Abstract : This paper proposes a variable frequency synchronous control system that utilizes a free communication protocol, uses a PLC as the master station, and communicates with multiple frequency converters via an RS-485 bus. The hardware configuration of the system is introduced. Based on the communication principle and protocol between the PLC and the frequency converters, a timed interrupt data transmission method is adopted for speed setting to achieve linear speed synchronization; while motor start-up and stopping can be achieved using broadcast or individual transmission methods. Finally, the system's software flowchart is given. Practice shows that this synchronous control system has advantages such as flexible, accurate, and reliable speed control compared to traditional synchronous control systems. Keywords: Free communication protocol, synchronous control, RS-485 bus module 0 Introduction Currently, variable frequency speed control systems are widely used in various industries such as machinery, metallurgy, and chemical engineering. Most traditional synchronous control systems are implemented using cascading or expanded D/A modules. Cascading reduces the real-time performance and accuracy of the entire system; while the expanded D/A module method is more suitable for applications with a small number of motors and relatively concentrated motor distribution. In this method, the PLC and frequency converter are connected via external wiring, preventing internal data transmission. Furthermore, the attenuation of speed command signals over long distances and susceptibility to external interference reduce the overall system stability and reliability. Additionally, the large number of D/A conversion modules increases system cost. Since most frequency converters now have RS-485 communication interfaces for system configuration and monitoring, using an RS-485 communication interface to control the frequency converter not only fulfills all the functions of traditional applications but also enables internal parameter data communication. This allows for convenient acquisition of various parameters of the driven motor, such as operating frequency, current, and voltage, from the frequency converter, which can be displayed in real-time with a human-machine interface. However, due to cost considerations in actual systems, the PLC and frequency converter are usually from different manufacturers; therefore, a free-port communication protocol should be used between them. [b][align=center]For details, please click: Synchronous Control of Multiple Motors Based on Free Communication Protocol[/align][/b]