In general a Control System is a collection of electronic devices and equipment which are in place to ensure the stability, accuracy and smooth transition of process or a manufacturing activity. It takes any form and varies from power plant to a semi-conductor machine. As a result of rapid advancement of technology, complicated control tasks accomplished with a highly automated control system, which may be in the form of Programmable Logic Controller (PLC) & possibily a host computer, etc. Besides signal interfacing to the field devices ( such as operator panel, motors, sensors, switches, solenoid valve and etc.), capabilities in network communication enable a big scale implementation and process co-ordination besides providing greater flexibility in realizing distributed control system. Every single component in a control system plays an importnat role regardless of size. For instance, as shown in Fig 1.1. the PLC would not know the happenings around it without any sensing devices. And if necessary, an area host computer has to be in place to co-ordinate the activities in a specific area at the shopfloor.