An OPC Client is a software module that enables applications to acquire data from an OPC Server or conduct supervisory control using an OPC Server. OPC is based on client/server architecture.

OPC Client applications can be used for data acquisition. In this scenario, they simply subscribe to data (or poll data) from an OPC Server and either display or archive the data, or even both. Software applications can then archive data from a PLC, DCS, process analyzer, etc.

OPC Client applications can also be used for supervisory control. HMI applications are typically programmed with OPC Clients. This enables the HMI to provide supervisory control setpoints in the OPC Server, which, in turn, sends the setpoints to the underlying control system (PLC or DCS).

When OPC Clients and Servers are used together, they can be used for supervisory control and data acquisition (SCADA).

OPC Clients can handle an unlimited number of OPC Server connections. In addition, OPC Clients can handle multiple OPC specifications simultaneously. For example, an OPC Client can support OPC DA (Data Access), OPC HDA (Historical Data Access), OPC A&E (Alarms & Events), and even OPC UA (Unified Architecture) inside the same software application.

See also: Client, OPC Server, SCADA