OPC Unified Architecture (OPC UA) is a platform-independent standard through which various kinds of systems and devices can communicate by sending Messages between Clients and Servers over various types of networks. It supports robust, secure communication that assures the identity of Clients and Servers and resists attacks. OPC UA defines standard sets of Services that Servers may provide, and individual Servers specify to Clients what Service sets they support. Information is conveyed using standard and vendor-defined data types, and Servers define object models that Clients can dynamically discover. Servers can provide access to both current and historical data, as well as Alarms and Events to notify Clients of important changes. OPC UA can be mapped onto a variety of communication protocols and data can be encoded in various ways to trade off portability and efficiency.
OPC UA provides a consistent, integrated AddressSpace and service model. This allows a single OPC UA 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 standard 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 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, an OPC UA 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 below, OPC UA is not targeted at just the telemetry server interface, but also as a way to provide greater interoperability between higher level functions.
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 standard 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 [UA Part 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 [UA Part 6]. Two data encodings are defined in this part:
- XML/text
- UA Binary
In addition, two transport mappings are defined in this part:
- TCP
- SOAP Web services over HTTP
Clients and Servers that support multiple transports and encodings will allow the end users to make decisions about tradeoffs between performance and XML Web service compatibility at the time of deployment, rather than having these tradeoffs 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 to migrate 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