Abstract : This paper introduces the RS485 communication protocol of Siemens MMV frequency converters and utilizes the ActiveX control MSComm in VC++ 6.0 to implement serial communication control between a microcomputer and the frequency converter under Windows. Keywords : Siemens frequency converter, RS485, serial communication In industrial applications, frequency converters are frequently used to control the speed and direction of AC motors. To address this need, some companies (such as Siemens (Germany), Toshiba (Japan), and Mitsubishi) have launched frequency converters with RS485 communication interfaces, allowing users to conveniently and flexibly select the powerful functions of the frequency converter and design their own industrial control systems. Developing industrial control software under Windows allows for the utilization of Windows' rich resources and the easy generation of various menus and aesthetically pleasing graphical interfaces. However, in the Windows environment, the system completely takes over various hardware resources and does not allow users to directly control the interrupt management of the serial port. How to develop the low-level resources of a microcomputer under Windows has become a major hot topic and challenge in industrial control software development. This paper successfully implements a serial communication interface between a Windows environment and a Siemens MMV frequency converter using the ActiveX control MSComm in VC++ 6.0. 1. System Overall Design Figure 1 shows the overall block diagram of the system, focusing only on the interface between the PC and the RS485 inverters. The RS485 driver can support 32 receivers. At a baud rate of 100 Kbit/s, the communication distance can reach 1200m; at a communication distance of 15m, the baud rate can reach 10 Mbits. RS485 is a widely used communication method in industrial settings. It should be noted that the PC connects to multiple inverters via an RS485 communication interface card. Each inverter is assigned its own address code for identification, allowing the host computer to control all the inverters connected via the RS485 communication line. [b][align=center]For more details, please click: Inverter Control System Based on VC++[/align][/b]