The Missing UART6 in the BetaFPV F4 AIO 12A/20A/35A FC

If you are using the BetaFPV F4 AIO 12A/20A/35A FC, and can’t find UART6 (TX6, RX6) in the ports tab, here is why. I had a tough time figuring this out, thought I should share to save people’s time.

Product Page

Missing UART 6?

Well, there is no UART6 on this board, simply never existed.

The labelled TX6 and RX6 in the pinout diagram are actually Softserial 1! I think this might have “tricked” some people into getting this FC and thought it had 3 hardware serial ports, but in fact it only has 2.

Further Reading: What’s a UART?

That’s why you don’t see UART6 in the ports tab in Betaflight, but there is Softserial 1, which they already set up for you. You need to connect your device to TX6 and RX6 to use Softserial 1.

This is annoying, but not the end of the world. You can still use softserial for a lot of things that aren’t “timing critical”, basically almost everything apart from RC links.

To put it simply, you can use Softserial for SmartAudio, SmartPort Telemetry, GPS, Camera Control, DJI UART, and so on, but you SHOULD NOT use it for SBUS, Crossfire etc.

Further Reading: What’s Softserial?

Wrong Softserial Config

The “fun” doesn’t stop there, they even configured Softserial wrong, so it’s not going to work properly :D

If you type Resource in CLI, you might find the pin B03 is assigned to both SERIAL_RX 11 and SERIAL_TX 11. If you need to use both TX and RX pin, you should assign B10 to SERIAL_RX 11.

Enter this in CLI:

resource SERIAL_RX 11 B10


SBUS and UART Conflicts

Finally, if you use the SBUS pin, don’t use RX1, or you might get jittering in RC signal (your quad doing random things).

The SBUS pin is basically RX1 with an inverter.

8 thoughts on “The Missing UART6 in the BetaFPV F4 AIO 12A/20A/35A FC

  1. Vincenzo Tarricone

    Hey Oscar, you saved me today and thousand other times.

    Many many many thanks to you for your precious work!


  2. Stephen

    Excellent. Looking forward to seeing if the betafpv 20A board with 1.25mm motor pins can support SBUS-RX + SmartAudio + RunCam-Device-Control.

    Ability to start/stop runcam or caddx HD cameras from a switch makes it much easier to upload a video as we don’t have to cut the video down afterwards. Alternatively it opens up the option of running a GPS unit. Basically anything that requires a full TX/RX pair is difficult to implement on these boards currently.

  3. Stephen

    Thanks for the great info! I have the BetaFPV 20A 2.0 with the 1.25mm pin connectors and can see “S5” which I guess is pin B03 but do you know where pin B10 is on this board?
    This is the model:

    Also it seems like RX2 is inverted to use for SBUS on this one?

    1. Oscar Post author

      I just received this board as well, I will check it out in the next few days and make a review on it.

  4. Utkarsh Verma

    Motor beeper is set. I like the old school buzzer beeps, also comes in handy for low voltage warning. Thanks :)

  5. Jeff Forhan

    I actually got a response from their tech support on this issue. Here is what they said:

    …” As the FC is updated, this UART6 is Softserial. Please map TX 11 to B03 and RX 11 to B10.”…


  6. Utkarsh Verma

    I have the very same board 12 A version. The issue I faced was that there are no buzzer pads, the ones shown as Bz+ and Bz- in the image in the post are +5v on my board. I could not figure out how to connect a buzzer to this one. Please let me know if any one has been able to do that successfully.


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