DragonLink Instruction Manual: Connections

Support related stuff, for various products
Daniel Wee
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Re: DragonLink Instruction Manual: Connections

Post by Daniel Wee » Mon 08 Mar 08 2010 9:11 pm

The D.S.C. port issue

Concerning the PPM from the JR/Spektrum/Graupner trainer port (also known as the D.S.C. port), I should mention that JR made some rather odd engineering choices. Not only is the PPM there very low level, just over 1V peak-to-peak, but it is also AC coupled through a capacitor. This means that the DC voltage actually floats around a bit. This makes dealing with this particular PPM stream a little more tricky. This is one of the major contributing issues to the problem of the PPM stream not being reliably detected or being interfered by stray RF. (So far, this only affects D.S.C. ports.)

Furthermore, the way it works with these radios is that you turn the radio on in trainer mode by plugging the cord into the port - meaning that the way to turn the radio on or off is by plugging-in or unplugging the PPM cord respectively. This, and the fact that the plug itself doesn't hold real well leads me to recommend drawing the PPM signal for the DragonLink from the module connector. That signal is way more stable, 8V p-p and is DC coupled. As such, this is a much more reliable PPM stream than the one available from the D.S.C. port. Besides, the wear and tear on the port will lead to lower reliability.

For those radios that do not have a removable module, you will need to open up the back of the radio and what you will find (usually) is a three wire header connected to the RF board (usually in the middle). From what I have seen, the three wires are usually red, yellow and black. This is to be differentiated from other headers with many more cables (usually red and white). I am not sure what voltage is present on the red wire but I suspect the yellow wire is the PPM signal. These three wires should correspond to the three wires on the DragonLink if the voltage is greater than 6.5V or so. You can see an example of the three-wire header from the photo of the innards of the Spektrum DX7 below:-
SpektrumDX7.jpg (129.57 KiB) Viewed 5974 times
Futaba 12FG (and 14MZ)

While on this subject, a number of people have asked about using the Futaba 12FG with the DragonLink. I understand that the 12FG can be coaxed into outputting either an 8-channel or 12-channel PPM stream. As long as the PPM frame is constant, it should work without problems with the DragonLink. But this requires the correct output mode to be selected (and tested). The DragonLink requires a PPM stream with constant frame timing and no more than 12-channels encoded.

More often, the issue is to get the PPM stream out of the module connector. The 12FG (and 14MZ) use a newer RF module (which is actually just an amplifier from the looks of it) that incorporates an on-board serial EEPROM. The sole purpose of this EEPROM is to identify the module and determines the frequency sets to be used, region, and so on. The problem here is that until the EEPROM is read and the module identified, these new radios will refuse to output any PPM stream. This leaves the user basically 3-options:-

1. Use it with the RF module in-place and operational
2. Use it with the RF module in-place but modify the module to disable the RF section so as to prevent power wastage
3. Build a daughterboard with an EEPROM to fool the radio

Option 3 is the most elegant but also the most involved approach.

More than 12-channels?

What about those radios with 14 or even 16-channels? Very often, the extra channels are not encoded into the PPM stream due to timing constraints. As such, these radios do not encode all 14 or 16-channels into the PPM stream and if so, they are no use to the DragonLink since the DL will only recognize whatever is encoded onto the PPM stream. Where the PPM frame timing (typically 20ms for 8-channels in Futaba, or 22ms for 9-channels in JR) is not constant, you will experience PPM detection errors indicated by the green LED going red. If this happens with the 12FG for example, you will need to select a different output mode. Some (but not all) Graupner radios also purportedly use non-constant PPM framing times when involving more than 9-channels or so.

So far, the Multiplex Royal Evo/Pro have proved to be pretty ideal in terms of PPM timing. Note also that when there are less than 9-channels encoded in the PPM stream, the DL receiver will output a corresponding (composite) PPM stream on the 9th servo channel. This is useful for people flying quadrocopters (such as the MikroKopter) which requires a PPM stream from the receiver. Such a PPM-stream is also used by the new DragonOSD+ as a way of reducing cable count to the OSD. Currently, the only way to enable this output is to make sure that the radio is sending the DragonLink TX a PPM stream with less than 9-channels encoded. This could present some problems with radios where the number of channels are fixed at 9-channels, for example. I will be looking into addressing this issue.


Ref: http://www.aeromodelismovirtual.com/sho ... php?t=3155

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