What is the working principle of a PLC control cabinet?

Release time:

2022-08-12

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Abstract

The process of the PLC control cabinet outputting corresponding control signals to the field execution part is generally divided into three stages: input sampling, user program execution, and output refresh. The completion of these three stages is referred to as the scan cycle. Throughout the operation, the CPU of the PLC scans at a certain speed.

　　PLC control cabinetThe process of outputting corresponding control signals to the field execution part is generally divided into three stages: input sampling, user program execution, and output refresh. The completion of these three stages is called the scan cycle. During the entire operation process, the PLC's CPU scans at a certain speed.

1. Input Sampling Stage

Repeat the above three stages. In the input sampling stage, the PLC reads the input status and data through a scanning sequence and stores them in the corresponding units of the IO image area. After input sampling is completed, it will enter the user program execution and output refresh stages. In these two stages, even if the input status and data change, the status and data of the corresponding units in the IO image area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal needs to be greater than one scan cycle to ensure that it can be read under any circumstances.

2. The program is executed by the user in accordance with program 40; when scanning each ladder diagram, it must first scan the control circuit composed of each contact on the left side of the ladder diagram, performing logical operations from left to right and top to bottom on the control circuit composed of contacts, and then refresh the state of the logical coils corresponding bits in the system RAM storage area based on the logical operation results; or refresh the state of the output coils corresponding bits in the IO image area; or determine whether to execute special function commands specified in the ladder diagram.

That is, during the execution of the user program, only the status and data of input points in the IO image area will not change. Although other output points and software devices' status and data in the IO image area or system RAM storage area may change, as well as the ladder diagrams listed above, the results of program execution will play an important role in ladder diagrams that use these coils or data; conversely, for ladder diagrams arranged below, refreshing the status or data of logical coils can only work on programs arranged above until the next scan cycle.

During program execution, if immediate IO instructions are used, IO points can be accessed directly. That is to say, if IO instructions are used, values will not be updated in the input processing image register. The program directly obtains values from IO modules, and output process image registers will be updated immediately, which is different from real-time inputs.

3. Output Refresh Stage Differences:

After scanning the user program, PLC enters the output refresh stage. During this period, CPU refreshes output latch circuits based on corresponding statuses and data in the IO image area and then drives corresponding peripheral devices through output circuits. At this time, it represents PLC's actual output.

4. Functional Characteristics of PLC Control Cabinet:

(1) The system composition is flexible and easy to expand; digital control is its characteristic; it can also perform PID loop control for continuous processes; it can form complex control systems such as DDC and DCS with upper computers to achieve comprehensive automation of production processes.

(2) Easy to use with simple programming. It uses simple programming languages such as ladder diagrams, logic diagrams or statement tables without requiring computer knowledge. Therefore, system development cycles are short and on-site debugging is easy. In addition, programs can be modified online to change control schemes without disassembling hardware.

(3) It can adapt to various harsh working environments with strong anti-interference capability and high reliability, far exceeding other models.

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