Hi all,

I'm trying to cut down on cables and move to a single cable ppm input on the APM 2.5.

I have the input coming into input 1 and a jumper on between the singles on pin 2 and 3.

I have the output coming out of the openlrsng receiver on pin 6 (channel 5) and set this up as PPM using the configuration.

Where am I going wrong?

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I don't know what's wrong, but I can point you to this other firmware and detailed setup instructions. I'm flying 12 km with it with full APM telemetry and RC support.


If you have anything other than 8-channels in your Tx settings, try setting 'Limit PPM Output to 8 channels' in Rx settings to yes. APM doesn't like PPM streams with +8 channels. It works on any of my dtfuhf receivers I tried, should work with HK ones too.

Could be due to the OpenLRS generating a falling edge for PPM instead of a rising (pause is high and pulses are low). 

The other problem could be the OpenLRS Telemetry generating to much noise on the output pin.. I have exactly the same HobbyKing Module and hooked it up to a scope (because i have similar problems with multiwii): https://www.youtube.com/watch?v=vw5nMU4zotQ

have you configured the RX to output PPM on that pin?

The Pin is the right one b.t.w.

any progress with setting up ppm?

I have the same problem,

Apm doesn't see openlrs's ppm signals.

Any progress made?

would be very helpful :)

Actually OpenLRS produces a very short 'low' pulse on the serial PPM, like 50 µs. For some systems this is not an issue, as the important data is the time between two pulses, not the pulses themselves.

But for APM (and Pixhawk), this is too short.

I made a special firmware to correct this issue and have normal 300 µs low pulses and this works fine with APM telemetry at 19.200 bauds. The only constraint is that it's made to use with two 1W TX modules. I've flown up to 12 km with APM and telemetry. All of this for less than 60 $ (2 x 1W OrangeRX TX)

If you're interested the details and firmware are here : http://www.itluxembourg.lu/site

And RCgroups discussion here : http://www.rcgroups.com/forums/showthread.php?t=2037442

If someone is interested I can consider adapting it for 'normal' 100 mW RX-TX.

Thanks for the reply ben,

It would be really cool if you could make the firmware for the normal receiver,

But if it's too much of an hassle than forget it,

I just wanted to see if it was possible, it would clean up my copter.

But oh well, plugging the cables only on the signal pins works as well

Hey Oliver

I struggled with this for awhile too. your wiring looks correct but as someone mentioned the minimum sync time could be a factor. My setup is as follows. OpenRX/TX 433mhz flashed through openlrsng google configurator. I had firmware 3.70.

I messed with sync time, standard for me was 3000 but I tried 4400 that didn't work then tried 5000 that didn't work so then for some lame reason I tried 4350 and it worked! That didn't really make sense to me why that value worked but what I think did make a difference and maybe you should try is to rebind the tx/rx once you have this configured. I tried the rebind once I did the 4350 value and it worked.

Good luck!

With OpenLRSNG PPM is on channel5 / port 6. 

I've exactly the same receiver as yours and it's work with 3.6.4 and 3.7.1 from Kha and google configurator but you have to choose in Rx side of google configurator to output PPM on channel 5 (choose 'PPM' : it's the only channel where you can choose it ) and limit to 8 channel. Without that it's not working at all. 

On APM (mine is 3DR 2.5), you must short signal Input 2 et 3 with jumper. 

I have a 9XR with openTx setup to default : 22.5 / 300usec and it's work. 

Ps: I've just tested version of Ben from  http://www.itluxembourg.lu/site and it's working very well !!! 

I'm using Orange OpenLRS TX 1 watt (JR) Hobbyking with Orange OpenLRS 9 CH 100 mw . 

In this case, you must plug PPM in OpenLRS RX channel 3/port 4 of OpenLRS RX to APM Input 1 with jumper between signal channel 2 and 3.

He just release a software updater (only beta 1.04 work at this time) and it's very simple and so easy to update RX and TX...


No need to config anything !!! it's just working simply ! Many thanks to Ben !

Just need to know how to output RSSI from RX to APM

Hi Thierry,

It's done ! just go to www.itluxembourg.lu/site and you'll find the details. You can use two 1W TX or one 1W TX and a 100mW 'RX'.

A new configurator is available which takes care of uploading the firmware.

update: with 1.0.5 firmware from Ben, RSSI is output on OpenLRS Rx 100 mW 'RSSI port'

I have the same setup.  I will look at my configuration when I get home from work.  But I'm pretty I have the that sync time set for 4000 something.  I also have it set for limit PPM to 8 channels.  Works flawlessly.

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