Abstract : This article first systematically introduces the USS communication protocol, then takes the Siemens MM400/3 frequency converter as an example to introduce the hardware interface for communication between a microcomputer and the frequency converter—RS485. Finally, based on the USS protocol, it implements a program to control the frequency converter using a microcomputer as the controller and the MSComm control in VC++ in a Windows environment. Keywords : USS protocol; microcomputer; frequency converter; RS485; control 0 Introduction An important direction in the development of frequency converter technology is the integration of communication modules that can communicate directly with microcomputers. This facilitates remote control of the frequency converter by a microcomputer, enabling one-to-many monitoring to improve automation. Simultaneously, it allows for monitoring, displaying, storing, and printing the operating status of motors to meet the needs of the production site. The rapid development of computer technology has led to increasingly abundant resources and powerful functions for microcomputers, while their cost has significantly decreased. These two aspects have promoted the application of microcomputers in the field of industrial automation. With the help of microcomputers, "intelligent" control of automated equipment can be achieved through a user-friendly human-machine interface. Especially with the development of network technology, microcomputers can independently control automated equipment and also act as workstations to transmit information with various units in the entire factory network system, which is conducive to the realization of "unmanned factories". Therefore, the application of microcomputers as controllers in the field of industrial automation has broad prospects. This article takes the Siemens MM400/3 frequency converter as an example to illustrate how to control the frequency converter with a microcomputer. 1 USS Communication Protocol The USS (Universal Serial Protocol) communication protocol is a communication protocol designed by Siemens for its automated equipment to realize a master-slave structure industrial control network. It determines the access method according to the master-slave communication principle of a serial bus to realize access and control of equipment. One master station and up to 31 slave stations can be connected on the bus, and each station is identified by a unique identifier. The master station selects the slave station to transmit data based on the address characters in the communication message. When the master station does not request it to communicate, the slave station itself cannot send data first, and the slave stations cannot directly transmit information to each other. The USS protocol has a maximum baud rate of 187.5 kbps, and its communication character format consists of one start bit, one stop bit, one even parity bit, and eight data bits. (Read full article: Application Research of Microcomputer-Controlled Frequency Converters)