How PLC works

How PLC works

Blog Article

PLC(Programmable Logic Controller) is a control device widely used in the field of industrial automation.What is the working principle of PLC?Its working principle mainly includes the following three stages:


　　1.Input sampling stage

　　In this stage,PLC reads the signal status from external devices(such as sensors,buttons,switches,etc.)through the input module and stores these status in the I/O image area.In this process,PLC reads all input status and data to ensure that the input signal can be correctly read in any case.

　　2.User program execution stage

　　Next,PLC scans the user program(usually expressed in the form of ladder diagram)in order from top to bottom.When scanning each ladder diagram,PLC will first scan the control line composed of contacts on the left side of the ladder diagram,and perform logical operations on the control line in the order of left first,right second,top first,bottom.According to the result of the logical operation,PLC will refresh the state of the corresponding bit of the logic coil in the system RAM storage area,or refresh the state of the corresponding bit of the output coil in the I/O image area,or determine whether to execute a specific functional instruction.

　　3.Output refresh phase

　　When the user program scan is completed,the PLC enters the output refresh phase.In this phase,the CPU will refresh all output latch circuits according to the corresponding status and data in the I/O image area,and then drive the corresponding peripherals through the output circuit.Only then is the real output of the PLC.
　　How does the PLC work?

　　Internal processing:When the PLC starts running,it first executes the fault self-diagnosis program,self-checks the CPU,memory,I/O components,etc.,and stops and displays an error if an abnormality is found.If the self-diagnosis is normal,continue to scan down.

　　Communication service:The PLC checks whether there is a communication request,and if so,it performs corresponding processing,such as computer software monitoring to update the display content,and the PLC and the touch screen communicate,and the touch screen keeps refreshing the display;

　　Input sampling:read all input status and data in sequence,and store them in the image register of the I/O image area.What this means is that,for example,the status of the PLC external button switch is all read into the PLC I/O image area.

　　Program execution:PLC executes user programs one by one,from top to bottom and from left to right,that is,performs logical and arithmetic operations according to the requirements of the user program,and sends the results of the operations to the output image register.

　　Output processing:After all instructions are executed,the state of the output image register is transferred to the output latch,and converted into a signal acceptable to the controlled device through the output module of the PLC to drive the external load.This is the actual output of the PLC.

　　Summary

　　The working principle of PLC can be summarized as:reading external device signals through the input module,executing user programs for logical operations,and finally outputting control signals to drive external devices.The entire working process is carried out in a cycle to ensure the stable operation of the industrial process.The advantage of PLC lies in its high programmability,flexibility and reliability,which can adapt to various industrial control needs.Unimat focus on industrial automation for 20 years,welcome to talk to us. Original link：https://www.unimatautomation.com/blogs/news-and-press/how-plc-works