RS-232: In telecommunications, RS-232 is a standard for serial binary data interconnection between a DTE (Data terminal equipment) and a DCE (Data Circuit-terminating Equipment). It is commonly used in computer serial ports. Because the application of RS-232 has extended far beyond the original purpose of interconnecting a terminal with a modem, successor standards have been developed to address the limitations.

OPC: OPC is basically a generic standaridized way of talking to PLC’s which controls a machine. OPC is open connectivity via open standards. Initially promoted by Microsoft, OPC is now driven by the www.opcfoundation.org Interoperability is assured through the creation and maintenance of open standards specifications. There are currently seven standards specifications completed or in development. Originally based on Microsoft's OLE COM (component object model) and DCOM (distributed component object model) technologies, the specification defined a standard set of objects, interfaces and methods for use in process control and manufacturing automation applications to facilitate interoperability. The COM/DCOM technologies provided the framework for software products to be developed. 

OPC DA: The first standard (originally called simply the OPC Specification and now called the Data Access Specification) resulted from the collaboration of a number of leading worldwide automation suppliers working in cooperation with Microsoft. There are now hundreds of OPC Data Access servers and clients.