What is the scanning cycle of the programmable controller?

The scanning cycle is an important indicator of PLC, and the scanning cycle of a small PLC is generally more than ten milliseconds to tens of milliseconds. The length of the PLC's scanning cycle depends on the scanning speed and the length of the user's program. Millisecond scanning times are generally permissible for general industrial equipment, as are short lags on input from PLCs.

However, for some devices that respond quickly, corresponding measures should be taken. For example, choose a high-speed CPU to increase the scanning speed; Choose a fast response module, a high-speed counting module, and different interrupt handling measures to reduce the lag time. For users, it is necessary to improve the programming ability and optimize the program as much as possible; When writing the control program of a large device, the program execution time can be reduced by minimizing the program length, selecting branch or skipping programs, etc.

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The main features of PLC are as follows. 1 High reliability, strong anti-interference ability PLC is specially designed for industrial environment, its high reliability, strong anti-interference ability, its average failure interval time can reach more than 5 years, mainly reflected in: 1) Strict screening and aging of components, I O interface circuit adopts optoelectronic isolation.

2) The structure considers the requirements of heat resistance, moisture resistance, dust resistance and earthquake resistance. 3) The hardware adopts measures such as isolation, shielding, filtering and grounding. 4) The software adopts anti-interference and fault diagnosis measures such as digital filtering.

5) The working mode adopts the cyclic scanning method. 2. Perfect function In addition to the basic logic processing function, the PLC can be used in the field of digital control with special functional modules, and can realize communication with the host computer. In various industrial control fields, PLC can be applied to achieve control requirements.

3 Easy to operate PLC has the characteristics of easy operation, which is mainly reflected in: 1) Easy to input and change the program. 2) A variety of programming languages are available.

3) Self-diagnosis function makes maintenance convenient. 4 Flexibility and scalability The flexibility of PLC is manifested in the following three aspects: 1) The programming languages used include ladder diagram, Boolean mnemonic, menu diagram, functional module diagram and language description programming language, as long as you master one of the languages, you can program.

2) According to the continuous expansion of the application scale, the capacity can be expanded, the function can be expanded, and the application and control scope can be expanded. 3) The workload of design, programming and installation is reduced, and the process monitoring and control is easy to achieve.

The process of executing the program in one scanning cycle of PLC is divided into three stages, namely the input sampling stage, the program execution stage, and the output refresh stage.

1) Enter the sampling phase.

In this phase, the PLC scans the input signals on all input terminals and stores the respective input states in the corresponding input image registers. At this point, the input image registers are flashed.

During the program execution phase and the output refresh phase, the input image memory is isolated from the outside world and its contents remain unchanged until the input scan phase of the next scan cycle, when the input signal is re-read.

It can be seen that when the PLC executes the program and processes the data, it does not directly use the input signal at the time of the site, but uses the data input into the image area during this sampling. In general, the width of the input signal should be greater than one scan cycle, otherwise the signal may be lost.

2) Procedure execution phase.

In the process of executing the user program, the PLC follows the principle of ladder program scanning, and in general, the PLC executes the program one by one from left to right and from top to bottom. However, in the case of a program jump instruction, the program jump address is determined according to whether the jump conditions are met.

During program execution, when the input and output states are involved in the instruction, the PLC "reads" the state of the corresponding input terminal from the input image register, and "reads" the current state of the corresponding component ("soft relay") from the output image register. The corresponding calculations are then performed, and the results are stored in the output image registers. For the output image registers, the state of each component (the "soft relay") changes as the program is executed.

3) Output refresh phase.

The results of the program execution phase are stored in the output image area and are not sent to the output port. In the output refresh phase, the PLC feeds the output variables in the output image area into the output latch, and then the latch generates the control output of the current cycle through the output module. If the status of the internal output relay is "1", the output relay contacts are closed and the external load is driven through the output terminal.

The state of all output devices is maintained for a scan cycle.

Answer]: a, b, d

Item A, according to the "Technical Manual of Electric Drive Automation" (3rd Edition) P877 "System Scanning Cycle" related content. The speed of scanning is related to the complexity of the control object and the programming skills, the same function, if the program is skillfully written, the program will be executed quickly, and if the program uses a lot of loop statements, the program execution will be slow.

B, the speed of scanning is related to the processor model used in the PLC, and the inherent de-segmentation scanning speed of different models and series of PLCs is different.

Item D, the length of communication time, the number of connected external devices, and the length of the user's program finger sequence will affect the length of the PLC scanning time. As far as the same type of PLC is concerned, the main factors affecting the scanning time are: the length of communication time and the number of connected external devices, the distance of communication, the communication mode, the communication medium, etc.

Answer]: a, b, d

According to the Royal Gaze: "Technical Manual of Automation of Trouser Removal and High Electrical Transmission" (3rd Edition) P877 "Scanning Cycle of the System".