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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  • That sucks blue monkey balls.

  • Developer

    I just received direct information from FrSky internal peoples and message goes: we know this problem but we will not fix it as new technology is coming soon. Till then use only 6 channels.

    That's a bit sad news but we do not have any support from FrSky for this issue.

  • I've sent an email to:

    Hello, I am contacting you to find out if you are working to solve the problem of the CPPM timing on your receivers.  I have bought many of your units (8 receivers, 2 transmitter modules, and telemetry display) and have been very pleased with the performance.   More importantly, I have given very strong recommendations to other people that they should buy your systems because they are a very good value and perform well.  I am a developer on the Ardupilot project, and using CPPM is important for reliability of the data link between the radio receiver and Flight Control computer.  It's also nice to eliminate wire clutter.

    But recently have found out about this problem where the frame width of 18ms is not capable of properly supporting 8 channels.  I have been using all 8 channels, and I have been lucky not to have had a problem!
    I would like to know if you are working on a firmware update to fix the problem?
    I think everybody, and I mean everybody, should sent them an email.
  • This is a bit disappointing. However, FRSKY is one of the ONLY companies out their that listen to their customers so I'm sure they will make the change if we ask. which I'm sure a lot of people have done already.

  • I.S. the problem is real.  As I said, I tested it, and it really happens.  When all the channels go high, the system locks up.

    Crashpilot: Are you sure that's what happened?  This only occurs if virtually ALL of your channels are high.  You need all 8 channels high, if even two of them are 1500 or less, the problem is virtually impossible to get, and if two of them are near 1000, it is impossible.  Why would you have had channel 8 high?  And you were holding full pitch, full roll, full rudder and full throttle when you tried to switch modes?

  • Developer

    Yes, all my multicopter are S.BUS or PPM these days. Comparing the clutter of eight wires to a single one there is no question. Especially when you want to move the receiver out on a arm for separation. But I also use FrSky receivers.. :(  Guess I have been very lucky so far. Time to power up the scope and have a look.

  • @Ellison The question is not how many autopilots need CPPM, the question is rather how many support invididual channel inputs. Apart from APM and maybe UDB almost anybody else only supports CPPM. Once you have done a CPPM setup with a small receiver and a single connecting wire, you will never want to go back. Feeding 8 channels with 8 wires is a huge wire mess, consumes a lot of volume and weight. And there is not a single good reason to do so, except for maybe reusing old gear.

  • (btw, the problem isn't new and exists since a long time, so i'm not very confident in a quick update/resolve)

  • until new firmware from frsky, i'm using this  cheap as hell, working great with frsky rx.

  • @Sandro
    BTW, have you tried to test this with real high PWM values.
    Maybe the system "widens" the period further than 18ms when needed. (crossing fingers)

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