2.4ghz: How to read 7 channels with one wire (NOT PPM) with Spektrum modules...



Since i got my new 2.4ghz spektrum radio (I FALL IN LOVE) i always wonder how the daughter receiver send data to the mother, maybe PPM? mmhh not! With a little help of an oscilloscope i started to reverse engineer the protocol used, well i saw only steady digital signal with lows of 8.6 useconds, that responded to the movements of my radio, so i quickly realize that is some kind of serial communication, and i did the calculations (1000000us/8.6us=116279bps) and i thought that maybe the serial speed was 115200bps. So i attached my FTDI cable and start analyzing the data, and i got this:



Spektrum daughter board is sending 16 bytes of information using a serial com. running at 115200bps. As you see in the picture above, the two first bytes are the preamble or sync bytes (0x03, 0x01). The data comes in integers that response precisely in the order is labeled in the picture...

So Paparazzi users this is a way to switch 2.4ghz (only in American), the back of my Specktrum radio says: "NOT FOR USE IN EU", uuh!

I hope somebody can do something usefully with it, enjoy!

Views: 19217

Comment by James Turner on March 15, 2009 at 2:45pm
I have a couple of these units and on some projects it could be very useful to know what the spektrum is receiving especially for the binding problems I have come accross. Thanks jordi.
Comment by Giles Barton-Owen on March 15, 2009 at 3:46pm
Spectrums are allowed in the UK, and thanks, this simplifies things, I wonder if one could find a similar signal in the transmitter. as i want to use it's handset form.

Developer
Comment by Jordi Muñoz on March 15, 2009 at 3:56pm
Oh that's nice! The transmitter has a port for the "buddy cable" but is PPM, is easy to decode but i prefer serial.
Comment by jaron on March 16, 2009 at 12:00am
This is really interesting, thanks Jordi.

Mikado is going to release a new version of their 3-axis stabilization system for model helicopters that uses a Spektrum satellite receiver only. It's called the Mini VBar.

The Spektrum radios are sold in the EU as well but they seem to be limited in the output power.
Comment by MercuryTree on July 16, 2009 at 8:55pm
How do we handle the binding aspect of this connection? Any input on that? Thanks.
Comment by bGatti on July 16, 2009 at 10:29pm
Is it possible the sync or preamble is in fact the transmitterID, and that "binding" means ignoring serial lines that start with other sync values?

It would seem weak to waste serial bandwidth reporting on excluded signals, but it seems possible? Are 4 bytes of data is probably enough to ensure uniqueness? 4096 unique codes. Probably not enough.

Developer
Comment by Jordi Muñoz on July 17, 2009 at 11:56am
bGatti,

Is just a preamble. The bind is another process that tune the devices to hear only our radio.

Developer
Comment by Jordi Muñoz on July 17, 2009 at 11:56am
I have found another signal port. I will try it... and has tx/rx
Comment by Max van Daalen on October 23, 2009 at 5:18am
Did anyone ever figure out how to tell the satellite to bind over the 3 wire link?
Comment by Max van Daalen on October 24, 2009 at 1:54am
Here we go, this post describes how to decode and bind a Spektrum satellite RX.

http://www.rcgroups.com/forums/showthread.php?t=922566

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