I'm having an issue with some XBee-PRO 900HP S3B modules. The DOUT and DIN pins are wired directly to the Rx and Tx pins of an off the shelf product. If the XBee is powered up with the DOUT pin connected to the Rx line of the target device then the pin seems to shut down and does not initialize (it should be in the pull up state), when measured it reads 1.2V (the target device Rx pin floats at 1.2V when disconnected). However, if I power up the XBee with the DOUT line disconnected, and then manually connect the line everything works fine (line is pulled up to 3.3V and communication works).
I did get this response from the target device vendor, although I don't fully understand it. "the Rx pin is controlled by current, so to ensure the reliable signal identification, the actual control node voltage of this pin will not be lower than 1.6V." I believe the device uses a STM32 MCU.