 Programmable logic controller, a programmable logic controller PLC or programmable controller is an industrial digital computer which has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, or robotic devices, or any activity that requires high reliability control and ease of programming and process fault. Diagnosis They were first developed in the automobile manufacturing industry to provide flexible, ruggedized and easily programmable controllers to replace hard wired relays, timers and sequencers. Since then they have been widely adopted as high reliability automation controllers suitable for harsh environments. The PLC is an example of a hard real-time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result.