1. System Introduction The blower control system of the Dalian Tiger Beach Wastewater Treatment Plant was designed by our company. The PLC is a Siemens product. There are six blowers, all speed-controlled by Siemens frequency converters. Six MPR-53S multi-function meters are installed in the system to measure the system's electrical power. The MPR-53S meter is a multi-function power meter with an RS-485 communication interface and pulse output. Measurable parameters include phase voltage, phase current, total current, active power, and line voltage. The PLC retrieves data from the meters; in this system, the PLC retrieves data from the MPR-53S meters for reading current and voltage. 2. Network Configuration The PLC is installed in the main control cabinet, and the six multi-function meters are installed on six local control cabinets, as shown in Figure 1. The CPU of S7-300 reads data from the instrument in S7-200 through the EM277 connected to the ProfibusDP port. The multi-function instrument MPR-53S has an RS-485 communication interface and supports the Modbus RTU protocol. S7-200 can form an RS-485 Modbus RTU network and can act as a Modbus master station. However, there is no ready-made instruction library in the S7-200 system that supports Modbus RTU master station mode. It is necessary to program it yourself. We will introduce the program we have prepared as follows. 3 Software and settings of S7-300 (1) In the S7-300 station, it is necessary to set up the hardware of this system and program the S7-200. The program for reading S7-200 data in S7-300 is placed in OB1, as shown in Figure 2. The software settings are shown in Figure 3. (2) The application program of S7-200 includes 1 main program, 2 subroutines and 2 interrupt service routines, as shown in Figures 4 to 6. (3) The instruction manual accompanying the MPR-53S multifunction instrument contains a MOdUbUS register diagram, with detailed correspondences in the table. If you use this instrument, you can select the data you want to read based on the register diagram. In this example, the read data is stored in DB60 of the S7-300. 4. Conclusion This system was debugged in August 2005 and put into operation in October 2008. Four months of practice have shown that the system operates stably.