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
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).
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.
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.
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.