XBee Shield + I/O Line passing

I have a Arduino + XBee shield that is sending data to a base XBee through the XBee I/O line passing configuration. It works if the shield jumpers are in the USB position, but if I put them in the XBee position, it sends one packet and nothing more. The problem is that when the jumpers are in the USB position, the Arduino can’t communicate with the XBee. I was hoping to be able to run Arduino code and communicate with XBee, while the XBee is sending I/O samples but it appears this is not possible or I am missing something. Any ideas? Thanks

I figured out the solution is to turn on serial in the setup function: Serial.begin(9600)
This is necessary even if you are not using serial at all, as was my case.