If you connect an XBee XSC (S3B) module into a third-party interface board (such as the Sparkfun USB Explorer, or Parallax USB Adapter) you may be unable to communicate to the module using X-CTU. This is because several third-party interface boards have an LED connected to pin 6 of the XBee header. This LED provides an indicator for the RSSI (signal strength) for most XBee modules. To verify that you are being affected by this particular issue, when viewing the UART of the radio, you will notice a sequential series of numbers being generated.
The XBee XSC (both S3 and S3B) modules have a slightly different pinout than the rest of the XBee product line. Pin 6 is used as a config line rather than an RSSI indicator. A legacy feature on the XSC is a diagnostic tool called "pitch mode", when the config line is pulled low during startup, it starts sequentially counting numbers and outputs it to the UART. This feature was originally used to perform a range test and allows further backwards compatibility with the XStream radio.
Because an LED indicator is connected to this CONFIG line on the third-party interface board, the XSC (S3B) module will enter pitch mode due to the pin being pulled low. There is no way to bypass this mode on the XBee itself. The best solution is to remove the current limiting resistor or LED on the interface board that is tied to pin 6, which will leave the pin unused. Please refer to the manufacturer's schematic of the interface board to locate these components.
The older XSC (S3) module should not be affected by this problem because it has a lower value internal pull-up resistor on that pin and can more easily overcome the current draw by the LED.
We have found that with some third-party voltage shift boards have an additional problem. The DIN pin of the XBee also acts as a config line. If this pin is pulled low upon startup, it will enter a "command mode" and all data sent to the XBee from the UART will be interpreted as a command. If you wait for 10 seconds, it will exit command mode and the XBee will return to normal operation.
Some of these regulator boards pull the DIN pin low and will cause a long delay in startup before the radio can be used. A simple fix is to use a 10k ohm resistor between DIN (pin 3) and VCC (3.3V), this will drive the pin high and can prevent this delay.