For a student project, I had to make an arducopter, all is fine ... without the Xbee modules.
In fact, I can't get them comunicate in the two ways. I succeeded to receive something from the APM board but I can't transmit something to it, so when I try to connect using Mission Planner, It says "Getting params[...]" and then in the console I can read "Lost xxx pkts xxx" and, after a while it says "Read timeout[...]".
I tried the test code and I can get the "port 3" message so the communication on this way is working fine.
I have two Xbee Serie 2 rev.F modules, A xtreambee FTDI adapter and another USB adapter for the computer.
I checked the connections between the FTDI adapter and the APM, all is ok.
As some people said, I solded the CTS and the VIN pins.
Every XBee network requires one node that is configured to perform the role of the Coordinator. This is true even if all you want to do is establish a transparent serial link between two modems.
I tried a range test and all is OK from the XBee's. The problem is between the UAV's XBee and the ArduPilot-Mega board, but I checked all the connections and all is ok.
After another test I can say that if I do a physical loop between the RX and the TX pins of the APM board, I can see the loop, so the problem is (I think) the communication between the UAV's Xbee and the APM board, whereas it's the same communication speed (tried with 9K6, 57K and 155K Baud).
Which APM do you have?
A fair amount of us have seen this problem with APM 2.0. From what I can tell the automux on APM2.0 cannot "drive" the loads of some Xbee carriers and thus this is the exact problem at least 5 of us had. In some cases, they replaced the Xbee carrier and that fixed it. There is a workaround to use the other uart0 port on the long edge of APM2.0 right next to the 2x3 pin header for ICSP .
See this thread here. http://diydrones.com/forum/topics/no-signal-on-apm2-0-uart0-and-xbe...
And as an update not posted in that thread, the same APM 2.0 that I thought was defective and my xBee module and even hack did not work with, does work just fine with the new 3DR radio, which supports my theory that the APM has a very weak output on the automux uart port 0/2 and thus some carriers for Xbee modules may not work with that port but do work with the direct uart port 0 that is hardwired from the mega 2560. Again, this is just a theory but given 2 APM2.0 boards and different carriers for Xbee and also the 3DR radios, I have done the side by side testing.
UART0 on APM 2.0