Why is Classic OPC Suitable for Old PLC Systems?
Classic OPCs are designed to offer a common bridge for process control hardware and software applications to interface SCADA and other HMIs. This protocol design allows access to field data from plant floor devices consistently regardless of data type and source. In old PLC systems using the traditional I/O, OPC Server communicates securely over Ethernet, serial, and so on. For instance, the OPC Server used by Allen Bradley PLCs communicates over DH+, DF1, DH485, and Ethernet/IP.
One of the reasons why classic OPC is preferred for old PLC systems can be explained using Allen Bradley’s DH+ electrical interface, which uses differential signaling. The electrical interface consists of two transmission channels that do not have the same ground potential, which carries data represented as the difference in voltage. Any common interference as noise is ignored since the data exchanged is encoded differentially, and all non-transmitting nodes go into receive mode when one node transmits. This process protocol is compatible with Server-Client interaction under classic OPC specification, where the Server only serves up data requested by the Client. OPC Server waits for a request, and OPC Client waits for data after sending the request.