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Relay is only used as switch which is used to control high voltage device using low voltage signal. And relay can not be programmed.
While PLC is Programmable Logic Controller having counter,timer,Input/Output,which can be programmed according to user requirement and hence interfaced device is controlled accordingly.
Reduces the Hardware wiringThe System can be made compactContact Failure rate is very lessSpeed of opeartion is very highEasy for Trouble shootingReduces the Maintainance costEasier way of Making logics
PLC provides the ability to change the operating logic without affecting the wiring
