Programmable Logic Controller

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Programmable logic lontroller (PLC) is a digital computing operation electronic system specially designed for application in industrial environments.

Programmable Logic Controller

It uses a programmable memory that stores instructions for performing operations such as logic operations, sequence control, timing, counting, and arithmetic operations, and controls various types of mechanical equipment or production through digital or analog input and output.

Development History


The development of technical requirements for the production of the US automotive industry has promoted the production of PLC.

In the 1960s, when General Motors Corporation adjusted the factory production line, it found that the relay and contactor control system was difficult to modify, large in size, loud, inconvenient to maintain, and poor in reliability. So the famous “General Ten” bidding index was proposed. .

In 1969, American Digital Equipment Corporation developed the first programmable controller (PDP-14).

After the trial on GM’s production line, the effect is significant;

In 1971, Japan developed the first programmable controller (DCS-8);

In 1973, Germany developed the first programmable controller;

In 1974, China began to develop programmable controllers:

In 1977, China promoted PLC in industrial applications.

The original purpose was to replace the mechanical switching device (relay module).

However, since 1968, the functionality of the PLC has gradually replaced the relay control board.

Modern PLCs have more features.

Its use extends from single process control to control and monitoring of the entire manufacturing system.


Microprocessors appeared in the early 1970s.

People quickly introduced it into the programmable logic controller, which enabled the programmable logic controller to add functions such as calculation, data transfer and processing, and completed the industrial control device with real computer characteristics.

The programmable logic controller at this time is a combination of the microcomputer technology and the conventional control concept of the relay.

After the development of the personal computer, in order to facilitate and reflect the functional characteristics of the programmable controller, the programmable logic controller is named Programmable Logic Controller (PLC).

In the mid-to-late 1970s, programmable logic controllers entered the stage of practical development.

Computer technology has been fully introduced into programmable controllers, making its function leap.

Higher computing speed, ultra-small size, more reliable industrial anti-jamming design, analog computing, PID function and high cost performance have established its position in the modern industry.

In the early 1980s, programmable logic controllers were widely used in advanced industrial countries.

The number of countries producing programmable controllers in the world is increasing and production is increasing.

This marks the beginning of the mature stage of the programmable controller.

From the 1980s to the mid-1990s, it was the fastest growing period of programmable logic controllers, with an annual growth rate of 30-40%.

During this period, PLC has greatly improved the ability to handle analog, digital computing, human-machine interface and network capabilities.

Programmable logic controllers are gradually entering the field of process control, replacing the DCS systems that dominate the process control field in some applications.

At the end of the 20th century, the development of programmable logic controllers was more adapted to the needs of modern industry.

During this period, mainframes and ultra-small machines were developed, various special functional units were born, various human-machine interface units and communication units were produced, making it easier to apply industrial control equipment for programmable logic controllers.

Basic structure

A programmable logic controller is essentially a computer dedicated to industrial control.

Its hardware structure is basically the same as that of a microcomputer, and its basic structure is:

  1. Power supply

The power supply is used to convert AC power to the DC power required inside the PLC.

At present, most PLCs are powered by a switching regulator.

  1. Central processing unit (CPU)

The central processing unit is the control center of the PLC and the core component of the PLC. Its performance determines the performance of the PLC.

The central processor consists of controller, operator, registers.

These circuits are concentrated on one chip and connected to the input/output interface circuit of the memory through the address bus and the control bus.

The role of the central processor is to process and run user programs, perform logical and mathematical operations, and control the entire system to coordinate.

  1. Memory

The memory is a semiconductor circuit having memory function.

Its role is to store system programs, user programs, logic variables and other information.

The system program is a program that controls the PLC to implement various functions.

Written by the PLC manufacturer and solidified into read-only memory (ROM), the user cannot access.

  1. Input unit

The input unit is an input interface between the PLC and the controlled device, and is a bridge for signals to enter the PLC.

Its role is to receive signals from the master and detection components.

The input types are DC input, AC input, AC and DC input.

  1. Output unit

The output unit is also the connection between the PLC and the controlled device.

Its function is to transmit the output signal of the PLC to the controlled device, that is, convert the weak electric signal sent by the central processor into a level signal to drive the actuator of the controlled device.

The types of outputs are relay output, transistor output, and gate output.

In addition to the above parts, PLC has a variety of external devices depending on the model.

Its role is to help with programming, monitoring, and network communications.

Commonly used external devices are programmers, printers, cassette tape recorders, computers, etc.

Working principle

When the programmable logic controller is put into operation, its working process is generally divided into three stages, namely, input sampling, user program execution and output refresh.

Completing the above three phases is called a scan cycle.

During the entire run, the programmable logic controller’s CPU repeatedly performs the above three phases at a certain scan speed.

  1. The input sampling stage

During the input sampling phase, the programmable logic controller sequentially reads all input states and data in a scan mode and stores them in the corresponding cells in the I/O map area.

After the input sampling is finished, it is transferred to the user program execution and output refresh phase.

In both phases, even the input state and data change.

The status and data of the corresponding unit in the I/O map area will not change.

Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scan period to ensure that the input can be read in any case.

  1. The user program execution stage

During the user program execution phase, the programmable logic controller always scans the user program (ladder) in order from top to bottom.

When scanning each ladder diagram, the control lines formed by the contacts on the left side of the ladder diagram are always scanned first, and the control lines formed by the contacts are logically operated in the order of first left, then right, first up and then down. .

And then refreshing the state of the corresponding bit of the logic coil in the system RAM storage area according to the result of the logic operation;

Or refreshing the state of the corresponding bit of the output coil in the I/O image area;

Or determine if you want to execute the special function instructions specified by the ladder diagram.

That is, during the execution of the user program, only the state and data of the input point in the I/O map area will not change, while other output points and soft devices are in the I/O map area or the system RAM storage area. Both status and data are subject to change.

Moreover, the ladder diagrams listed above, the program execution results will work on the ladder diagrams that use these coils or data below;

Conversely, in the ladder diagram below, the state or data of the refreshed logic coil can only go to the next scan cycle to work on the program that is listed above.

I/O points can be accessed directly if immediate I/O instructions are used during program execution.

Even if I/O instructions are used, the value of the input process image register will not be updated, the program will take values ​​directly from the I/O module, and the output process image registers will be updated immediately.

This is somewhat different from typing immediately.

  1. The output refresh phase

When the scanning user program ends, the programmable logic controller enters the output refresh phase.

During this time, the CPU refreshes all the output latch circuits according to the corresponding state and data in the I/O map area, and drives the corresponding peripherals through the output circuit.

At this point, it is the true output of the programmable logic controller.

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