PLC Ladder Diagram (Programmable Logic Control Ladder Diagram) is a programming language commonly used to control programmable logic controllers (PLCs) in industrial automation systems. Ladder diagrams are used by electrical engineers and programmers to write PLC programs that perform different logic flow controls on equipment and machines.

The execution sequence of the PLC ladder diagram generally follows the following steps:

1. Read input signals: Before starting execution, the PLC will first read the status of all input signals, including sensor signals and manual control signals.

2. Perform logical processing: The PLC will perform logical operations, comparisons, calculations, etc. on the input signals according to the logical sequence on the ladder diagram.

3. Execute output operations: Based on the results of the execution logic processing, the PLC will perform corresponding output operations, such as driving actuators, controlling switches, lights, etc.

4. Loop execution: The PLC will continuously loop through the above three steps, monitoring input status, processing logic, and executing output operations until it stops running or is reset.

It should be noted that the execution order of the PLC program may vary depending on the specific application scenario. For example, when multiple input signals and output signals are involved, the execution order may be adjusted.

The programming rules of PLC ladder diagram are as follows:

1. Ladder diagrams must be written from left to right.

2. Each running branch of the ladder diagram must be clearly visible and not confusing.

3. The coil and contacts must match, and the coil must be on the same vertical line as the contacts.

4. All ladder diagram programs must be commented.

5. It is forbidden to use duplicate coils in the program.

6. Ladder diagram programs must be structured. Codes must be grouped according to functional blocks and must not be disordered.

7. All coils must be protected to ensure that they are not disturbed by non-control related variables in the program.

8. There cannot be loops in ladder diagrams.