OPC (Classic)

OPC Classic is a set of legacy Windows-based standards for exchanging industrial process data between control systems and software applications. Based on Microsoft COM/DCOM technology, its most common variant, OPC Data Access, provided a standard way to read and write live tag values from diverse devices.

Before OPC, every software product needed a custom driver for each PLC or device. OPC Classic standardised this with server-client interfaces for current data, alarms, and history, letting any compliant client read from any compliant server. Its reliance on DCOM made it difficult to secure and route across networks, which drove the move to the platform-independent OPC UA. OPC Classic still matters because vast amounts of installed equipment depend on it.