Try Sending "AT\r"

Thank you for your response
I send "AT\r" but I get uncorrect response from sim900.
my program in mikroc PRO Compiler is:
////////////////////////////////////////////////
#define LED1 PORTA.B0
#define LED2 PORTA.B1
#define Buzzer PORTB.B4

char receive;
void main()
{
int i=1;
TRISA.B0=0;
TRISA.B1=0;
TRISB.B4=0;
TRISC.B6=0;
TRISC.B7=1;

UART1_Init(9600);
delay_ms(400);
UART1_Write_Text("AT\r");
delay_ms(200);

while(i==1)
{ //First while loop
if(UART1_Data_Ready()==1)
{
receive = UART1_Read();
if((receive=='K')||(receive==0x4B)||(receive=='k')||(receive==75)||(receive=='O')) LED2=1;
else if(receive=='A') Buzzer=1;
else LED1=1;
}
}
}

After power up micro Buzzer and LED1 goes on.this indicate sim900 response is A and another character(perhaps T).
my schematic is in attachment pdf file:

Try: "AT\n" ..? or possible "AT\r\n"

This code should do the same as yours with less clutter:

You could try this too:

thank you
I correct my program as you say:
#define LED1 PORTA.B0
#define LED2 PORTA.B1
#define Buzzer PORTB.B4

char receive;
void main()
{
int i=1;
TRISA.B0=0;
TRISA.B1=0;
TRISB.B4=0;
TRISC.B6=0;
TRISC.B7=1;
UART1_Init(9600);
delay_ms(400);
UART1_Write_Text("AT\r\n");
delay_ms(200);

while(i==1)
{ //First while loop
if(UART1_Data_Ready()==1)
{
receive = UART1_Read();
switch( receive ){
case 'O':
case 'K':
LED2=1;
break;
case 'A':
case 'T':
Buzzer=1;
break;
default:
LED1=1;
break;
}
}
}
}

but when connect to sim900 Buzzer go on and indicate AT is response of sim900 module.
but why?

Your device echo everything you send back to you + its response. To turn it of use "ATE0" (or ATE1 to turn it on).

Btw, please edit your posts to use the "[ code] .. [/code]" tag.

-----Btw, please edit your posts to use the "[ code] .. [/code]" tag. -----??

---------- Post added at 23:05 ---------- Previous post was at 22:37 ----------

I edit program as :
#define LED1 PORTA.B0
#define LED2 PORTA.B1
#define Buzzer PORTB.B4

char receive;
void main()
{
int i=1;
TRISA.B0=0;
TRISA.B1=0;
TRISB.B4=0;
TRISC.B6=0;
TRISC.B7=1;
UART1_Init(9600);
delay_ms(400);
UART1_Write("A");
delay_ms(10);
UART1_Write("T");
delay_ms(10);
UART1_Write("E");
delay_ms(10);
UART1_Write("0");
delay_ms(10);
UART1_Write(0X0D);
UART1_Write(0X0A);
delay_ms(200);
UART1_Write_Text("AT\r\n");
delay_ms(200);

while(i==1)
{ //First while loop
if(UART1_Data_Ready()==1)
{
receive = UART1_Read();
switch( receive ){
case 'O':
case 'K':
LED2=1;
break;
case 'A':
case 'T':
Buzzer=1;
break;
case 'E':
case 'R':
{
Buzzer=1;
DELAY_MS(100);
Buzzer=0;
}
break;
default:
LED1=1;
break;
}
}
}
}

in this case LED1 goes on and micro doesn't get OK.
in correct response LED2 must be On.