AC 4.1 and VR Ubrain

hello there,

I still use a VR Ubrain 5.1 on my system and with the new AC 4.1 with Fport support I would like to know if the main port for RC in will then support the Fport function.

The board has both RX and TX pins, currently only RX is used with SBUS received.




We’ve got a little bit on the wiki here about FPort in case it helps.

I think maybe @andyp1per is the expert on this area.

@yaapu is the expert!

Hi Lorenzo,
@andyp1per is one of our hwdef gurus and will correct me if I’m wrong so here I go…

FPort requires a bidirectional half duplex UART/USART configured with SERIAL_PROTOCOL=23.
If you have a spare serial port you’re set but if you want to re-use the RCIN pin you’ll need to adjust your board configuration and build your own firmware.

Take a look at the hwdef, RCIN defaults to timer capture i.e. unidirectional softserial read only input, in order to do fport on that pin you need to repurpose it to USART6.

If both RX (RCIN) and TX are wired to the MCU you might try to experiment a bit with the hardware definition by changing PC7 and PC6 in your hwdef.dat, something similar to the KakuteF7 hwdef where we have an extra UART by using alternate config 1.

In your hwdef you should add USART6 to the end of SERIAL_ORDER directive

# order of UARTs (and USB)

and redefine PC6 and PC7 like this

# extra USART for RC protocols

That “should” get you an extra serial port (SERIAL4) supporting speeds up to 230k baud.
Once you have the extra serial port, depending on the receiver, you might need an external inverter, just follow the wiki for hints on the actual wiring.

good luck,


Hi Alex,

many thanks for your reply.

ATM I think this is out of my knowledge. Compiling firmware is not for me. Already tried in the past


For 4.1 tracking purposes can you give me a short-form of this outcome? I guess we are saying that FPort is not support on the VR Ubrain boards? I wonder if this needs to be explained on the wiki… e.g. an explanation that only some boards support FPort?

@rmackay9 fport should work just fine on any board >= F4/F7 provided you use a serial port by setting SERIALn_PROTOCOL=23 for that port.

I had the feeling @Lorenzo_Pessah wanted to specifically use the RCIN pin for he probably did not have a spare serial port so I went into the rather complex description on how to repurpose that pin, but it’s not required, if you have a spare serial port for fport then you’re set!

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The main problem is that vrUBrain has only 2 serial available ( GPS) and telemetry port that is almost only dedicated to old hopeRF board. There were rumours about a second serial bit need to be wired directly to processor pins.

That why I think make sense convert rcin into a full serial and enable fport.

Same situation with vrBrain 5.2( the big brother) where the 3rd serial port is available from GPS 2 board. Also on this board convert the RC into into serial/fpost could make sense.

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