What exactly does a plc do?

PLC is a programmable logic controller. It uses a type of programmable memory to store programs internally and perform logical operations, sequence control, and timing. , counting and arithmetic operations and other user-oriented instructions, and control various types of machinery or production processes through digital or analog inputs and outputs.

Basic structure

The programmable logic controller is essentially a computer dedicated to industrial control. Its hardware structure is basically the same as that of a microcomputer. Its basic structure is:

1. Power supply

The power supply of the programmable logic controller plays a very important role in the entire system. Without a good and reliable power supply system, it cannot work properly. Therefore, manufacturers of programmable logic controllers also attach great importance to the design and manufacturing of power supplies. Generally, the AC voltage fluctuation is within 10% (15%), and the PLC can be directly connected to the AC power grid without taking other measures.

2. Central Processing Unit (CPU)

Central Processing The unit (CPU) is the control center of the programmable logic controller. It receives and stores the user program and data entered from the programmer according to the functions assigned by the programmable logic controller system program; checks the status of power supply, memory, I/O and alert timers, and can diagnose syntax errors in the user program. When the programmable logic controller is put into operation, it first receives the status and data of each input device on site in a scanning manner, and stores them in the I/O image area respectively, and then reads the user program one by one from the user program memory. After the command is interpreted, the result of logical or arithmetic operation is performed according to the instructions and sent to the I/O image area or data register. After all user programs are executed, each output status of the I/O image area or the data in the output register is finally transferred to the corresponding output device, and this cycle continues until the operation stops.

In order to further improve the reliability of programmable logic controllers, dual CPUs are used to form a redundant system for large programmable logic controllers, or a three-CPU voting system is used. In this way, even if a certain CPU fails, the entire system can still run normally.

3. Memory

The memory that stores system software is called system program memory.

The memory that stores application software is called user program memory.

4. Input and output interface circuit

1. The on-site input interface circuit consists of an optical coupling circuit and a microcomputer input interface circuit, which functions as the input of the interface between the programmable logic controller and on-site control. aisle.

2. The field output interface circuit is integrated by the output data register, strobe circuit and interrupt request circuit, and functions as a programmable logic controller to output corresponding control signals to the on-site execution components through the field output interface circuit.

5. Functional modules

Such as counting, positioning and other functional modules.

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