Abstract : This paper introduces a monitoring scheme for variable frequency speed control in a water treatment system using a host computer, an S7-200 series PLC, and an MMV frequency converter. The main components and characteristics of the system are analyzed. The automation part of the system mainly implements the following functions: dynamic data acquisition, data analysis and processing, automatic control, centralized display, process parameter setting, data storage, query, reporting, and printing. Keywords : Frequency converter; PLC; PID control; USS protocol; PPI protocol; Configuration software 0 Introduction This water treatment experimental system uses membrane technology for water treatment. This system can treat tap water into purified water and surface water into tap water. The system requires constant pressure or constant flow and needs to dynamically store a large amount of data for analysis. Manually adjusting the pressure and flow is not only cumbersome but also prone to errors and energy waste. This system uses a host computer, PLC, frequency converter, and configuration software to form a constant pressure water supply system control scheme, realizing automatic monitoring of the system and meeting the needs of the experimental system. 1. Hardware Configuration The system uses an S7-216 PLC, an EM231 module, and an MMV420 frequency converter. The host computer is a standard model (due to the favorable experimental environment). It includes 11 solenoid valves, 2 flow transmitters, and 3 pressure transmitters. The monitoring software is KingView 6.5. The system composition is shown in Figure 1, and the electrical principle is shown in Figure 2. [b][align=center]For more details, please click: Application of Frequency Converters and PLCs in Water Treatment Experimental Monitoring Systems[/align][/b]