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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  • From ch2 signal pin.

    "...still get RSSI..." what you mean by still ? How you get RSSI before upgrade ?

  • MR60

    After upgrading A D8DR II Plus with this 27ms CPPM firmware, will APM and mission planner still get RSSI information ? Or Do I need to wire something else and/or recompile the Arducopter code ?

    Same question if Pixhawk is used instead of APM ? How do I get RSSI from a D8DR receiver upgraded with CPPM firmware ?

  • If only 1` wire from your receiver to the APM you better do the 27 mS software update.


  • Moderator

    Are you using one wire from the Rx to the APM, or are you using one wire per channel from the Rx to the APM?

  • thank you, yes after i purchased i dint do any mod to it, so it should be PPM.

  • My guess is that you are using D8R as a normal receiver, and not in CPPM mode.

    The issues described in this thread are for CPPM mode only, not apply for normal receiver usage.

  • Hello Guys,

                     I just came across this thread, as I have a D8R-II plus /DJT frsky module, using along with er9x, i guess am using PPM, should i be worried ? am a noob :).. pls let me know..



  • Hi all, quick question and apologies if this has been answered. Once your FRSky D8R receiver has been updated to 27ms firmware, must you set PPM FRLen to 27ms from the default 22.5ms? Will it cause issues set at 22.5 or 27, or will it make no difference to the signal from transmitter to module to receiver to APM?


  • I tried both D8R-II and D8R-II plus. Both accept the 27 mS firmware. As a bonus, they both function as intended! Beauty! 

    Once programmed with this firmware, *if* channel 3 and 4 (signals) are shorted on power up, channel one becomes serial PPM and channel 2 becomes RSSI. Great!! 

  • Yes, was my special design criteria for Frsky when XP firmware was issued.

    The reasons to move CPPM and related signals to lower channels pins, instead ch8 as before, are obvious:

    - people who need CPPM decodes at least the first four channels in their FC/OSD, so these pins on receiver are useless

    - not many FC/OSD decodes more than 6 channels, so at least channels 7/8 must be available decoded on receiver, for those using head trackers or similar devices.

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