The PLC works by means of "sequential scanning, continuous cycling". That is, when the PLC is running, the CPU prepares a program that is stored in the user memory according to the control requirements, and performs periodic cyclic scanning according to the instruction step number (or address number). If there is no jump instruction, the first instruction starts. The user program is executed sequentially one by one until the end of the program, and then the first instruction is returned again, and the next round of new scanning is started. In each scanning process, the sampling of the input signal and the refresh of the output state are completed.
Six principles of PLC ladder design:
1. Arrangement of contacts
The contacts of the ladder diagram should be drawn on a horizontal line and not on a vertical branch.
2. Serial and parallel processing
When several series circuits are connected in parallel, the series circuit with the most contacts should be placed at the top of the ladder diagram. When several parallel circuits are connected in series, the parallel circuit with the most contacts should be placed on the leftmost side of the ladder diagram.
3. Coil arrangement
The contact cannot be drawn to the right of the coil and the coil can only be connected to the right of the contact.
4. Do not allow double coil output
If the coil of the same component is used two or more times in the same program, it is called a double coil output. At this time, the previous output is invalid, and only the last time is valid, so the double coil output should not appear.
5. Rearrange the circuit
If the circuit structure is more complicated, it is easier to re-use some contacts to draw its equivalent circuit and then program it.
6. Programming sequence
For complex programs, the program can be divided into several simple blocks. Each segment starts from the leftmost contact, is programmed from top to bottom, and then connects the program piece by piece.
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