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).
What's it look like?
PLCs come in various sizes. Generally, the space or size that a PLC occupies is in direct relation to the user systems and input/output requirements as well as the chosen manufacturer's design/packaging capabilities.
The chassis of a PLC may be of the open or enclosed type. The individual modules plug into the back plane of the chassis.
The electronic components are mounted on printed circuit boards (PCBs) that are contained within a module.