Connectivity Framework
7 : Connectivity Standards
in the operations domain and is also being applied to information and application domains ( see Figure 1-1 ).
OPC-UA is an evolution of the classic OPC ( Object Linking and Embedding for Process Control ) standards . It unifies the various original OPC specifications and is an evolution from an API to a network protocol . Adapters are available to bridge between OPC-UA and classic OPC . OPC is operational in thousands of factories globally . Traditionally , OPC was used to configure and query plant-floor servers ( usually Programmable Logic Controllers ( PLCs )). Actual device-device communication was then effected via a hardware-based fieldbus 1 such as Modbus or Profinet .
OPC-UA retains some of that flavor ; it connects and configures plant-floor servers . The UA version adds better syntactical data typing ( see section 4.1.3 ) and semantic information modeling capabilities . There are many companion specifications that define information models for various device types . For example , Field Device Integration ( FDI ) defines a model that represents all fieldbus device types . A remote client such as a graphical interface can browse the device data controlled by a server on the floor . By allowing this introspection across many servers , clients can build a directory with cross-references of all the devices on the floor . Additionally , OPC-UA also addresses the specific needs of device-device communication and therefore does not anymore rely on additional fieldbus solutions . Its scalability allows for implementation on devices with very restricted hardware resources , such as sensor and actuator devices .
OPC-UA divides system software into clients and servers . The servers usually reside on a device ; they provide a way to access the device through a standard “ device model ”. There are device models for dozens of types of devices from sensor to feedback controllers . Each manufacturer is responsible for providing the server that maps the generic device model to its particular device . The servers expose an object-oriented , remotely-callable API that implements the device model .
Generic device models are central to the OPC-UA architecture . For example , the object model for a motor starter includes methods for setting parameters , reading data and operating the starter . Thus , applications can control a starter directly without being dependent on the manufacturer ’ s particular implementation .
OPC-UA is developing a “ pub-sub ” capability . This will provide direct device-to-device connection . There will be several “ profiles ” using different underlying protocols . The UDP profile supports multicast for efficiency . It targets simple implementation and does not attempt advanced functions like fragmentation control ( sending large data types in pieces ), reliability or quality-of-service control . Another profile is designed for connection to cloud-based data analytics . There is work on a DDS profile that will provide more sophisticated pub-sub functionality .
OPC-UA targets all kinds of manufacturing , including automotive , oil and gas , pharmaceuticals , food & beverage , medical machines , machine tools . It connects applications at the factory-floor
1
A “ fieldbus ” is the name of a family of industrial computer network protocols used for real-time distributed control .
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