Abstract: This paper focuses on the synchronous rotation system of the DMQ4035 gantry crane, which combines frequency converter drive and PLC control, and utilizes a host computer for optimal control. It introduces the basic principles and specific design methods of PLC-host computer communication, and presents the program design for implementing serial communication using ActiveX controls provided by Visual C++. Keywords : Serial communication; Real-time measurement and control; Control To solve the problem of asynchronous operation between two motors, a system scheme is proposed for the DMQ4035 multi-purpose gantry crane, which uses frequency converter drive and PLC control, and coordinates with a host computer for optimal control. The PLC collects the encoder angular displacement pulse signals of the two motors through a high-speed counting module. The host computer continuously communicates with the PLC, reads the pulse signals, and analyzes and processes them. Once the angular displacement difference exceeds the allowable range, the host computer calls the optimal control algorithm, transmits it to the PLC, and the PLC then performs corresponding control. 1. Communication between PLC and Host Computer The PLC uses Mitsubishi A-series products, which support multiple communication types. A dedicated protocol computer connection mode is selected, using an RS232C port for serial communication. Figure 1 shows the port connection between the Mitsubishi AJ71UC24 communication module and the host computer. 2. Communication Protocol between PLC and Host Computer In the communication between the AJ71UC24 and the host computer, the host computer always has initial transmission priority. To know the angular displacement error in real time, the encoder input data is stored in D350 (motor #1 angular displacement) and D351 (motor angular displacement). At this time, the read frame is activated: [b][align=center]For details, please click: Communication between PLC and Host Computer in a Door Machine[/align][/b]