So I posted earlier this month (and commented on other discussions) about how I was supposed to hook a 9Xtend radio to the APM2 and get telemetry data (and potentially autopiloting, etc.).  I've finally found at least one solution (UPDATE:  Solution #2 is at the bottom of this post), as follows:

Solution #1 (not very useful solution):

1. Make sure both 9Xtends are on their dev. boards and have the following settings changed from default (using the X-CTU utility):

Baud Rate - 57600

...

RF Data Rate - 115200

TX Power Level - 500 MW

MAKE SURE you WRITE this data to the module using the "Write" button, otherwise it won't remember the settings.

2. Hook up one dev board to the computer and make sure it's powered on (just the red light on).  You don't need to change ANYTHING else from the original settings when you got the boards.

3. Hook the other dev board up to a RS-232 to TTL adapter, then hook the TTL side up to the APM2 (GND, VCC, RX, and TX).

4. Power the APM2 AFTER you hooked up the dev board to the APM2.

5. You should see a lot of lights flashing (particularly make sure the TX and RX lights are steady and/or flashing).

6. If you open up the X-CTU and try a range test, you should get some strange-looking (lots of weird symbols, etc.) "bad packets" (according to the X-CTU).  This is good--you're actually getting lots of packets/info from the 9Xtend that's hooked up to the APM2.  X-CTU just thinks it's bad, but it's not.

7. Fire up the APM Mission Planner software (make sure both the APM2 firmware and the Mission Planner software are both up to date).  Connect through whatever port your dev board is plugged into (probably either COM1 or COM2, depending on your computer).  Set the Baud to 57600 (like you set it to in X-CTU).

8. You should be able to Connect and see your telemetry data flying through the modules.

Hope this helps everyone!  Let me know if you have questions--I'll try to use my limited knowledge to answer as best I can (understand, however, that I work a 40hr a week job, so answers could be sporadic).  =D

PS:  I'm going to look into using a breakout board + logic level converter with the 9Xtend so I don't have to use the dev. board for telemetry (just the module alone on the APM2 side).  I'll update this post as I do more tests.

UPDATE:

I've been trying (and I think succeeding) in powering the module alone through the APM2 (instead of hooking it up to the APM2 via the dev. board).  The RX and TX LEDs will light up when it's hooked up, so I'm certain that the APM2 recognizes the device and is communicating with it.  However, on the computer side (with the other module still hooked up to the dev. board), it won't pick up the module that's on the APM2.  I'm still unsure of why this is.  To my understanding, that's how other people got theirs working.  Any thoughts?

UPDATE 2:

So I finally got a "mobile" solution (the first solution required you to hook the APM2-side dev board to a wall socket), so that the Xtend module alone is connected to the APM2 (no dev board involved with that side) and the other Xtend module is on a dev board connected to the computer.

Solution #2 (better solution):

1. Configure both of your Xtend radios to these settings using X-CTU:

Baud Rate - 57600

...

RF Data Rate - 115200

TX Power Level - 10 MW

(10 MW because we don't want the modules to heat up too much (higher transmit power will do that)

2. I highly suggest using an Xtend breakout board (URL:  http://www.sparkfun.com/products/9596) for this step (if you want you can just directly use the module's pins, but it'll be messy).  Also, you should grab two 10-pin 2mm headers (I got mine from an XbeePro telemetry kit).  I had to sand down the sides of both headers to get them to fit onto the Xtend module snugly without any bending of the module's pins.  Then solder the headers onto the breakout board and you'll have an easily connected breakout board for your Xtend module.  This makes it easy to disconnect the module and change settings using the dev. board.

3. Next, solder wires to the GND, VCC, DI, and DO holes.  Also, solder a single wire from the SHDN hole and just attach it to the VCC wire (like the bare, protruding wire coming out of the board).  THIS IS ABSOLUTELY NECESSARY--the SHDN signal MUST be connected (to the VCC, so it will shut down when it pulses low), otherwise the Xtend module will NOT function (SHDN is one of the required pins to connect in order to operate according to the manual).

4. Now plug the four wires (GNC, VCC, DI, and DO) to the appropriate pins on the APM2 board's telemetry pins.  Remember to cross the RX and TX wires (just switch where they're plugged in).  This is important because you want the APM2's output to be the Xtend's input, and vice versa.

5. Now power up the APM2 board, fire up the Mission Planner, and connect to your dev board (probably COM1) w/57600 baud.  You should be able to connect fairly easily and see your wireless telemetry data.

Basically, the ONLY thing that was holding me up before was that I wasn't connecting the SHDN signal to the VCC.  I think this is the only thing holding other people up, as well.  Also, remember to keep the transit power low in order to keep the modules from getting too hot (I'd suggest 10 mW, like I said in the guide above).  Hope this solution has been helpful, and as always, let me know if you have questions and I will do my best to answer them.

PS:  A logic level converter is NOT necessary for this solution (though I guess it may vary from case to case and how you power your board).  The APM2 outputs 5V and the Xtend needs 5V in my case, so no converter was necessary for me.

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I also was unable to find any solution to this when attempting to use a microhard n920F / n1320, and after I figured out I needed a RS-232 to TTL converter rather than trying to get the other methods I was working on, I couldn't find anyone who had stated this would work. So yeah for anyone using generic radio modems rather than Xbees or 3DRs, this is a necessity.

Yeah, I think a logic level converter is necessary to use the module apart from the dev. board.

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