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Plc Basic Structure

- May 17, 2018 -

Folding central processing unit


The central processing unit (CPU) is the control hub of the PLC controller. It receives and stores the user program and data entered from the programmer according to the functions assigned to the PLC controller system program; checks the status of the power supply, memory, I/O, and watchdog timer, and can diagnose syntax errors in the user program. When the PLC controller is put into operation, it firstly receives the status and data of each input device in the field by scanning, and stores them in the I/O mapping area respectively. Then the user program is read one by one from the user program memory, and the command is interpreted. The result of executing logic or arithmetic operations as specified in the instruction is sent to the I/O map area or data register. After all the user programs have been executed, the output status of the I/O image area or the data in the output register is finally transmitted to the corresponding output device, and the cycle is repeated until it stops operating.


In order to further improve the reliability of the PLC controller, in recent years, large PLCs have also used dual CPUs to form redundant systems or three-CPU voting systems. In this way, even if a CPU fails, the entire system can still operate normally.


Folding memory


The storage system software memory is called system program memory.


The memory for storing application software is called user program memory.


Folding power


The power supply of the PLC controller plays an important role in the entire system. If a good, reliable power system is not working properly, PLC manufacturers are also very concerned with the design and manufacture of power supplies. Normal AC voltage fluctuations are in the range of +10% (+15%), and PLC controllers can be directly connected to the AC network without taking other measures.


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