Key Considerations for Writing PLC Programs

The PLC application program should maximize the control requirements of the controlled object. After conceptualizing the framework of the main program, it should be the main line to write the programs that achieve each control function or sub-task one by one. Through continuous adjustments and improvements, the program can complete the required control functions. In addition, the PLC application program should generally include the following aspects:

1. Initialization Program: After the PLC is powered on, some initialization operations are generally required. Its role is to make necessary preparations for starting and to avoid misoperation of the system. The main contents of the initialization program are: clearing certain data areas and counters; restoring certain data areas to required data; setting or resetting certain outputs; displaying certain initial states, etc.

2. Signal Detection, Fault Diagnosis, Data Display, Equipment Communication, etc., can all be reserved as routine. These contents can be added when the basic design of the program is completed. They can also be relatively independent program segments.

3. Protection and Interlocking Programs. Protection and interlocking are essential parts of various application programs. They can prevent control logic confusion caused by illegal operations, ensuring that the system operates more safely and reliably. Therefore, careful consideration should be given to protection and interlocking issues. Generally, interlocking and protection measures should also be set up outside the PLC.

4. Main PLC Program and Subprograms. This is directly related to the logic and calculations of process control, which can generally be classified into subprograms according to space or time. Each subprogram should ideally allocate enough independent data storage areas or data blocks in advance, and the program variable names should be uniformly planned, with detailed comments in complex areas. Only by thoroughly understanding the process and fully collecting equipment parameters and operator requirements can a qualified program that is easy to accept be written.

For the same control requirements, even if the same model of PLC is used, the programs written with different design methods may have different structures. Although several programs can achieve the same control function, the quality of the programs may vary greatly. The quality of a program can be measured by the following aspects:

The fundamental aspect of a good application program is correctness. A correct program must withstand the test of system operation practice; without this, any evaluation of the program is meaningless.

A good application program can ensure that the system operates safely and reliably under both normal and abnormal (short power outages, certain controlled quantities exceeding standards, faults in certain links, etc.) working conditions, and can also ensure that there are no system control errors when illegal operations (such as pressing or inadvertently activating buttons that should not be activated) occur.

One of the advantages of PLC control is its flexibility, making it easy to change certain functions of the system by modifying programs or parameters. For example, some systems may need to change certain control parameters (such as timer or counter settings) under certain conditions, so it is essential to consider how to write the program to facilitate modifications.

The written program should be as concise as possible, reducing the number of statements, which can generally reduce the program scan time and improve the PLC’s response speed to input signals. Of course, if too many long-execution-time instructions are used, sometimes the number of program statements may be small, but the execution time may not necessarily be short.