PLC

The National Electrical Manufacturers Association (NEMA) defines a PLC as a "digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions, such as logic, sequencing, timing, counting, and arithmetic to control through digital or analog I/O modules various types of machines or processes."

To understand how PLC working, you need to understand the working principles of relay, a relay has a coil which is supplied by a voltage source and it is connected to two terminals.

The principle of the relay: while connected to voltage source then all contacts will change their status. NO means contacts ‘close’ and NC means contacts ‘open’.

The principle of PLC signal is the device input (on/off) will activate the coils. All inputs are reflecting each input device. (In this case, it is stored in a memory data input) coil will control all of these conditions on / off internal contacts that arranged in a program PLC / ladder diagram (programming & processing).