This document discusses different configurations for providing redundancy when using OPC (OLE for Process Control) servers and clients. It describes four common scenarios: 1) no redundancy with a single PLC, OPC server, and client; 2) redundancy on one computer with redundant PLCs and OPC servers; 3) networked redundancy across multiple computers with redundant PLCs, OPC servers, and DataHubs; 4) multiple client redundancy across networks with redundant PLCs, OPC servers, and multiple OPC clients connected via DataHubs.