Frsky problem.

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Re: Frsky problem.

Postby takilara » Thu Aug 12, 2010 12:11 pm

I think maybe there are separate issues. My issues are solved. (jittering/choppy movement, digital servos). This was fixed for me by changing the shift polarity in the Tx. I never tried it in the air though, and i suspecct that the problem would not have been noticeable in the air.
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Re: Frsky problem.

Postby JMP_blackfoot » Thu Aug 12, 2010 12:44 pm

JMP_blackfoot wrote:
takilara wrote:Switching from Negative shift SFTN (According to the manual, used by Hitec and Futaba) to Positive shift SFTP (used by JR, Airtronix and Multiplex) PPM resolved all my jittering, and choppy behaviour! Now the servos are as smooth as ever, and no buzzing at neutral...

To explain this, my theory is that the timing from coders is accurate and jitter-free on one edge of the channel separating pulses, while the actual width of the pulses is maybe not so consistant in some brands.
The FrSky module must be edge triggered (positive-going or negative-going, I don't know which). That is how it accomodates both positive and negative signals.
Depending on the polarity of the PPM signal from the coder, the FrSky module will react to the leading edge of the separating pulses or to the trailing edge.
If the pulse width is jittery (as distinct from the pulse position), polarity of the signal to the FrSky module will make a difference, as shown in this case.

It would be interesting to monitor the width of the PPM channel separating pulses fom the coder of this transmitter on a scope to confirm whether their width itself is jittery.
Then the problem would be the same with a 72 MHz autoshift receiver when changing polarity at the transmitter

It would show conclusively that it is a host transmitter problem, not a FrSky module problem.

Unfortunately, I don't have a Hitec Optic 6 transmitter to check the presence or absence of jitter of the separating pulse width myself.
Has anyone had the opportunity to put a scope on the PPM signal from this transmitter - maybe at the teacher/student plug?
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Re: Frsky problem.

Postby takilara » Thu Aug 12, 2010 1:01 pm

JMP_blackfoot wrote:Has anyone had the opportunity to put a scope on the PPM signal from this transmitter - maybe at the teacher/student plug?

I have a scope, and an optic 6, but i wouldn't know what line the ppm is. Is this one of the lines from the tx to the tx module ?
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Re: Frsky problem.

Postby JMP_blackfoot » Thu Aug 12, 2010 1:21 pm

Yes. You would want to have just one pulse on the screen (about 300 to 400 us wide) and see if the width is steady or jittery.
Another check would be with a non-2.4GHz module (35, 40, 72 MHz) and an autoshift receiver, and the same servo which gave trouble.
Change the PPM from negative to positive and see if the problem appears.
Then we know that the problem is actually with the coder in the transmitter.
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Re: Frsky problem.

Postby Fly2High » Thu Aug 12, 2010 2:40 pm

I use a Jr 9303. don't think I can change the shift like hitec so i have been waiting to see if there is a problem, which radios/ modules/etc. are affected and if a solution can be found.

Until this issue, I was all set to purchase a 2 way module and several receivers.

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Re: Frsky problem.

Postby takilara » Thu Aug 12, 2010 2:55 pm

Would anyone have a pinout for the wires between the Tx and the Tx module?
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Re: Frsky problem.

Postby JMP_blackfoot » Thu Aug 12, 2010 3:13 pm

1 - PCM/PPM Signal from Radio
2 - V+
3 - not connected/Rf detect
4 - GND
5 - RF Out
(In case the coder output is open collector, you may need a 10k resistor from PPM output to +V)
Source:
http://www.fpv-community.com/forum/show ... 254&page=2 (see post #16)
and:
http://www.rcgroups.com/forums/attachme ... tid=524885
http://static.rcgroups.com/forums/attac ... 1120039564
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Re: Frsky problem.

Postby takilara » Thu Aug 12, 2010 4:13 pm

JMP_blackfoot wrote:1 - PCM/PPM Signal from Radio
2 - V+
3 - not connected/Rf detect
4 - GND
5 - RF Out
(In case the coder output is open collector, you may need a 10k resistor from PPM output to +V)
Source:
http://www.fpv-community.com/forum/show ... 254&page=2 (see post #16)
and:
http://www.rcgroups.com/forums/attachme ... tid=524885
http://static.rcgroups.com/forums/attac ... 1120039564

Thanks.
Is there a way i can check if the coder output is open collector or not?

I'll see if i can manage to scope it this weekend. probably won't be today as i have a fever and not feeling great.
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Re: Frsky problem.

Postby JMP_blackfoot » Thu Aug 12, 2010 7:03 pm

If the signal is very small or non-existent between ground and PPM output, then it is an open collector.
You could try to check between +V and PPM ouput, when you should see an inverted signal.
Or you could check between shield and pin 2 of the round DIN socket:
http://users.belgacom.net/TX2TX/tx2tx/e ... 2txgb7.htm
(Hitec and Futaba supposedly the same)

Hope you get better 8-)
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Re: Frsky problem.

Postby RCModelReviews » Thu Aug 12, 2010 9:30 pm

Interestingly enough, I noticed that when I was doing the review of the FASST compatible receiver, the servos which were most jerky with the FrSky appeared just as jerky with a genuine Futaba FASST transmitter and receiver.

I'm continuing to investigate.
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