I am wanting to make my own UEXT USART6 i/o to USB. I am using the FDTI FT234XD (USB TO BASIC UART IC). I can see serial output from the UEXT using my 'scope. And I can plug the USB into a computer and it is seen as a tty.USB0 from Linux, for example.
But I cannot transfer data. I am wondering if I need a level translator between the UEXT TX/RX pins and the FDTI FT234xd chip? I cannot see why I need this, since both chips operate at 5v.
The STM32F407 chip in STM32-E407 design is powered by 3.3V and this makes the UART pins 3.3V too. I recommend you to measure the voltage levels with your scope to confirm it.
I had an earthing problem. All solved.