Receiving data on XBee module on a XBee explorer from a XBee module on an Arduino

_x000D_ _x000D_ I've a network formed by: A: XBee S2 module mounted on an Arduino UNO using a XBee shield v1.1, configured as Zigbee router/end device AT mode. B: XBee S2 module mounted on a XBee explorer conntected to a PC with XCTU software, configured as Zigbee coordinator AT mode. I uploaded the following code to Arduino. As you can see, A is waiting to receive data. When A receives data, starts to send data to B, and B receives it. void setup() { // initialize serial communication: Serial.begin(9600); establishContact(); } void loop() { Serial.println('A'); // It is shown in B delay(2000) } void establishContact() { while (Serial.available() <= 0) { delay(300); } Serial.readString(); // Read data from B } Problem: If Arduino runs the following code, A sends data to B before receiving any data from it, then B doesn't receive data (no led blink) from A. void setup() { // initialize serial communication: Serial.begin(9600); } void loop() { Serial.println('A'); // It isn't shown in B delay(2000); }