging protocols (e.g. the ISO/IEC AMQP 1.0 protocol) with JSON data encoding supports the cloud integration path and readily allows handling of the information in modern stream and batch analytics systems.

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