How to enter into AT mode ?

1 Preparation
Steps

------ Insert SIM800H module onto Arduino UNO or any other Arduino card;
------ Connect UNO with computer through USB cable
------ Connect UNO power jack with an external power supply, 6-12V @ >500mA. ----- Trochę potrzebuje zasilania
------ Set the swith to USB.

2 Boot SIM800H in two ways, anyone of them will work.
Press the button boot for 2 seconds
Upload the sketch below.

void setup(){
//Set pin 12 as output
pinMode(12,OUTPUT);
//GSM power on sequence
digitalWrite(12,HIGH);
delay(2000);
digitalWrite(12,LOW);
}
void loop(){
}

3 Choosing the right board and Serial port at arduino IDE menu bar. Open the serial monitor or other Serial Assistant software. Choosig"Both NL & CR". Wait for the NET led to blink as finding net, then you could send some AT commands. Otherwise please check the external power supply/ SIM card/ switch choosing /NET led if it's on and Reset/ Reboot module.