Why FRSky CPPM signal is so disappointing. [UPDATED]


[ Update at the end ]

You bought a brand new FRSky with 8 channel and a promise of a helpful PPM Sum output.

Be warned that you cannot use this with 8 channels. Only 6 channels could be used with some risks (only 5 for real safety). [Note #1 at the end] It's really disappointing.

Yes. FRSky's CPPM signal has a BIG problem: It has a period of only 18 milliseconds. What does that means? Here we go:

A PPM Sum signal usually has a period of 20ms. As each channel uses up to 2ms so you need 16ms to fully accommodate the data from 8 channels (8 * 2 = 16ms).

Now comes an important element: the Sync Pulse. It needs to be wider than all other ones  to indicate the start of a new PPM train. Any 8 channels system based on a 20ms has room for a 4ms pulse (16 + 4 = 20ms). Even with all the channels at 100% a system like that still gives you a perfect sync pulse.



That's the BIG problem with FRSKy CPPM. If you start using some switches and knobs you are pretty much risking to lose the sync on your autopilot. Because the sync pulse is squeezed until having the same size of any channel.



I hope they can fix that with a firmware update sooner, because I believe it's not acceptable.

Until that, you cannot use it on your autopilot without risks. [Note #1 below]

[ NOTE #1 ]

This is far away from the ideal, but there is a cheat to eliminate the risks when using just 6 channels by suppressing CH7 and CH8 from CPPM.

At least on a ER9X or ERSky9X radios there is this way:
By changing your model's setup to use a Proto PPM 6CH it will not output CH7 and CH8.
(The frame space (300uSec) though is just ignored. I did not see any changes.)
3689479086?profile=originalI've verified on Oscope. It works!!! The CPPM was outputed from RX without CH7 and CH8.
Is still a shame and disappointing using just 6 from 8 channels. But that seems to eliminate the risk.

[ NOTE #2 ]

Jani, from jDrones took this issue to FRSky's GM/CEO and got a response from them. They are now baking a solution to release a new firmware. Probably the new CPPM frame period would be 27ms (the next available number dictated by the hardware's clock division).


Yesterday (28th September)  Jani came up with a beta firmware from FRSky. It does output CPPM frames at 27ms. So far we have some positive feedback from DIYDrones dev team's tests. I'll not update this post anymore. A new post instead will show some results followed by a mini updating tutorial for those CPPM capable receivers. Stay tuned! ;)


This post shows how to fix it.


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  • Really interested in a CPPM as well.
    FrSky really needs to fix it.
    This is the chance to show their level as a trustful company.

  • Hello, I have forwared this post to ParkeFlyer and ask if he can forward this to his suppliers to fix this.

    Hopefully he can help.

  • Now i know why my flightmodeswitch failed me once! I am using frsky d4fr in ppm mode on apm2.0. In one FPV flight i switched to alt hold an the quad kept climbing. Analyze of the log and watching the groundrecorder vid (minimosd overlay) showed that the apm never got the althold signal from my switch! I suspected a bad mixer setup in my transmitter - but it was ok - but i did it sloppy (i thought: who cares?) and distributed the flightmodechannel over the whole range (1000-2000) skipping several "boxes" in missionplanner. Instinctively i corrected my tx mixer to upfollowing boxes in missionplanner (so it stays clearly away from 2000) and it worked. Now i know WHY! Thank you very much.

    So long

    Kraut Rob

  • I use frsky with cppm on everything - autoquad will only accept a cppm input, so I am very keen for a fix!

  • Doubtful that FrSky will fix this.  How many people actually use CPPM mode anyways?  How many flight controllers besides APM 2.x support CPPM?

    Also, from the user point view, this problem should not affect people that are still using the channels individually and not in CPPM mode.

  • How hard could it be to fix?  Not knowing much about how it's done... but it seems pretty simple?  Just make the period longer?

  • Developer

    I am still working and trying to convince FrSky to fix this issue. So let's hope the best that they will listen us and fix it asap.

  • Developer

    I've got many hundreds of dollars invested in FRSKY equipment, and I've recommended it to friends. Unless we can get a fix for this, I'll be looking for alternatives.

  • I'm reading a bit more into this.  Apparently one idea is to use very low values for your switches.  Say, 1000-1500.  So that would work on Ch 7 and 8.  Now, since most people don't use Ch 8 anyway...

    I wonder how low we can go on Ch7 and still have it work?  Would 1000-1200 work?

    I have just redone my radio setup, I set Ch8 permanently low (well, it's on the trainer switch, best I could do on the 8CAP?)  And I used endpoints to set Ch7 from 1000-1500. In this configuration, I can only invoke the problem with all 7 channels at their EXTREME limits.  It doesn't happen until the last little bit of the last channel.  I'm reasonably happy with that, but I am not sure Ch7 switching is actually working, I can't test it. 

  • I had to test this because not only am I heavily invested in this (I think 6 of these Rx's?) but I've also been a huge proponent of the system.  I booted up my quad which is using CPPM on a D8R Rx with the PPM firmware.  It was not hard to replicated the failure.  If I set Ch 5, 6, 7 and 8 high, you can move the sticks and find the point where the signal is lost and basically all the controls are locked.  You do not regain control until you back off one of the channels.

    Not good!  I'm really disappointed in FrSky on this one.  Hopefully they fix it!

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