MPI communication is used for small-scale, decimal point field level communication. S7-300 CPU integrates MPI communication protocol. The physical layer of MPI is RS-485, and the maximum transmission rate is 12mbit / s. The user interface of step 7 provides the communication configuration function, which makes the communication configuration very simple. The MPI connection bus that each CPU can use is related to the CPU model, which is 6 to 64.