[STM32F407G-DISC1 USB CDC VCP] How to judge USB is ready?

[one V]

STM32F407G-DISC1 USB CDC VCP. Cubemx 5.2.1. STM32Cube_FW_F4_V1.24.1. The COM port can be recognized on the PC side. The MCU can receive the data sent by the PC. The MCU can also send data to the PC-side serial port assistant. However, if the CDC_Transmit_FS function is called, the COM port will not be recognized by the PC after re-plugging the USB port. If the CDC_Transmit_FS function is not called, after re-plugging the USB port, the COM port can be recognized by the PC.