Abstract : This paper introduces a dosing control system composed of an industrial control computer, a Siemens PLC, and a frequency converter. The system utilizes the USS protocol based on an RS485 interface to achieve serial communication between the PLC and the frequency converter. A BiCo (Binary Interconnect) method, which flexibly combines input and output functions, is employed to set the frequency converter parameters. The method and programming for controlling the frequency converter using the USS protocol are discussed. This system can monitor the operating parameters of each frequency converter online and perform remote control, effectively reducing system wiring and improving the system's anti-interference capability, providing a low-cost and high-performance solution for water plant dosing systems. Keywords : USS protocol; frequency converter; PLC; PID With the development of fieldbus-based control networks, many electrical manufacturers have launched products with data communication functions, which can be configured using LonWorks, PROFIBUS, CAN, etc., but require separate purchase. Siemens frequency converters can use the USS protocol based on an RS485 communication interface as their serial communication protocol. Since Siemens' USS protocol is open, it provides feasibility for developing user-defined software based on the USS protocol. Users can directly read and write to the frequency converter using the USS control protocol without purchasing additional accessories for system configuration. This not only helps reduce costs but also fully meets the needs of specific projects. In a water plant's dosing system, a small PLC controls four Siemens MM420 frequency converters via the USS protocol, reducing the complexity of wiring connections and avoiding interference from various electromagnetic fields on the control equipment. 1. System Network Composition System hardware composition (see Figure 1): Communication with the four MM420 frequency converters is achieved via the USS protocol and the CPU226's RS485 interface. The CPU226 integrates 24 inputs/16 outputs, totaling 40 digital I/O points. It can connect to seven expansion modules, expanding to a maximum of 248 digital I/O points or 35 analog I/O points. It has 13KB of program and data storage space, six independent 20kHz high-speed pulse outputs, a PID controller, two RS485 communication programming ports, and supports PPI, MPI, and free-mode communication protocols. Featuring numerous input/output points, strong modular expansion capabilities, fast operating speed, and enhanced internal integrated special functions, it is fully adaptable to some complex small and medium-sized control systems. For more details, please click: Application of Variable Frequency Control Systems Based on the USS Protocol