Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

To learn PLC, you must have a deep understanding of the scanning process and execution principle of PLC in order to write programs reliably and without errors. In layman’s terms, the PLC program is executed sequentially and cyclically from top to bottom and from left to right. It requires three processes to actually output and realize external actions.

To learn PLC, you must have a deep understanding of the scanning process and execution principle of PLC in order to write programs reliably and without errors. In layman’s terms, the PLC program is executed sequentially and cyclically from top to bottom and from left to right. It requires three processes to actually output and realize external actions.

The first step is to batch update the state of the external switch signal to the I input image area.

In the second step, the CPU executes the program sequentially from left to right, from top to bottom. In each scan cycle, the subsequent register status will be changed, but when the program is not scanned, there will be no actual physical properties immediately. Output.

In the third step, after all programs are executed, the output Q image area is refreshed uniformly, so as to execute the Q external execution structure action.

The following is a single-key start-stop program to analyze the status of each scanning process, so as to realize the single-key start-stop function.

The following uses M0.1 to replace the start-stop switch

Action process: Every time M0.1 is turned on, the output Q0.0 is turned on and off alternately, realizing the single-key start-stop function.

Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

Analysis process:

1. When M0.1=1 is pressed for the first time: M0.0 only turns on one scan cycle through the rising edge

In the first scan cycle, M0.1=1, M0.0=1

Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

Then the upper circuit in network 2—M0.0=1, and the closing point of Q0.0 is still 0 initially, then the upper circuit is on and the Q0.0 coil=1 is turned on.

The status of the lower line-M0.0 closed point open = 0, Q0.0 open point closed = 1, the lower line is blocked.

The result of the first scan cycle is: M0.1=1., M0.0=1, Q0.0=1. As shown in the figure below

Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

In the second scan period, even if M0.1 continues to be turned on=1, since the rising edge is only turned on for one scan period, the line after P is blocked, so M0.0 is blocked=0, as shown in the following figure:

Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

The status in network 2 now becomes:

Upper line: M0.0=0, Q0.0 closed point is open and cannot be opened, then the upper line is closed
Lower line: M0.0=0, the closed point is connected, Q0.0=1, the open point is connected, then the lower line is connected.

Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

In the third scan cycle, even if M0.1=0 is released, M0.0 continues to be 0, and the state in network 2 is the same as that in the second cycle, and Q0.0 continues to remain at 1.

Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

2. When M0.1=1 is turned on for the second time: a scan cycle is turned on after the rising edge P

The first scan cycle: M0.1=1, M0.0=1

Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

When M0.0 is reconnected:

Upper circuit: M0.0=1 is connected, and the closed point of Q0.0 remains open.The upper line is blocked
The lower line: if the M0.0 closed point is open, the lower line will also not work. Q0.0 loses power,
Therefore, the Q coil does not pass

Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

In the second scan period, even if M0.1 continues to be turned on=1, since the rising edge is only turned on for one scan period, the line after P is blocked, so M0.0 is blocked=0
In network 2, the upper line: M0.0=0, Q closed point connected=1, the upper line is blocked

Lower line: m0.0 closed point is open, Q open point is disconnected, and the lower line is blocked.

As shown below:

Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

Talking about the Three Scanning Processes of PLC from the Single-key Start-stop Program

The Links:   LM8V33 SKM300GB12T4

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