Generally, the power module is inserted in the first slot with the number 0, the PLC is inserted in the second slot, and the input and output modules of various types are inserted in the other slots. A printed circuit board with connectors is installed on the back of the guide rail of the I/O module to connect the slots that are inserted into the I/O module. The upper and lower parts of the slots can make the inserted modules line up. A guide with an I/O module and processor is called a local I/O guide. Remote I/O rails with I/O modules, power supplies, remote I/O communication cards and installed away from the local I/O rails are called remote I/O rails. The number of remote I/O rails depends on the number of variables that different processors can control. The communication between the remote I/O guide rail and the processor can be realized in a variety of ways, including coaxial cable, biaxial cable, shielded twisted pair, etc. If the distance is relatively long and the problem of noise suppression needs to be considered, optical fiber communication can be adopted.