Today I've discovered another small but irritation limitation of Airbot F4 / Flip32 F4 also known as CC3D REVO for unknown reason. Looks like, by default, this board is unable to share UART1 RX line and PPM input. So, if you would like to connect, for example, OSD or GPS to UART1, you would have a problem. I've discovered that PPM input does not blocks UART1 totally, but makes UART1 transmission erratic and unreliable.
This is because PPM input pin is connected to both UART1 RX (pin PA10 of STM32F405) via inverter and PPM input pin (PB14) without inverter. Any electrical signal applied to PPM input would also be sent to UART1 RX.
Luckily, there is pretty simple, hardware, solution to allow UART1 and PPM input function simultaneously. Jumper called SBUS located near SBUS/PPM input has to be removed with soldering iron. This operation breaks the connection between PPM/SBUS input and UART1 RX.
There is a drawback of this solution too. If jumper is removed, SBUS will not work. To make SBUS works again, jumper would has to be closed again. With a blob of solder for example…