Getting Uninverted SBUS and Smart Port from Frsky Receivers

In this guide we will show you how to get uninverted SBUS and Smart Port signals from many different Frsky receivers.

Why bother to get the “uninverted signal”?

For some strange reasons, Frsky SBUS and Smart Port Telemetry are both inverted at the output, which cannot be recognized by microprocessors directly. This has caused a lot of problems for the users and Frsky is the one to blame (Futaba was the first to use SBUS and it’s not inverted).

F3 and F7 processors have built-in inverters, so you can just connect SBUS and Smart Port to any UART and it will just work. However this is not the case for F1 and F4 flight controllers, and they need these signals “un-inverted” for them.

Back in the days (e.g. Naze32) external inverters were used between the receiver and FC. Many modern F4 flight controllers these days have built-in inverters on the flight controller for the UART’s, making this as easy as “plug and play”, such as the Kakute F4.

However, if your flight controllers don’t have built-in hardware inverters for SBUS and S.Port, and you don’t want to use external inverters, you might find this guide useful.

In this article we will show you where you can get the original, uninverted signal on the RX (before the inversion), and you could solder a wire to it and connect it to the FC directly.

You can also use Softserial to connect SmartPort to your flight controller without doing the inversion hack.

Setting up SBUS and Smart Port after connecting

If you don’t know how to setup SBUS and Smart Port in Betaflight/Cleanflight, here is how: How to Setup SBus, SmartPort Telemetry


X4R-SB

SBUS

For the Frsky X4R-SB, you could get the uninverted SBUS signal from the chip (Pin A as labelled), or you could also get it from the pin out on the side (also labelled as A).

Smart Port

For Betaflight, Cleanflight and some other FC firmwares, all you need is tapping to pin A (red). You just need to solder a single wire to this pin and connect it to the TX of a spare UART on the flight controller.

However, for other FC firmwares you might also need pin B (yellow), middle pin of the 3, and connect this to the RX of the same UART.


XSR

SBUS

Like the X4R-SB, the XSR’s uninverted SBUS is at a similar location: middle pin of the 3 (A).

Smart Port

Same as the X4R-SB, for Betaflight or Cleanflight, all you need is the the pin A (red). You just need to connect this pin to the TX of a spare UART on the flight controller.

For other FC firmwares, you might or might not also need pin B (yellow), middle pin of the 3, and connect this to the RX of the same UART. Try this if you have trouble getting SPort to work, and consult the FC firmware support.


XM+ (PLUS)

XM+ review

The XM+ doesn’t have smart port but only SBUS and here is where the uninverted signal is.


XM

XM doesn’t have smart port, only SBUS and here is where the uninverted signal should be.


R-XSR

Uninverted S.Port, is circled in red. This is discovered by someone in my Facebook group, I have not tested it myself.

In the new batch of this receiver, Frsky has added 2 solder pads for uninverted SBUS and Smart Port, so hopefully it’s easier to work with.

16 thoughts on “Getting Uninverted SBUS and Smart Port from Frsky Receivers

  1. Matt Brown

    Anything new on getting Smartport inversion working on XSR-M RX? I have just installed one on my latest build with DYS F4. I really want telemetry!

    Reply
  2. drew

    I can confirm that the NEWER R-XSR Uninverted S. port pad shown does indeed work! Make sure you are updated to the most recent Firmware as well. It is a super small pad. I had to scrape off some clear protective film on the pad to get solder to stick, just an FYI.

    Reply
  3. Mathias

    Hi! Is it also possible to get an uninverted signal from the X8R receiver? Do you have pictures showing he right pin maybe?
    Thank you!

    Reply
    1. Oscar Post author

      sorry i don’t have the X8R to find out for you, do you have one? can you take some close up pictures (both sides) and send them to me? Maybe i can help you pin point a few places you can try.

      Reply
    2. Chris Eilbeck

      This is for a x8r rev5 but if you desolder the two boards from each other and find an npn transistor with the top mark of j6, you connect the uninverted smartport signal to the side that has one pin on it. I’ve done this with a Matek F405 AIO and it works perfectly. Sorry, don’t have any pictures, didn’t have a camera when i had it in bits.

      Chris

      Reply
  4. Filip

    Hey! I am wondering about the same thing in XSR-M / XSR-E, which I own and I am still using without telemetry. Where can I find pin A for that one? I would be very grateful for some tips, thanks!

    Reply
    1. Oscar Post author

      it’s very hard to tell without getting one to actually test it.
      I will update this post when I can confirm this. Meanwhile feel free to let me know if you find out the answer to this before I do :)

      Reply
    2. Rick

      I think on the XSR-M/E, the S-port chip is the one labeled J6, it’s the only chip that has three legs. Two legs on one side and one leg on the other side. Solder your telemetry tap on the single leg side. I’m waiting for my XSR-M from Banggood to arrive and I will do the mod as soon as I get it and I can confirm then.

      Reply
      1. Oscar Post author

        it would be awesome if you could share with us some photos indicating which pin is the correct one Rick :)
        Thanks alot!

      2. Andrew

        I too have a few xsr-e/m and am attempting to get uninverted smartport telemetry from this receiver. I have soldered to the single leg of J6 yet unable to get telemetry on my CLRacing F4 Pro on tele pad.

      3. Szper

        Same here. J6 and no result. Perhaps quad chip which is on R-XSR too. Easier is to solder directly to CPU. My next planned try is to do mod according to this manual:
        dutch-rc.nl/wp-content/uploads/2017/06/XSR-M-XSR-E-smartport-pin-300×269.jpg

  5. Luca Filippi

    Very good article, as always. Can i ask you a thing? Can you reupload excel tables of Inverse kinematic for robot?
    ‘Cause links on dropbox are expired. Thank you and have a good day. Happy Flying!!!

    Reply
    1. Oscar Post author

      LOL, sure, I have updated the link now. The file didn’t move/change, not sure why the original link didn’t work.

      Reply

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