History of the OPC standard In 1995, four providers of industrial automation solutions created a working group to standardize the data access protocols of their devices, which at that time mostly used the Microsoft operating system. The objective was to summarize the communication protocols of programmable logic controllers (PLC/PLC). The working group, by summarizing these different protocols, was able to create a standardized interface that works as a middleman. This interface converted read or write requests between the human-machine interface used by operators to control machines via PLC and SCADA (Supervisory Control and Data Acquisition). In 1996, the working group released version 1.0 of the standard, called OPC (later renamed OPC Classic), which other hardware and software vendors began to adopt. The same year the OPC Foundation is created in order to formalize the compliance, certification, interoperability and validation procedures. OPC Classic Specifications The first version of the OPC standard was based on a Microsoft technology for exchanging data between software components called DCOM (Distributed Component Object Model), a subset of COM (Component Object Model). The original meaning of OPC was “OLE for process control”, OLE (Object Linking and Embedding) which refers to a Microsoft COM technology designed to embed and link documents and other software objects. In 1998, the OPC Foundation began converting the initial specification to web services. There are three specifications of OPC Classic: -OPC Data Access (OPC DA) defines OPC client-server technology and how software can read and write data, as well as the available data types and structures. -OPC Alarms and Events (OPC AE) describes the procedures by which OPC server software can monitor systems and send alarms to client software. -OPC Historical Data Access (HDA) defines queries and analysis that can be applied to time-stamped historical data provided by devices.
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