An OPC client is software written to communicate with OPC connectors. It uses messaging defined by a specific OPC Foundation specification.
What does an OPC Client do?
Conceptually: OPC Clients represent a data-sink. They initiate and control communications with OPC Servers based on what the application they are embedded in requests of them. OPC Clients translate a given application’s communication requests into an OPC equivalent request and send it to the appropriate OPC Server for processing. In turn, when OPC data returns from the OPC Server, the OPC Client translates it back into the application’s native format so the application can properly work with the data.
Technically: OPC Clients are software modules used by an application to enable it to communicate with any compliant OPC Server visible to it on the network. Typically, OPC Clients are embedded in applications such as HMIs, trending packages, historians, and report writers to make them inherently OPC-enabled.
It is common to refer to the application with an OPC Client embedded in it as the “OPC Client” even though only the OPC implementation is the true OPC Client.