Servers by jDrones

Connection between pin 15 of RFD900x and FRSKYX8R SBUS


I have a question regardin connecting pin 15 of RFD to a FRSKY receiver.
it mentionned that a logic level shifer / diode or resistor mght be needed as pin 15 of RFD is not 5v tolerant.

I have seen many people connect pin15 directly to X8R, L9R or RXSR from frsky.

Do you know if it is okay and could you explian why ? and how to detect receivers that are compatible ?

Best Regards

I am confused about your question.

The RFD900 connects to Tele port on the FC normally.

I never power RFD via FC due to high current draw.

Can you elaborate on your question and where did you see that pin 15 reference

It’s Sbus.

I was going to link another thread but I see you are already posting there.


I am not talking about air connection between RFD and FC, rather talking about ground connection between TARANIS —> X8R----->RFD in order to use SDUS PASSTHROUGH

I would like to know if pin15 of RFD can be connected to sbus of X8R direcly without using a logic level shifter or pullup resistor.


@dave as I have mentioned in PM and the previous post yes you can its a simple connection pin 15 to signal on the X8R sbus. Then you need to make sure and hook the ground as well. there is no need to use anything but straight wires to hook this up.

Can you kindly draw a hand sketch of what you are trying to do here. Sorry, I have not personally ever tested what you are trying to accomplish here.

Pin 15 on RFD900x is a Digital I/O, PPM 1/O pin. As this is an I/O pin, 3.3V is ok, because most outputs do not go higher than 3.3V when active. Only power source on Rx or FC will go to +5V. So no need for any pull down resistors.

"GPIO pins will configure for 40k Ohm pull down or pull up based on setting as input or output. They can sink or source approximately 5 mA each."

The RFD900x manual is very poorly written probably by an engineer, not a technical writer:)

What the above statement means is that on PCB board itself, they have used 40 ohm resistors to keep the voltage from going above 3.3Volts.

Read this thread. Now I understand what you are doing.

yes, it will work.

On the Taranis side as the output voltage is between 7.2 to 8.4V (Please measure it first) from the module plug, you will need a like 9V to 3.3V converter from ebay.

Don’t use a resistor unless you know what your are doing.

As per the above thread i am also not seeing a practical application of this solution.

Why not just buy an R9M module for Taranis for 45 dollars, use a frySky Rx with FC and run Yaapu telemetry if that is the objective to get telemetry data inside the Tx, unless you are using RFD to control the UAv or some device inside your UAV.

Just my 2 cents.

I amnot connecting the RFD to the back side of the Taranis
I am connecting the x8R sbus to oin 15 of the RFD

Ok…and what is your objective here? Sorry, never tried such a setup.


if you look in this photo I have the R-XSR frsky set to sbus and connected directly to the pin 15 on the rfd I also have radio boxes for all my UAS that have a X8R connected directly to the pin 15 on the rfd. There is no need to do anything special just hook the x8r sbus signal pin to the pin 15 on rfd and set rfd to sbus in and you are good to go, also make sure you set the rfd on the vehicleside to rcout it does not need to be set to sbusout to work rcout will also work. Ignore the teensy as all that does is pass the mavlink messages to the taranis so I can have a onboard telemetry screen made by @yaapu & @Eric_Stockenstrom

Thanks @Matt_C that is very clear.

Just did that today and it works like a charm.
I noticed that when testing failsafe on the ground by disconnecting the ground rfd, arduplane goes to circle then rtl as expected.
But when i plug the rfd back in, I noticed sometimes (happens very rarely), that the rtl mode is stuck and can’t change it.
I need to unplug the rfd on the ground and plug back in, in order to go back to fbwa.

Do you know what might cause this ?


Servers by jDrones