The control scale represents the control ability of PLC. It depends on how many input and output points it can control and how many analog circuits it can control. The control scale is related to speed. Because the scale is large, the user program is long, and the speed of executing instructions is not fast, it is bound to prolong the PLC cycle time, and it is bound to prolong the PLC response to the input signal. In order to avoid this situation, the working speed of PLC should be fast. Therefore, the working speed of large PLC is always faster than that of Small PLC. The control scale is also related to the size of the memory area. Large scale and long user program require a larger user storage area. At the same time, the number of points is large, and the signal area of memory input and output (input and output relay area or input and output mapping area) of the system is also large. This area is large, and the internal devices (see explanation later) will also increase, which requires a larger system storage area. The control scale is also related to the number of input and output circuits. If the control scale is 1024 points, there must be 1024 i/o circuits. These circuits are integrated into i/o modules, and there are always a number of i/o points in each module. Therefore, the scale is large and there are many modules used. The control scale is also related to the PLC instruction system. The large-scale PLC has a large number of instructions and strong functions, so it can meet the needs of controlling the system with many points. Control scale is an index that restricts other performance indexes of PLC; PLC is also divided into micro, small, medium, large and extra large 5.3 modules. Although the structure of PLC can be divided into box and module, from the qualitative point of view, the box is also a module, but it integrates more functions. Here, it is advisable to regard the module composition of PLC as the structural performance of all PLCs.