PLC's full name is: Programmable Logic Controller, a digital operation of the electronic system, designed for application in the industrial environment. It uses a class of programmable memory for its internal storage of programs, the execution of logic operations, sequence control, timing, counting and arithmetic operations and other user-oriented instructions, and through digital or analog input/output control of various types of machinery or production processes. It is the core part of industrial control. 

The ESSENCE of PLC is a special computer for industrial control, its hardware structure is basically the same as microcomputer, the basic composition is: 

Power Supply 

The power supply of PLC plays a very important role in the whole system. If there is not a good, reliable power supply system is unable to work normally, so the PLC manufacturer of power supply design and manufacturing is also very important. General AC voltage fluctuation in the range of +10%(+15%), can not take other measures and the PLC directly connected to the AC grid 

Central Processing Unit (CPU) 

Central processing unit (CPU) is the control center of PLC. It receives and stores the user program and data typed from the programmer according to the function given by the PLC system program; Check the status of power supply, memory, I/O and alert timer, and diagnose syntax errors in user programs. When PLC in operation, the first it receives in the form of scanning field state of each input device and data, and deposited in the I/O image area respectively, and then passes from the user program memory read the user program, after command interpretation shall be governed by the provisions of the directive logic or the result of the arithmetic operation into the I/O image area or in a data register. After all the user program is executed, the data in the output state or output register of the I/O image area is transmitted to the corresponding output device, and the cycle runs until it stops running. 

memory

Storage system software memory is called system program memory; The storage that holds application software is called user program storage.