PLC is represent an equipments control electronic created from result of solidarity of among computer technology " solid-state " and " traditional sequence controller " . If visible from computer used, PLC is special purpose group. Computer special purpose is designed special tools to process certain problem, for example inspection equitment.
Basically PLC have function to replace job from relays mechanic and timer or switch manual. Becaused by excellence from microcontroller equipments develop hardware, hence PLC can operate arithmetical, converting data from analogue to digital or on the contrary, and can finish control problem having the character of complexs. To correspond to job circumstance, hence PLC designed to can operate on dusty industry environment and mount high pollution, with temperature change 0 - 60oC and the dampness relative at 0% - 95%.
National Electrical Manufacturesers Association ( NEMA ) giving the meaning of PLC :
" The electrics equipments operating digital by using " programmable memory" to is depository of instructions internal in the place of job from equipments having functions specific, like " logic, squence, timer, counter and arithmatical", and to control job from machines or process, through input output module analogously to digital"