Figure 3.3.1 shows the basic elements of a PLC. It is basically a microproceeor based control system. Microprocessor communicates with the outside world with input/output devives via a circuitry. This circuitry protects the microprocessor and other elements of PLC from the high voltages and currents coming to the PLC. Microprocessor does its basic functions of taking decisions according to the instructions written in the programs which are stored in the memory. PLC scans a set of sensor inputs rapidly and repeatedly. Then it evaluates their logic relationships to defined outputs according to a logic program. At last it sets the outputs according to the programmed logic. Figure 3.3.2 shows an industrial PLC with input and output ports.