[ 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.)
I'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).
[ UPDATE ]
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! ;)
[ UPDATE ]
This post shows how to fix it.
--Sandro
Comments
@Oliver: No, unfortunately the D8R-II isn't on their list and I really don't know about its hardware.
About the RSSI: It's not compatible as it is. Like I said the RSSI signal from those RX is a PWM at 9.2us period. So, you need to add a "Low Pass Filter" before feeding an APM's analog port. I'm running tests right now with some different filters ("faster but rippling" and also "slower but cleanest"). It will become a tutorial with lots of infos, waveforms, graphics, etc.
That's great! =)
Time to upload the tutorial in few mins.
Status update to all: After several days we gave green light for FrSky for this new custom 27ms software and I am happy to let you all know that software is now publicly available for download.
So if you need to have 27ms and 8 channel output from your PPM capable receivers. Point your browser to FrSky pages and get latest software upload.
For people who did download test files from jDrones website, old files will be now removed and there are only one official software.
FrSky download page for official software is: http://www.frsky-rc.com/download.asp?id=23
Files are under names of:
- CPPM 27ms for D8R-XP
- CPPM 27ms for D4R-II
@Oliver: The advantage of using CPPM on APM is due to use just one cable for all the 8 channels and also on having access to the RSSI output signal (which is a pwm pulse running at 9.2 microseconds period i.e. around 109kHz).
D8R-XP firmware has pwm RSSI output from factory. D8R-SP doesn't have it. However they share the same FCC ID: "XYFD8TD8R". So, by updating any SP with a XP firmware it would remap the SP pins to XP's layout allowing you to enable RSSI or CPPM at the same way.
PWM at 18ms has more resolution in terms of time (more updates per second) and also has no problem of space between the channels so you can safely increase the wide of channels (more resolution in terms of steps between min and max values from sticks, knobs, etc).
The big difference is when you compare CPPM periods:
CPPM at 27ms has more space between the data from channels(less updates per second). However, it allows you to safely extends the wide of the pulses of all channels (more resolution in terms of steps from the sticks, knobs, etc).
CPPM at 18ms has less space between the data from channels i.e. more resolution in terms of time (more updates per second). However, there is no space for full 8 channels neither for expanding some of them safely.
@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
@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.
"[...] 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)
@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.
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