STM32F407 FreeModbus USART STM32F407 FreeModbus USART 实测，

Read Input Register (0x04) Read Holding Registers (0x03) Write Single Register (0x06) Write Multiple Registers (0x10) Read/Write Multiple Registers (0x17) Read Coils (0x01) Write Single Coil (0x05) Write Multiple Coils (0x0F) Read Discrete Inputs (0x02) Report Slave ID (0x11)