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
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: