A PLC is a new type of general-purpose industrial automation control device with extremely high reliability. It offers advantages such as powerful control capabilities, flexible configuration, simple programming, and ease of expansion. As a primary means of industrial control and a vital automation control device for both the present and the future, it is seeing rapid development and widespread application across various industries. However, beginners often encounter difficult-to-understand challenges when learning PLC programming. The following summarizes several common issues in PLC programming.
1. Programming cables for PLCs of the same brand are not compatible and cannot be used interchangeably.
2. PLC programs are written not in assembly language or C, but in ladder diagrams and instruction lists, which must be compiled into binary machine language.
3. When a PLC is connected to a personal computer, settings can be configured directly on the PC. A host computer refers to a computer from which a user can directly issue control commands; it is a personal computer that must have the corresponding PLC programming software. There are no particularly high system requirements; generally, an ordinary computer is sufficient.
4. Plan function blocks and write subroutines based on actual engineering requirements.
In a PLC, a subroutine is a relatively independent program written for specific control purposes. Subroutines are executed using instructions such as CALL. If the conditions for calling a subroutine are not met, the program's scan proceeds only within the main program and skips that subroutine, thereby reducing unnecessary scan time.
5. Control Outputs by Transferring Word or Double-Word Data to DO Points
PLC applications typically involve a large number of output controls. Controlling outputs by transferring word or double-word data to DO points can improve speed. By reasonably allocating output addresses and converting control output words according to actual application requirements, the number of execution steps in the PLC program can be significantly reduced, thereby accelerating the program's execution speed.
6. Pulse-Triggered SET and RESET
In a PLC, the SET instruction only needs to be executed once; it does not need to be executed during every scan, making it well-suited for use with pulse output (PLS/PLF) instructions. Some engineers overlook this issue and use conventional methods to trigger the SET instruction, inadvertently increasing the PLC program's scan execution time.