Uninverted SBUS and Smart Port on Frsky Receivers

In this guide I will show you where to get the uninverted SBUS and Smart Port signals on many different Frsky receivers, including the R-XSR, XSR, X4R-SB and XM.

What is “Uninverted Signal”?

Frsky SBUS and Smart Port Telemetry are both inverted at the output, which cannot be recognized by some older processors directly. This has caused a lot of problems for the users.

Newer processors like F3 and F7 have built-in inverters so you can 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.

If you don’t want to do the tricky soldering, you can also use Softserial to connect SmartPort to your flight controller without doing this inversion hack.

If you don’t know how to setup SBUS and Smart Port in Betaflight/Cleanflight, here is a how-to: 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.

36 thoughts on “Uninverted SBUS and Smart Port on Frsky Receivers

  1. Christoph Kloeti

    Hello Oscar
    many thanks for all the help for providing very useful information about uninverted ports out of Frsky receivers. If you are interested, I have found the uninverted SmartPort/F.Port on the R9 receiver and made some pictures I can provide to you for posting them in your blog.

    Just let me know if you are interested and I’ll send them to you.

    Best regards
    Christoph

    Reply
  2. LitterBug

    Hi Oscar,
    Any chance you can add the R9 Mini to this excellent infomation? They are now out in the wild.

    Cheers!
    LitterBug

    Reply
  3. Bobby

    I thought I was starting to understand all this until I read this article, lol. What? So if I put the XR to the P7J-Q85fB transistor modulator uninverted XSR-M-XSR-E over the J8 FC RSSI inpput to the PCCM SBUS then it will fly?

    Just kidding, do I need to know about this uninverted hack to get my first quad to fly?

    Reply
  4. snake675

    Thanks a lot Oscar, for putting this helpful information together in in such a clear overview! Hacking FrSky receivers is even more important since we use F.Port, since this only works with inversion hacks. Hopfully FrSky does integrate soldering pads for this in the future on all of their receivers…

    Reply
  5. Quannd

    I have a FC F4 Pro V2, and I do not know how to weld my XSR into it, please guide me with a specific image, would appreciate. I tried but did not see the light on the RX.

    Reply
  6. Chad FPV3D” Taul

    Really want to use new FrSky R9 Slim Long Range Rx with F4 FC (Hobbywing xrotor) and without being able to figure out anyway to get softserial activated, I therefore really need to figure out where the uninverted smartport pin (solder point) is on this particular Rx (R9 Slim 900mhz LR). Anyone that knows, please update page with photos because I have a feeling this will become a very popular combo that will be used – R9 Slim + F4 FC…. thanks in advance.

    Reply
  7. HereComesWhitey

    Hey, I have a question about getting an un-inverted signal off of a MOY DS801. banggood.com/MOY-DS801-DSMX-DSM2-Mini-Compatible-Receiver-with-SBUS-PPM-Output-p-1142298.html?rmmds=search&cur_warehouse=CN

    How do you do it?!?!? Haha
    I’ve got one of those Super_s F4 boards that I’m trying to use this Rx on but I have no idea how to get an un-inverted signal.

    Thanks for any help you can give!

    Reply
  8. Dave

    Hi! Wondering a bit about the X8R. Haven’t received mine yet but have been looking for uninverted smartport solution on the web with no luck. There seem to be two boards on top of each other in the X8R. If I would desolder the pins and split the boards apart and provide hi-res pictures of each side of both boards, do you think you would be able just by looking at the pictures to figure out where to solder on a wire for the uninverted smartport signal?

    /Dave

    Reply
  9. dudu du76

    Does someone know how to get un-inverted signal from the kingkong and/or griffonFPV one ? (the lightest i know of )

    I mean this one :
    banggood.com/Kingkong-Mini-Frsky-D8-Compatible-Receiver-SBUS-PPM-For-X9D-X12S-X9E-Transmitter-p-1094782.html

    and this one ;
    nwareno.blogspot.fr/p/nwa-events-store.html#!/Griffon-fpv-8CH-frsky-micro-receiver-V1-0/p/96708135/category=26401124

    Reply
  10. Sebastian Lemieux

    Super helpful. With the Betafight F4 only UART 6 is inverted, so you have to use the inverted s port signal if you want telemetry at all, and I’m using an R-XSR, so I’m really glad they added those little uninverted pads.

    Reply
  11. Jan

    Hallo @ all,

    I’m trying to connect a R-XSR to a Hobbywing F4.
    SBUS i s working great but i can not get the LUA script to work…
    I can do LUA with an XSR receiver when i do connect A and B but
    on the R-XSR i don’t know wher the B connectio is located…
    Can somebody help me with this ?
    I also have tried with softserial… no luck so far
    Thanks in advance

    Reply
  12. famtory

    FRSKY-XSR-M unverited pins: rcgroups.com/forums/showthread.php?2995696-HOW-TO-MOD-no-inversion-telemetry-FRSKY-XSR-M-Eachine-XSR-E

    Reply
  13. 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
  14. 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
  15. 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
  16. 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

  17. 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

Leave a Reply

Your email address will not be published. Required fields are marked *

Are you Robot? *

I only check blog comments once or twice a week, if you want a quick reply you can post your question on this forum IntoFPV.com... You might get a faster response from me there (multirotor related only).