This system written in VB6. It should how to communicate using serial com port. It s good for reference

使用VB6.0编程，通过串口与伺服器通信时，按照MODBUS协议编写的求解LRC校验码的程序-Using VB6.0 programming through the serial port to communicate with the server, in accordance with the MODBUS protocol checksum written procedures for solving the LRC