89S52 not able to receive complete AT Response from SIM900 GSM MOdem

Hello
I have connected SIM 900 (GSM Modem) with 89S52 Controller, via RS232 Communication.
If I send AT from controller to Modem , it is replying with
AT
OK
but my controller is just able to receive character “A”, (in below given cod if “A” is replace by “T” then it will not work)

I am using method in which , after a time period Interrupt will generate and it will check if data is received from RS232 .
Please help me where m I wrong.

Code is given below :

//Interupt code
void sEOS_ISR() interrupt INTERRUPT_Timer_2_Overflow { TF2 = 0; // Must manually reset the T2 flag if(i == 1) { check_connection(); //sending "AT" Command to Modem, send this command only 1 time time_stamp = 0; i = 0; //i is global variable ones it become 0 again it will not become 1 } rec_from_wire(); }

void rec_from_wire(void)
{

if(RI == 1) { RI = 0; temp = SBUF; }

if(temp == 'A') { port_0_0 = 1; //LED 0 ON port_0_1 = 0; //LED 1 OFF

} else { port_0_0 = 0; //LED 0 OFF port_0_1 = 1; //LED 1 ON }

}

void check_connection(void)
{ send_on_wire('A'); send_on_wire('T'); send_on_wire(0x0D); send_on_wire(0X0A);
}