wonderful post ..nowadays plc is replaced by DCS.distributed control system (DCS) is a control system for a process or plant, wherein control elements are distributed throughout thesystem. This is in contrast to non-distributed systems, which use a single controller at a central location.
A DCS typically uses custom designed processors as controllers and uses both proprietary interconnections and standard communications protocol for communication. Input and output modules form component parts of the DCS. The processor receives information from input modules and sends information to output modules. The input modules receive information from input instruments in the process (or field) and the output modules transmit instructions to the output instruments in the field.