An OPC or 'OLE for Process Control' get the devices to talk to the HMI software. OPCs are only workable in windows systems.
An OPC server is a window application that allows window program to communicated with industrial hardware devices. The task of an OPC server is to convert the hardware communication protocol of PLC into OPC protocol.
OPC run as a background process and do the data exchange, both read and write. An OPC can directly talk to the field devices such as PLCs, HMIs and PACs.
OPC connects only with digital devices that do not include in 4-20mA devices. So the digital devices for an OPC are the PLCs.
An OPC client is the HMI packages like RS viewer, Win CC.
The architecture of an OPC system is shown above. An OPC server takes the information from the digital field devices and the gives the data to the HMI, reports, Excels etc.
Benefits of OPC:
Integration costs will be reduced because all software and hardware components are there to a single, standard communications protocol.
OPC servers will replace proprietary I/O device driver software for their hardware.
OPC will provide a single, consistent communication interface so that software suppliers can focus on adding new features to their software instead of developing along the list of proprietary hardware device drivers.
DCS, SCADA, HMI, plant scheduling, maintenance, and other applications can act as OPC clients and servers to permit data exchange between cooperating applications.