I have two Series One XBees talking to each other “directly”; --switches on one, LEDs on the other. When DI0 on transmitter is made analog the output on the receiver’s pin 7 (RSSI) shows a pulse width modulated signal, but it randomly changes value. It alternates about one per second which is in sync with an LED blink on pin 3 (receiver).
You need to change the RSSI to PWM for output. That is done with the P0 command.