Old name for what is now simply OPC Classic. This is a set of standards/protocols originally based on Microsoft OLE/COM technology for data exchange between industrial software applications. An OPC Server would talk to devices (like PLCs) and expose their data in a standard way, so different client software (HMIs, historians) could all access it uniformly. OPC Classic includes specs like DA (Data Access), HDA (Historical Data Access), A&E (Alarms & Events). It was huge for interoperability but being COM/DCOM-based, it had all the Windows quirks (and DCOM security issues). It’s largely been succeeded by OPC UA, but many legacy systems still use OPC DA servers to bridge devices and software.
