Abstract: This paper introduces a frequency converter control system constructed using the USS communication protocol between a Siemens PLC200 and an MM440 inverter. The method and program design for controlling the inverter using the USS protocol are discussed. This system can perform on-line monitoring of the remote parameters of each inverter and remote control, effectively reducing system wiring, improving the system's anti-interference capability, and providing a low-cost and high-performance solution for frequency converter control systems. Keywords : USS protocol, frequency converter, PLC, PID, KingSCADA Keywords : USS protocol, MM440 inverter, PLC, PID, KINGVIEW. In industrial control, frequency converters are increasingly used for AC motor drive control. Generally, frequency converters offer panel control, terminal control, and communication control methods. Conventional control typically uses terminal control; the inverter's start/stop, direction, alarm, fault indication, and fault reset usually use digital control, while speed control uses analog input. This method requires a large amount of wiring and numerous control modules. With the development of fieldbus-based control networks, many electrical drive manufacturers have launched products with data communication capabilities, supporting various fieldbuses such as LonWorks, PROFIBUS, and CAN for convenient system configuration. However, these are generally sold as accessories and require separate hardware. In the automatic control system for the comprehensive utilization of alumina in a pilot plant, an S7-200 PLC is used to control 18 SIEMENS MM440 frequency converters via an RS485 interface using the USS protocol. The MM440 (MicroMaster 440) is Siemens' fourth-generation high-performance vector inverter. It borrows the flexible parameter structure, Bico technology, and high-precision control performance of the Master Driver Vector engineering inverter, and integrates an RS485 interface of up to 115.2 kbd for USS communication. This greatly reduces the complexity of wiring connections and improves system scalability. [b][align=center]For more details, please click: Variable Frequency Control System Based on USS Protocol[/align][/b]