A seed- metering device laboratory table with sand -paving conveyer belt was designed. As to its complex parameters, control system based on RS485 was designed. In this system, PC was adopted as the HOST. Variable - Frequency Technique was applied to control the speed of then belt. Stepping motor was also adopted in this system. This paper introduced the structure and principle of the system, discussed the control method and the selection of motors. Analyzed the communication protocol of frequency converter; Analyzed the control method of the s...