You have the FTDI and Xbee connected to the Pro and you have the switch in the UART position, correct? If so, what you are seeing has much more to do the fact you are running the FTDI and the XBee together on the same UART lines, not that level shifting resistors are 1k. Changing them to 10k helps, but that isn’t the issue. The series resistors on the pro are there so you don’t create bus contention on the UART lines when you have an FTDI and something else connected to the UART lines of the atmega. The same resistors are on all Arduinos EXCEPT the resistors are parallel to the Xbee lines instead of in series. The functional difference being the Pros will allow the FTDI to work but not the XBee and other Arduino’s (Leo, Uno, etc.) will allow the XBee to work, but not the FTDI (or 8/16U2) connection. I’m not sure why the Pros are different, but it doesn’t matter, you shouldn’t use two devices connected to one set of UART lines. If you use software serial and move the switch to DLINE, you shouldn’t have this problem.
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