Uninverted SBUS and Smart Port on Frsky Receivers

by Oscar

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.

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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


Table of Contents


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.



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+ review

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


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


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.

R9 Mini

Credit to David Marzocca he found the “uninverted” SmartPort on the original R9 Mini receiver. I haven’t tested it myself however so use it on your own risk.


R9 MM is a new version of the R9 Mini. The only difference is now the R9 MM has an “uninverted” SmartPort output you can easily access (but Frsky call it Inverted S.Port so don’t get confused!). It replaces the SBUS_IN pin.

This saves a lot of trouble soldering the tiny wire! Buy the R9 MM here:

R9 Slim

Credit to AJ Karki for posting this picture. According to him in the comment: this is uninverted sbus out point where you can connect and use uninverted sbus out. He’s using this rx uninverted sbus out without any problem. Use enamel coated thin wire for easy to soldering after that use some glue to protect the connection and wire.

I haven’t tested it myself however so use it on your own risk.


Thanks to voodoo for pointing out the missing XSR-M and provided info and sources regarding the uninverted SBUS and Smartport pin location.

Edit History

  • 2016 – Article created
  • Jul 2017 – added info about R-XSR, XM, XM+
  • Oct 2018 – added info about R9 Mini, R9 MM, R9 Slim
  • Apr 2019 – added XSR-M

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Sergio 9th February 2024 - 5:30 pm

Hola, haz el favor de poner información en la web sobre el receptor frsky archer sr6, gracias

Martin Greiner 5th February 2023 - 5:14 pm

Do you also know the inverted SBUS pin on an Archer R4?

Michael Sutherlin 14th January 2023 - 9:42 pm

Do you know where the uninverted signal pin for a Archer R6 is ?

Jeff Grills 3rd December 2021 - 9:55 am

I tracked down the uninverted SBUS and S.Port pins on the Diatone MSR, which is basically the FrSky R-XSR in a different form factor; it’s even firmware compatible. The photos you have of the R-XSR made it quick to track them down too, thanks for that. Here’s a link to a photo of the pins I identified photos.app.goo.gl/f9fNgv6Fn6bhNByt5

Alan Kitching 18th September 2022 - 4:38 pm

Hi. I moved to FrSky from Spektrum because I wanted better telemetry, but I am having the same difficulty (i.e. I have failed) in getting telemetry data from the TELEM2 port of my Pixhawk 4 via an FrSky R9MM inverted S.Port to the telemetry page of my FrSky Horus X10.

I am not a programmer, but ok with hardware, and have now spent about 20 hours investigating and reached a dead end

I have re-flashed the Pixhawk 4 with the latest firmware (V13), and have set the “TEL_FRSKY_CONFIG” parameter to be TELEM2.

The inverted S.Port of the receiver is connected directly to the Rx(in) (pin 3) of the TELEM2 connector, and the Tx(out) (pin 2) of TELEM2 is connected through a diode (cathode to TELEM2 Tx(out)) back to the inverted S.Port of the receiver. ie data are “diode ORed” together, and time division multiplexed.

The scope and logic analyser appear to show that after the receiver outputs two bytes (the first of which is always seems to be 0x7E), the Flight Controller (PixHawk 4) responds with 8 bytes. The idle level is “HIGH” for all data

However, this does not result in any telemetry being delivered to the transmitter, other than the receiver signal strength and the battery voltage.

It appears that the Pixhawk 4 is using 2 wires for communication, rather than half duplex.

Can anyone suggest what I may be doing wrong?

Many thanks.

Alan K.

Robin Haswell 18th September 2021 - 12:39 pm

Hi Oscar, do you know if this mod is possible with the Radiomaster R81?

Gatze 12th March 2023 - 3:48 pm

Bit late response, but I used a inverter via a some resistors and a BC547 and it worked, could not find directly a inverted signal on the board (very tiny), used something like this: google.com/search?q=inverting+sbus+schematic&client=firefox-b-d&sxsrf=AJOqlzV4MJwkU_x5eG0b1UotIfao3Ygxpw:1678636051910&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjf99LM3tb9AhUPtaQKHeYwD-cQ_AUoAXoECAEQAw&biw=1536&bih=721&dpr=1.25#imgrc=6u21vN6SfaP9YM

Manjunath B 6th September 2021 - 7:49 pm

Recently I tried to update my rxsr and r9mm (2018) to F port firmware(latest) available at FrSky website using taranis x9d plus Jr bay pins. After the update only the telemetry was not working in both the receivers. So I tried to flash older firmware using taranis, I received a message saying receiver not responding. I tried serial pass-through method using betaflight, it didn’t work. So I bought stk s. Port tool to flash them. This didn’t work.
Later I flashed my R9 module with Express lrs. Then I tried to flash r9mm through serial pass-through it failed. So I tried ST Link method. Through the ST link app I disabled the read write protection and then I flashed the Elrs firmware lt worked.
Now with rxsr is their any pin out for current, Ground, clock, data, so that I can disable the read write protection as mentioned above and flash older versions and use it.

Krystian 25th February 2021 - 12:32 pm

REMEMBER TO CHECK YOUR FIRMWARE IN RXSR!! FPort firmware requied if u use Uninverted Smartport, not normal!

Jesse 21st October 2022 - 6:29 pm

Hi Oscar thanks for knowledge… Im getting Failsafe arming flag when my Transmitter is connected and the channels are right and I’m stuck at what it could be for 3 days now.. Its the F4 Happymodel Dimond FC FR 1 S Spi rx Frsky and can’t get it to arm..??? Any help would be greatly appreciated….

Jean Vautrin 26th October 2020 - 4:07 pm

Hi Oscar
Thank you so much for all you tutos.
I am getting confused with the inverted being non-inverted and the F4 inputs.
I try to set a FrSky R9MM with the Transtec F411 of a Beetle HD with DJI air unit.
Should I connect what is labelled S.Port/F.Port or what is labelled inverted S.Port to the UART 1 TX ?
How should I set up Betaflight ports ? Should I touch the CLI ?
Thanks again.

Robert 19th October 2020 - 7:26 pm

Hi,,, is there similar points in a X9D+ ? Horus X10S?
The JR bay have a S-port out but inverted,, need to find the uninverted “Tap”

Alb 16th October 2020 - 3:44 pm

Hi, but for Jumper R1+ where is the uninverted pad?

famtory 22nd January 2020 - 4:21 pm


Don Sales 4th December 2019 - 5:10 am

Hello Oscar, if you need to update the R9 slim uninverted s.port, I can share it to you.

Giovany Arana 15th October 2019 - 1:19 am

This is great information. I solved my Rx problem with this information. I have a Mamba F405 mini V1 and the SBUS pad got ripped of in a crash. I, then, connect the SBS to the UART6 but nothing moved on the Receiver tab in Betaflight. Now, with this explanation you gave, it works again. Really hard to solder that thin spot, but it worths it. Thank you.

Aleksey 13th September 2019 - 6:59 pm

Hello. Does any body find uninverted sbubs on Jumper R1 ?

Joe Cosentino 27th August 2019 - 5:55 pm

Hi Oscar, Just wanted to drop you a note that the FrSky XM+ Sbus mod works like a charm. Yes its a challenge to solder a wire to the center pin on that circuit but the reward is a operational RX. It took time to find the info on one of your web pages but I am happy that this now works. Its one more step to get my first racing FPV quad into the air.

Thanks so much for posting this information!
Joe C.

Teo 2nd August 2019 - 1:17 pm

What if a r9mini is just flashed with the mm firmware? Given there are no hardware differences between the two, this should give us inverted s.port on that sbus_in pin.

Oscar 12th August 2019 - 11:00 am

no, i believe the schematics are different, so flashing wrong firmware will lead to unexpected result.

Franz P. Val 9th May 2019 - 1:24 pm

The XSR-M uninverted mod don’t work. The pin on the “J6” part is something like a TX/RX switch… I don’t find a mod that keeps the “normal” / inverted S.Port pad functional… Soldering direct to the STM32F103 µC is not possible with a “dick” silicon wire.

RICHARD ROH 24th January 2019 - 5:54 am

Hello Oscar, I am a long time follower and I come back often to read up on your literature. I recently bought several R9mm and a R9m module bundle to use on my quads. I soldered my r9mm mini using sBus out to the sbus pad on the Clracing f4s board and the Inverted S.port to the telemetry pad on the FC. I get telemetry an Rssi, but I cannot use my Qx7 in OSD to change anything. When I move the yaw/throttle to mid and left an pitch forward I don’t get the menu. What am I doing wrong?

Oscar 10th February 2019 - 6:46 pm

Did you enable Betaflight OSD?
Are your stick end points calibrated properly as suggested in this guide?

AJ Karki 20th November 2018 - 8:00 am

Thanks for sharing R9-slim rx uninverted soldering photo of mine. But this is not uninverted smartport connection, this is uninverted sbus out point where you can connect and use uninverted sbus out. I’m still using this rx uninverted sbus out without any problem.
Use enamel coated thin wire for easy to soldering after that use some glue to protect the connection and wire.

Oscar 23rd November 2018 - 4:57 am

Thanks AJ

DJ 27th October 2018 - 11:49 pm

I recently upgraded to a x9d, I am curious since I was coming from a radiolink where I needed to invert the signal for sbus. Can I use the same inverter chip to and us a UART that has an uninverted signal for Telemetry?

Dean Hallal 15th September 2018 - 11:38 pm

May be a dumb question but is RSSI and battery voltage the only reason to connect SPort? I know on the VTX SPort it allows me to change setting on the VTX.

Oscar 18th September 2018 - 6:14 pm

It’s also necessary for LUA script to work (for changing Betaflight settings and VTX settings)

Christoph Kloeti 10th July 2018 - 9:12 pm

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

Oscar 24th July 2018 - 6:17 pm

Hey Christoph, yes I am interested, please can you share that with me?

Trevor Long 25th July 2018 - 2:30 am

Christoph, could you please post the r9 information somewhere? I lifted my sbus pad and I need an alternate source.

Leonard Burns IV 25th July 2018 - 5:38 pm

Can you post a link to an image showing where? I can’t find any on my R9 Slim

LitterBug 8th June 2018 - 8:48 pm

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


esher 5th July 2018 - 11:55 am

AFAIK there is no uninverted pad on R9 Mini.

David 2nd September 2018 - 11:01 pm

I found the uninverted pin on the R9 mini, checked and it works. Beware, it’s a really difficult soldering job on a micro pad. Photo here: dropbox.com/s/2ps3y7j18pku58t/R9mini_uninverted.jpg

N D 29th May 2018 - 9:42 pm

Can you add the XSR-M to this article? or is it already uninverted?

Bobby 16th May 2018 - 2:12 pm

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?

Oscar 17th May 2018 - 5:22 pm

Only if you use Frsky radios and F1/F4 FC :)

snake675 11th May 2018 - 4:41 am

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…

Quannd 17th April 2018 - 4:19 am

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.

Chad FPV3D” Taul 19th March 2018 - 2:27 am

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.

Javier 28th May 2018 - 6:35 am

I am in the same situation. Anyone figured out how to do it?

Junmar A Sales IV 8th June 2018 - 8:07 pm

R9 slim has uninverte s.port and sbus.

HereComesWhitey 28th January 2018 - 4:46 am

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!

Dave 13th January 2018 - 3:17 am

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?


dudu du76 11th January 2018 - 3:38 pm

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 :

and this one ;

Gabriel Burchard 9th January 2018 - 7:26 am

Here is the Missing XSR-M

Sebastian Lemieux 9th January 2018 - 5:17 am

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.

Jan 26th December 2017 - 5:06 pm

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

Oscar 3rd January 2018 - 1:41 pm

Check the last diagram for uninverted SmartPort.
Make sure your RX is updated to the latest firmware.

famtory 7th December 2017 - 3:39 am

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

Matt Brown 17th November 2017 - 10:27 pm

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!

Oscar 18th November 2017 - 7:57 pm

Just use Softserial :) here is the tutorial.

drew 5th November 2017 - 7:32 pm

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.

Oscar 6th November 2017 - 6:37 pm

thanks for the update, yes I personally tested it as well and it’s working fine!

Mathias 15th August 2017 - 5:24 pm

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!

Mathias 15th August 2017 - 10:20 pm

P.S.: Sorry! Forgot to say that im mainly interested in the smart port uninverted signal!

Oscar 21st August 2017 - 2:30 pm

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.

Chris Eilbeck 16th November 2017 - 10:07 pm

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.


Z 9th January 2018 - 3:35 pm

Chris: Can you get some pics, or more info on getting the smart port from a X8R?

Filip 25th July 2017 - 2:16 pm

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!

Oscar 3rd August 2017 - 2:33 pm

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 :)

Rick 9th August 2017 - 8:59 pm

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.

Oscar 11th August 2017 - 4:03 pm

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

Andrew 22nd October 2017 - 6:37 am

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.

Szper 24th October 2017 - 12:26 am

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:

Luca Filippi 17th July 2017 - 12:11 pm

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!!!

Oscar 24th July 2017 - 3:35 pm

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

David Posea 3rd September 2018 - 7:11 pm

If you send me an e-mail I’ll send you good pis of my dead X8R, or I can send it to you. I burned the pads removing the daughter board. I realize this is old but I couldn’t find any better info .

David Posea
dposea AT yahoo.com