PPM data streams

Stuff I am working on
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Daniel Wee
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PPM data streams

Post by Daniel Wee » Thu 26 Feb 26 2009 10:30 am

The attached file is the scope trace of the PPM data stream from the JR9XII (equivalent to the JR9303). The trace is taken relative to the shield line from the D.S.C. port behind the radio. The vertical recticle is 500mV, and the horizontal recticle is 2.0ms.
JR9XII_PPM.jpg
JR9XII/9303 PPM stream trace
JR9XII_PPM.jpg (53.6 KiB) Viewed 6623 times
You will notice that the voltages are not centered around 0V, indicative that the output line probably passes through a large valued capacitor (AC-coupled). This could present problems for devices expecting a TTL compatible ground relative (DC-coupled) signal.
JR9XII_PPM2.jpg
JR9XII/9303 PPM frame trace
JR9XII_PPM2.jpg (66.75 KiB) Viewed 6623 times
The frame time is precisely 22.0ms with 9-channels represented in the PPM frame. For this model at least, it does not appear that you can change the number of channels represented in the PPM frame. It's fixed at 9-channels.

Daniel

Daniel Wee
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Posts: 2073
Joined: Wed 25 Feb 25 2009 8:00 pm

Multiplex Royal evo9 PPM data stream

Post by Daniel Wee » Thu 26 Feb 26 2009 10:54 am

The following trace is from the Multiplex Royal evo9. The first thing you notice is the large voltage swing - up to 8V relative to ground (0V). This could pose issues to inputs expecting TTL compliant levels or inputs that are not tolerant to the higher voltages. In this trace, the vertical recticle is 2V and the horizontal recticle is 2.0ms. I have shifted the trace down by two recticles for a better zoomed-in view, so 0V is two recticles down.
Evo9_PPM.jpg
Multiplex Royal evo9 PPM data stream trace
Evo9_PPM.jpg (53.03 KiB) Viewed 6621 times
The second thing you notice is that there are only 7-channels represented in this trace and that is due to the number of channels in use. On the evo9/12, you can actually vary the number of channels in use from a minimum of 6 up to 9 (for the evo9) or 12 (for the evo12). This will affect the total frame time, with 8-channels giving 22.0ms, and 9-channels giving 24.2ms.
Evo9_PPM2.jpg
Multiplex Royal evo9 PPM frame trace
Evo9_PPM2.jpg (68.05 KiB) Viewed 6621 times
Daniel

Daniel Wee
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Posts: 2073
Joined: Wed 25 Feb 25 2009 8:00 pm

Re: PPM data streams

Post by Daniel Wee » Fri 31 Jul 31 2009 7:16 pm

The trainer port output of the Futaba T9CAP seems to provide a healthy rail-to-rail swing as follows:-
Futaba-Trainer.jpg
Futaba-Trainer.jpg (95.05 KiB) Viewed 6596 times
The output from the module connector pin-1, however, is open collector so if you want to see a decent voltage, you should provide a 10k pull-up resistor to the +ve line.

Daniel

Daniel Wee
Site Admin
Posts: 2073
Joined: Wed 25 Feb 25 2009 8:00 pm

Futaba T12FGA PPM stream

Post by Daniel Wee » Wed 08 Sep 08 2010 11:57 pm

The Futaba T12FGA comes with two types of PPM modes, 8-channel and 12-channel. The 8-channel stuff is nothing to shout about but the 12-channel mode is rather peculiar, and appears to suffer from a bug.

Mode: 12-channel
Frame-timing: ~17.68ms (fixed)
Channel-width: 466us to 1.053ms
Channel-neutral: 760us
Channel-marker: 50% duty-cycle
Futaba T12FGA 005a.PNG
Futaba T12FGA 005a.PNG (86.66 KiB) Viewed 6311 times
The bug occurs only when all channels are reduced such that the total channel widths are less than about 12 * 496us = 5.952ms (rounded to6ms). When that happens, the PPM output essentially becomes garbage and is un-decodable.
Futaba T12FGA 001a.PNG
Futaba T12FGA 001a.PNG (85.54 KiB) Viewed 6311 times
The detection of this mode depends on fulfilling three criteria:-
1. there are 12-channels of data
2. none of the channels exceed 1.0ms
3. frame-period is not more than 18.0ms

I also found that contrary to popular belief, the T12FGA will quite happily put out PPM even without a module attached. It seems that nearly all the information I've been getting on this radio have been wrong thus far.

Daniel

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