After burning the sample code, the serial port output cannot be initialized?

(1) Check whether the external power supply is 7~12V, powered through the DC2.1 black round hole on the main board, or through the vin, gnd port on the expansion board. 

(2) Check whether two antennas are installed. 

(3) Check the TX>D8, RX>D7 dip switch on the expansion board. 

(4) See if the red power indicator light on the expansion board is always on, and the NET indicator light is blinking. 

(5) Check whether the NB card is inserted. After checking, press the Boot button to restart and observe.