Hello! I did try to use Virtual COM port project from archive STM32_USB-FS-Device_Lib_V3.3.0, loaded from site. But have problem - when MCU do sending "bytes string" from MCU to PC with length 0x40, 0x80, 0xC0 - PC didn't receive anything. If MCU do sending 0x3F, 0x41 bytes or other except "maxlength" - all is okey, PC will receive bytes with success! One else - If MCU did send "single string" with 0x40 bytes length and (after pause) send some else bytes - PC will receive old 0x40 bytes with these some else bytes! Can you are check it on your testboard?