Generally, the power module is inserted into the first slot, numbered as 0, the PLC is inserted into the second slot, and various types of input/output modules are inserted into the remaining slots. The back of the I/O module rail is equipped with a printed circuit board with connectors, which can connect the various slots inserted into the I/O module. The top and bottom edges of the slots can make the inserted modules align in a straight line. The rail with I/O modules and the processor is called the local I/O rail; the rail with I/O modules, power supply, remote I/O communication card and installed away from the local I/O rail is called the remote I/O rail. 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 rail and the processor can be achieved in various ways, including coaxial cables, dual-axis cables, shielded twisted pairs, etc. If the distance is long and noise interference needs to be suppressed, optical fiber communication can be used.