1. Trapezoidal stairs start from the horizontal line and end up with a vertical line (generally you can omit not to draw, only draw the left busbar). On the left side of each row is the junction combination, indicating the conditions that drive the logic coil, while the logic coil indicating the effect can only be connected to the bus on the right. The contact must not appear on the right side of the coil.
2. The contact should be drawn on the horizontal line, not on the vertical line, and the contact between the contact X005 and other contacts in the following figure (a) cannot be identified. For this kind of bridge circuit, according to the one-way principle from left to right and from top to bottom, all the ways should be drawn alone.
3. When the parallel blocks are connected in series, the path with many contacts should be placed on the left side of the ladder diagram (the principle of left weight and right lightness); When the tandem blocks are connected in parallel, the parallel path with many contacts should be placed above the ladder diagram (the principle of heavy at the top and light at the bottom). Doing so makes the program concise and then reduces the scanning time of the instructions, which is especially important for large programs.
Fourth, it is not suitable to use double-coil output. If the coil of the same component is used two or more times in the same ladder diagram, it is called a two-coil output or repeated use of the coil. One of the drawbacks of humbucker output is that beginners in general ladder diagrams are committed. In the case of humbucker output, only the last coil is valid, while the front coil is invalid. This is determined by the scanning characteristics of the PLC.
The CPU of the PLC is a cyclic scanning operation method. It generally consists of five periods (as shown in the figure): internal diagnosis and treatment, communication with peripherals, input sampling, user program execution, and output rewriting. When the method switch is at STOP, only the first two periods are executed: internal diagnosis and treatment, and communication with peripherals.
1. Enter the sampling period
The PLC reads the situation at each input and stores it in an inner unit called the input image register. When entering the program execution, such as the input situation changes. The corresponding unit information of the input image area will not be changed, but will only be changed during the input sampling period of the next scan cycle. As a result, the PLC ignores pulse changes in the switching amount at the input that is less than the scan period.
2. During the execution of the program
The PLC starts from step 0 of the program, scans the user program according to the order of first up and then down, first left and then right, and performs logical operation. The PLC performs logical operations according to the contents of the input image area, and writes the calculation effect to the output image area, instead of directly outputting it to the terminal.
3. Output rewrite period
The PLC changes the output terminal according to the contents of the output image area. This is the practical output of PLC.
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