Programmable logic controller (PLC) is a kind of digital electronic equipment with microprocessor, which is used for automatic control. It can load control instructions into memory at any time for storage and operation. The programmable controller is composed of internal CPU, instruction and data memory, input and output unit, power supply module, digital analog unit and so on. PLC can receive (input) and send (output) various types of electrical or electronic signals, and use them to control or supervise almost all kinds of mechanical and electrical systems.

The International Electrotechnical Commission (IEC) defines PLC as:

Programmable logic controller (PLC) is an electronic system operated by digital operation, which is specially designed for application in industrial environment. It adopts a kind of programmable memory, which is used to store program in it, to execute user oriented instructions such as logic operation, sequence control, timing, counting and arithmetic operation, and to control various types of machinery or production processes through digital or analog input / output. Programmable logic controller and its related external equipment are designed according to the principle that it is easy to connect with industrial control system as a whole and expand its functions.

In 1968, General Motors Corporation of the United States put forward the famous “general ten” bidding index, which is also the characteristic of PLC at present

The programming is convenient and the program can be modified on site;

It is easy to maintain and adopts modular structure;

Reliability is higher than relay control equipment;

The volume is smaller than that of relay control equipment;

The data can be directly sent to the computer;

The cost can compete with relay control equipment;

The input can be AC 115V;

The output is AC 115V, above 2a, which can directly drive solenoid valve, contactor, etc;

When expanding, the original system needs only a small change; The capacity of user program memory can be expanded.

In 1978, NEMA defined programmable controller as:

Programmable controller is a kind of digital electronic equipment, which uses programmable memory to store instructions, run functions such as logic, sequence, timing, counting and calculus, and control various mechanical or working programs through digital or analog input and output modules.

PLC has the characteristics of strong versatility, easy to use, wide adaptability, high reliability, strong anti-interference ability, simple programming and so on.

All PLC consists of three main parts: input, CPU and output.

The concept of programmable logic controller

PLC structure

Suppose we have a device with motors and buttons. We want the motor to turn on when the button is pressed. When you press the button again, you want the motor to turn off. This function is similar to the function of the button on the TV remote control. To do this, we must first write a program to perform this operation on the computer, and then download it to the CPU. Then we connect the button to the input and the motor to the output. Now, if we press the button, the electrical signal will be sent to the PLC. The PLC input will first transmit this signal to the CPU, and it must be written in a format that the CPU can process.


Because the CPU is a computer, the system can not process the original electrical signal. The CPU will process the received data and send the result to the output. The output converts the data into the original electrical signal, applies it to the motor again and turns it on. Now, if we press the button, repeat the process and the motor will turn off.

At present, most PLCs have the function of expandable communication network module. Simple PLCs communicate with each other by bus cable or RS-232, while high-end PLCs communicate with each other by USB or Ethernet. It enables the exchange of information between PLC and PLC, between PLC and personal computer and other intelligent devices to form a unified whole and realize decentralized and centralized control. Now almost all the new PLC products have the function of communication network. It has RS-232 interface like computer. It can exchange information within several kilometers or even dozens of kilometers through twisted pair, coaxial cable or optical cable. Of course, the communication network between PLC is special for each manufacturer. For the communication between PLC and computer, some manufacturers adopt industrial standard bus and approach to standard communication protocol, which will make it convenient for communication and network between PLC of different models and between PLC and computer.

PLC communication protocol specifications can be divided into RS-232, RS-422, rs-432, RS-485, IEEE 1394, IEEE-488 (GPIB), of which rs-432 is the most rare. At present, the most commonly used communication protocols in the world are modbus-ascii mode and MODBUS-RTU mode, which are the communication protocols formulated by Modicon company. PROFIBUS is developed by Siemens. Mitsubishi Electric of Japan launched CC-Link communication protocol.

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