In addition to basic instructions for switch control, timing, and counting, PLCs also have a large number of application instructions, with some PLCs having hundreds of application instructions.
For application instructions, these instructions can be divided into the following types:
1. Instructions that are present in almost all computer languages, such as instructions for data transfer, comparison, shifting, looping, mathematical operations, word logic operations, and data type conversion.
These types of instructions are very important, as they relate to fundamental computer knowledge (such as number systems, data types, and addressing modes). You should understand their basic functions through examples and experiments. Once you've mastered these instructions for one type of PLC, learning similar instructions for other PLCs will be much easier.
2. Instructions related to sequential control programs. Among these instructions, the STL instruction in fx is the best designed. There is a clear correspondence between the ladder diagram and the sequential function chart designed with the STL instruction, making it easy to understand and use. The designed program is shorter than that designed by other methods, which can save a lot of design time.
The S7-200's SCR (Sequential Control Relay) instruction and Omron's step instruction (Step/SNXT) are also used to write sequential control programs.
3. Instructions related to advanced PLC applications, such as those related to PID control, motion control, high-speed input/output, and communication, are also important. Some instructions require specialized knowledge to understand and use correctly.
4. Instructions related to PLC-specific hardware and software, such as instructions for reading and writing special modules and analog potentiometers, and instructions for reading the length and number of data blocks in S7-300/400.
5. Instructions related to specific engineering applications. For example, FX's cam sequence control instructions and rotary table control instructions.
