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Re: [Paparazzi-devel] First power-up of Autopilot OK, some questions

From: Gareth Roberts
Subject: Re: [Paparazzi-devel] First power-up of Autopilot OK, some questions
Date: Thu, 14 May 2009 23:10:12 +0100
User-agent: Thunderbird (X11/20090409)

Martin, I also use 9600 with my XBEE 2.4GHz pros.
The problem you are having I suspect (and the problem I had with my 868's) is that the DUTY CYCLE of the XBee Pro 868 is only 10%. This means it can only transmit for 10% of the time, and needs to rest for the remaining 90%. If you run it at 9600, that means you'd only get 960 baud realistically, and at their maximum rate you'd get 2.4kb/s, which is not enough for proper telemetry. Also, Paparazzi is designed to stream data continously, and there is no easy way of "saving" all the data and then burst sending it, because the rate is too slow.

If your modem is not working when you launch GCS, the screen will go blank as you describe - after about 4-5 minutes of streaming, the XBee (usually on the aircraft side as it sends more) senses it's duty cycle is too high and just stops working, after which it'll need a reset or power cycle.

That would explain why on first launch it works. Once GCS is populated, leave it about 10 minutes and come back. If the link indicator is red and displays a number (seconds since link lost) it's your xbees.

This has bitten a lot of people, as Maxcomm are advertising them as drop-in replacements for XBee 2.4GHz (which can support a 100% duty cycle) when in fact they are for different uses. One chap managed by having paparazzi reset the airbourne xbee every few minutes, but this seems a bit extreme. There is also work ongoing if you look at DiyDrones to try and cool the chips to stop them cutting out, but nothing successful so far.

To be honest, I'd recommend getting some 2.4ghz's to start with, they are fairly cheap and are good to over a mile. You can also test by connecting the serial port of your tiny directly to an FTDI convertor - if it works, it's your XBee's that are the problem.

Hope this helps!

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