What is the programmable logic controller?

In the field of automation the latest controller is PLC (Programmable Logic Controller).
before that to control an electrical system the most popular Relay logic system has been used.
for a smaller area of control, Micro controller is still the best choice among engineers.
PLC is a controller which is a kind of mini computer having various parts like CPU (central Processing Unit), FM (Function Module), SM(Signal Module), CP (Communication Processor), AM (Analogue Module) and Power Unit.
Signal modules is divided into two parts input parts that are dedicated to collect information from the field and output parts that are dedicated for to deliver signal to the field after calculation from CPU.
PLC operates either12V/24V DC in generally to receive and deliver information.
But for getting analogue information from the filed or to deliver analogue information based on CPU calculation to the field (0~10mA or4~20mA).
This system is faster than other controller system; because in PLC, dedicated modules are used to collect and deliver data-eventually the data process time is very low which make a machine system faster.
this PLC system is also very much reliable than other controller system.