RDF900x PPM Documentation for APM


Is the RFD900x PPM feature documented anywhere for use with the APM telemetry link?

I am only finding “bits and pieces” on the APM forums. It seems like it would be a good promotion for APM to integrate both R/C and Telemetry into a single wireless link that has less latency than the current Joystick/Gamepad via Mission Planner.

Perhaps I am missing something…

nope, you are not missing anything, it’s a great solution. :slight_smile:
datasheet is here
http://files.rfdesign.com.au/Files/documents/RFD900x%20DataSheet.pdf or
( note the AT&R command as well as RT&R command )

more clues here:

also, it is recommended to use firmware V2.4 or later when using PPM, as earlier version/s had some minor bugs.

for setting up failsafe values, note that “you cannot set the default signal correctly until you have an active ppm stream going to your vehicle, otherwise there is no stream to record.”

google has more details if you look,. :slight_smile:

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Hi David,

Thanks for the feedback. Do you know if anyone has interfaced the RFD900x to a Taranis yet? I read about some confusion with the PPM voltage levels and never saw a resolution.


I don’t fly a taranis, but it’s easy to check what max voltage/s your transmitter output/s on it’s PPM output with something like an ocilloscope or similar. You can then match that to the RFD900X PPM input ( which is 3.3v rated) by choosing two suitable resistors. ( a resistor divider is a very common approach, and very cheap and easy and flexible. ) . google knows more if you don’t. :slight_smile:

Ok, good to know. So the competition is Dragonlink and the TBS Crossfire? My assumption is that these solutions are for the R/C link and still require a separate telemetry link for APM?

I did some more research on Dragonlink and TBS Crossfire. If this information is still valid, it makes the RDF900x solution look even better for APM.

It appears that Dragonlink is only on the 433MHz band requiring an Amateur (HAM) Radio license in the USA.

From posts on FPVLAB by TBS in January 2016, it seems that only the PX4 stack supports the Crossfire, not the APM stack. I am not sure if there is newer information.
APM stack does not support it yet.

I did find a mention of Crossfire in the Wiki but not much information.
FrSky Telemetry

Adding Ultimate LRS as another solution.

Hi Greg, Another interesting product is the CUAV Hack Link which is supposed to integrate RC control, telemetry and video. But I have not found a single user to actually get it to work with the RC control signals. To me the holy grail is getting all these on one transmission.

I have used ULRS for a couple of years and it works really well for me.


Hi Mike,

Thanks for the link. I agree with your “holy grail” goal but the current solutions seem to be pricey and lack the distance.

At this point, I have been using 900MHz for Telemetry, 2.4GHz for R/C, and 5.8GHz for video. Combining Telemetry and R/C at 900MHz in the US would be sweet but I’m also open to using the ULRS. I’m curious if the video distance greatly increases using the licensed 1.3GHz band over the 2.4GHz or 5.8GHz bands. I suspect it does…

I definitely recommend ULRS. It is dirt cheap to play with and will certainly get you out 5 miles with a reasonable set of antennas. But as you note I have my amateur radio license and that can be a problem for someone wanted to follow all the laws. Not very many of those in RC;)

1.3g is definitely going to be less line of site for you. The nice thing about ULRS at 433mhz is being able to use 2.4ghz for video. For me it is the perfect combo. 2.4ghz for better long range video yet antennas that are a lot smaller than 1.3ghz.