Dear friends, can anyone say about the general operation process of the PLC, about 100 words, thank

PLC works in a "sequential scan, continuous loop" approach.

1. During each scanning process, the input signal is collected centrally, and the output signal is refreshed centrally.

2 Input refresh process, when the input port is closed, when the program is in the execution stage, the input has a new state, and the new state cannot be read. The new state is read only when the program performs the next scan.

3 A scanning cycle is divided into input sampling, program execution, and output refresh.

4 The contents of the component image registers change as the program is executed.

5 The length of the scan cycle is determined by three strips. (1) The speed at which the CPU executes the instruction (2) The time occupied by the instruction itself (3) The number of instructions, the current PLC scanning speed is very fast.

6 Due to the mode of centralized sampling and centralized output, there is a phenomenon of input and output lag, that is, the input and output response delay.

In each scanning cycle, the PLC judges the content of each input or address according to the conditions according to the changes in each time, makes corresponding logical judgments according to different requirements, and makes relevant reactions or outputs, so as to achieve the purpose of logic control, and complete the corresponding results according to the logic control requirements!

When a programmable logic controller is put into operation, its working process is generally divided into three stages, namely input sampling, user program execution and output refresh. The completion of these three phases is called a scan cycle. Throughout the operation, the PLC's CPU repeats the above three phases at a certain scan speed.

The PLC operation method is as follows:

1.In-depth understanding and analysis of plant process conditions and control requirements.

a Controlled objects are controlled machinery, electrical equipment, production lines or production processes.

b Control requirements mainly refer to the basic mode of control, the actions that should be completed, the composition of the automatic work cycle, the necessary protection and interlocking, etc. For more complex control systems, the control task can also be divided into several independent parts, which can be simplified, which is conducive to programming and debugging.

2.Identify the IO device.

According to the functional requirements of the controlled object for the PLC control system, determine the user input and output equipment required by the system. Commonly used input devices include buttons, selector switches, travel switches, sensors, etc., and commonly used output devices include relays, contactors, indicator lights, solenoid valves, etc.

3.Choose the right type of PLC.

According to the determined user IO equipment, count the number of input signals and output signals required, and select the appropriate PLC type, including the selection of models, capacity, IO modules, power modules, etc.

4.Assign an IO point.

Assign the input and output points of the PLC, compile the input and output distribution table or draw the wiring diagram of the input and output terminals. Next, PLC program design can be carried out, and at the same time, the design and on-site construction of control cabinet or operating console can be carried out.

5.Design a ladder program for the application system.

According to the work function diagram or state flow chart, etc., the ladder diagram is designed, that is, programming. This step is the core work of the whole application system design, but also a more difficult step, to design a good ladder diagram, first of all, we must be very familiar with the control requirements, but also have a certain practical experience in electrical design.

6.Enter the program into the PLC

When using the Simple Programmer to input the program into the PLC

, you need to convert the ladder diagram into a command mnemonic first for input. When programming on the computer using the auxiliary programming software of the programmable controller, the program can be transferred to the PLC through the connection cable of the upper and lower computers.

7.Conduct software testing.

Once the program has been entered into the PLC, test work should be carried out first. Because in the process of programming, there will inevitably be omissions. Therefore, before connecting the PLC to the field equipment, it is necessary to carry out software testing to eliminate errors in the program, and at the same time lay a good foundation for the overall debugging and shorten the overall debugging cycle.

8.Debugging of the application system as a whole.

After the completion of PLC software and hardware design and control cabinet and on-site construction, the online debugging of the whole system can be carried out, if the control system is composed of several parts, local debugging should be done first, and then the overall debugging; If the control program has a large number of steps, you can debug it in sections first, and then connect it to the general tune. Problems found in debugging should be eliminated one by one until debugging is successful.

The working process of PLC is generally divided into three stages: input refresh, program execution, and output refresh.

1. Input refresh stage: The PLC works in a scanning mode, and the input circuit monitors the input status at all times and temporarily stores it in the input image register.

2. Program execution stage: PLC scans the program serially in order, and obtains the required data from the input image register and the output image register respectively for operation and processing, and then writes the result of program execution into the output image area of the register execution result for saving.

3. Output refresh stage: After executing all the user's programs, the PLC will send the output result of the operation to the output image register.

There are three main phases in how PLC scanning works, namely the input sampling phase, the user program execution phase, and the output refresh phase. When the user program scan is finished, the PLC enters the output refresh phase.

Take S7-200 as an example to illustrate the working process of PLC. The S7-200 completes its task in a scan loop.

The S7-200 performs a series of tasks over and over again. The task loop is performed once and is called a scan cycle. During the draw cycle, the S7-200 performs some or all of the following:

1. Read input: S7-200 copies the state of the actual input to the process image input register.

2. Control logic in the execution program: S7-200 executes program instructions and stores values in different storage areas.

3. Handle communication requests: S7-200 performs all tasks required for communication.

4. Perform CPU self-test diagnosis: S7-200 can ensure that the firmware, program memory and all expansion modules work correctly.

5. Write Output: Write the values stored in the process image output register to the actual output.

The execution of the user program depends on whether the S7-200 is in STOP mode or RUN mode. In run mode, execute the program; At.

In stop mode, the program is not executed.

When the PLC is put into operation, its working process is generally divided into three stages, namely input sampling, user program execution and output refresh. The completion of these three phases is called a scan cycle. Throughout the operation, the PLC's CPU repeats the above three phases at a certain scanning speed.

a) Input Sampling Phase In the input sampling phase, the PLC reads all input states and data sequentially in a scanning manner and stores them in the corresponding unit in the I-O image area. After the input sampling is complete, it moves to the user program execution and output refresh phase. In both phases, even if the input state and data change, the state and data of the corresponding cells in the iO image area do not change.

Therefore, if the input is a pulsed signal, the width of the pulse signal must be greater than one scan period to ensure that the input can be read in any case.

ii) During the user program execution phase, the PLC always scans the user program in sequence from top to bottom (ladder diagram). When scanning each ladder diagram, the control circuit composed of each contact on the left side of the ladder diagram is always scanned first, and the control circuit composed of contacts is logically operated in the order of first left and then right, first up and then down, and then according to the result of logical operation, the state of the corresponding bit of the logic coil in the system RAM storage area is refreshed; Or refresh the state of the corresponding bit of the output coil in the Io image area; or to determine whether to execute the special function instructions specified in the ladder.

In the process of user program execution, only the state and data of the input point in the IO image area will not change, while the state and data of other output points and software devices in the IO image area or the system RAM storage area may change, and the ladder diagram ranked above, the program execution result will play a role in the ladder diagram of all coils or data used in the below; Conversely, the ladder below, the state or data of the refreshed logic coils can only be used for the programs above it until the next scan cycle.

During the execution of the program, if the immediate IO command is used, the IO point can be accessed directly. Even if the IO command is used, the value of the input process image register will not be updated, the program will take the value directly from the IO module, and the output process image register will be updated immediately, which is different from immediate input.

3) Output refresh phase, when the scanning user program is finished, the PLC will enter the output refresh phase. During this period, the CPU flushes all output latch circuits according to the corresponding state and data in the I-O image area, and then drives the corresponding peripherals through the output circuits. At this time, it is the real output of PLC.

The working process is:

1. Input field signal: under the control of the system software, scan each input point sequentially and read the status of each input point;

2. Execute the program: scan the instructions in the user program sequentially, and perform logical operations according to the input status and instruction content;

3. Output control signal: According to the result of logic operation, the output status register (latch) sends the corresponding control signal in parallel to each output point to realize the required logic control function.

PLC is an industrial control computer, you can use a computer to program PLC, you can also use a manual programmer to program, now the teaching is generally used to teach PLC programming, what PLC you learn to install what PLC programming software, when running this software, your computer is the PLC programmer.

PLC has its own program, which he executes in cycles.