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.


E-mail me when people leave their comments –

You need to be a member of diydrones to add comments!

Join diydrones


  • David: I didn't know that PWM was still output on ch 4-8. Very interesting. Thank you.

  • You are correct Oliver.  I have a D8R-II Plus here in front of me with the D8R-XP 27ms firmware that is connected via CPPM pin1 to the APM and working fine.  Just jumper pins 3&4 and CPPM comes out pin1, and pin2 has RSSI, just like the XP.  Interestingly, pins 4-8 still output PWM, mirroring the CPPM channels 4-8.  However, note that FrSky hasn't tested this and does not support it...but it seems to work fine.

  • Apparently you can flash the D8RII-Plus with the latest D8R-XP firmware. This will enable all channels over PPM:

  • I am currently using the D8RII-Plus on my three quads but would love to lose some of the wires and get rid of the extra weight and vibes. Are these updateable to add CPPM or would I need to purchase new modules? Sorry for the dumb questions!

  • D8R-II+ does not output CPPM natively, so you shouldn't be worried because you can't use it in this mode as is.

    However, the 27ms firmware can be applied on your receiver.

  • I have a D8R-II+

    Should i be worried? Because there isn't a firmware update for this receiver.  Or this product isn't affected?

  • When it comes to RC low pass filters, there is no such thing as optimum.

    Use any resistor in 10-47k range and capacitor between 1-10uF, the product of their values must be bigger than 0.02, the bigger the better, unless your OSD has a RSSI input impedance lower than 100k, in this case you need a buffer.

  • @Sandro: any recommendation for what the optimum resistor/capacitor filter combo is. No rush, just checking.

    Also, is it still necessary to recompile the Arducopter firmware to enable RSSI input via APM_Config.h (as per these instructions: I just noticed that the wiki entry for APM2board lists A8 as "RSSI / Motor LED" => (2nd pic)

  • ANY D8 receiver, since the first days in 2010 can be flashed with any Frsky firmware, nothing changed at hardware level, only pin ports added or removed.

  • Is it safe to assume that a D8R-II PLUS can be flashed with the new XP firmware?

This reply was deleted.