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Old Sep 10, 2008, 05:52 PM
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capture PPM signal from DX7 with radio on?

Hi everyone....a question for you...

What is the best way to capture a PPM signal (ideally TTL level), from a Spektrum DX7 during flight?

The trainer (DSC) port outputs PPM...but the radio section turns off when you plug in a cable. Also, these threads indicate that the trainer port is AC coupled for just 1.3V peak-to-peak:
http://www.rcgroups.com/forums/showthread.php?t=28323
http://www.rcgroups.com/forums/showthread.php?t=723410

This thread from April 2007 claims that the yellow wire on the radio board has an 8-channel PPM signal...
http://www.rcgroups.com/forums/showthread.php?t=671129
...but no mention of levels, or of whether that signal is pre- or post-mixing.

So...has anyone here done this? Or do you have other ideas? Sorry, I don't have access to a scope right now, so I can't check it myself.

Thanks in advance for any help!

-Jeff

P.S. This is so I can record micro helicopter flights for post-flight analysis and correlation against FDR data. If I could record the servo signals onboard, that would be great...but afaik an onboard recorder would be too heavy (>10g), expensive (time or money), or coarse (< 1024 bits/channel, 50hz). I'm open to ideas though.
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Old Sep 10, 2008, 06:22 PM
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Fluffy Monster's Avatar
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A real easy way would be to use a second RX and log those signals. This would avoid opening up your TX.
Just a thought outside the box
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Old Sep 10, 2008, 06:55 PM
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Thanks Fluffy. That would be the ideal approach, if it is possible with a Spektrum radio system. Is it? Can you bind two Spektrum rx's to the same model memory on a single Spektrum tx? And how do you get a PPM signal from a Spektrum rx?

-Jeff
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Old Sep 10, 2008, 08:57 PM
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As I understand it, the binding process is telling the RX what TX to listen to. So binding multiple RX's should not be an issue.

PPM is just a serial (20ms frame) version of what comes out of your RX. ie a pulse between 1 & 2ms. Plenty of articles on the web to fully describe it.
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Old Sep 10, 2008, 10:21 PM
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Quote:
Originally Posted by spinup
Can you bind two Spektrum rx's to the same model memory on a single Spektrum tx?
Yes you can bind two Spektrum AR9000 receiver to one model on a DX7 (and even Futaba/Spektrum modules). I never tested this with two AR7000 receiver though... but it certainly should work.
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Old Sep 10, 2008, 11:10 PM
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Quote:
Originally Posted by Fluffy Monster
As I understand it, the binding process is telling the RX what TX to listen to. So binding multiple RX's should not be an issue.
Yes! I just confirmed this too with two AR6100s, v1.2 firmware. The RX's also send a signal when binding...but I guess they just identify themselves as DSM2, no GUID.


Quote:
Originally Posted by Fluffy Monster
PPM is just a serial (20ms frame) version of what comes out of your RX. ie a pulse between 1 & 2ms. Plenty of articles on the web to fully describe it.
Correct. The question is, how do you get a PPM signal from a Spektrum AR6100? Does it output the PWM channels sequentially, with enough delay that they can be simply muxed together? I seem to remember reading that the AR7000 slams out the pulses edge-to-edge, too close to just mux...but again, I don't have a scope to check this...

Regardless, I'd still like to know if anyone has pulled a PPM signal out of the DX7. Then I won't have to buy another $50 receiver...yet...

Thanks all...

-Jeff
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Old Sep 11, 2008, 12:29 AM
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I guess the RX option was a little OT

You would not be able to just mux it together, specially if 'servo sync' was on. As 'Servo Sync' is designed to align the servo movements.

I would use a micro to capture the pulse lengths and log them. Format of logging them (PPM, stream like used for FMS serial etc) would depend on final application.

So if you did it serially (FMS format), you could fly Virtual and Real at the same time
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Last edited by Fluffy Monster; Sep 11, 2008 at 12:34 AM.
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Old Sep 11, 2008, 01:52 AM
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Oh, I think it was a great suggestion, thanks!! Really, I'll use whatever approach works best/fast/cheap.

FWIW, this is the thread about AR7000 PWM-to-PPM problems: http://www.rc-cam.com/forum/index.ph...pic=2064&st=20. I'm just guessing the TX approach is easier...but we'll see. Hopefully someone else has done it already.

-Jeff
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