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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  • Developer

    @John: Those FRSky's TF Series are full compatible with FASST 2.4G Systems and they have a PPM framing at 20ms. So, you're free of this problem.

  • Does this affect the FrSky TFRS receivers as well? If so can you point me to the link how do update the firmware?
    Thx John

  • What's the main advantage of using the CPPM signal with APM 2+? being able to also access the RSSI pin on D8R-xx series receivers?

    Also, is there a increase or decrease of resolution when using CPPM (vs PWM)?

  • Developer

    @David, you can update D4R-II easily to use this new software or old. Both are working fine. I have here several D4R-II receivers running with new software.

    And for new firmware that we are testing, so far we have not found any issues with it. It looks like we have something that can be released for public. Will discuss with FrSky on beginning of next week when holidays in China are over and people returns back to work.


  • @ David:

    Thank you much!  D4R-II on order, sounds like it will solve all my problems with no need to flash anything :)

  • Developer

    "[...] 50% decrease in control [...]"
    @Kenn. That isn't true.

    It's 17% more faster than you said. And 24% more, considering the standard.

    37Hz = 67% of 55Hz (non-standard 18ms)

    37Hz = 74% of 50Hz.(standard 20ms)

  • @David, @R_Lefebvre--

    Do you actually have any data to back up the claim that a 50% decrease in control rate is not noticeable for average users? I suspect that this is not correct. I would hate to encourage people to think that this is tested if it is not.

  • @Dale, per the FrSky sales rep, and this thread, the only two receivers supporting the new 27ms CPPM mode for 8 channels are the D8R-XP and D4R-II.

    and I agree with R_Lefebvre about CPPM latency...if you're just leading an autopilot around, you'll probably never notice.  If you're Alan Szabo Jr. flying a 3D heli in aerobatic competition, you would probably notice right away. But in that case, you could simply use the direct PWM outputs and you're good to go with low latency again.

  • The only disadvantage I'm aware of is a bit more latency and lag in the controls, but I'd be really surprised if anybody other than The Flash could actually feel it.

  • I should add, I need CPPM to enable all 8 channels on a MultiWii as parralel channels will only allow for two AUX

    so I NEED 8 channel, thus CPPM.

    Are there any disadvantages tot he 27ms fix?


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