#include <reg51.h> 
#include <string.h>
 #define INBUF_LEN 4 
 unsigned char inbuf1[INBUF_LEN];
 unsigned char checksum,count3; 
bit read_flag= 0 ;
 void init_serialcomm( void ) 
{
 SCON = 0x50 ; //SCON: serail mode 1, 8-bit UART, enable ucvr 
TMOD |= 0x20 ; //TMOD: timer 1, mode 2, 8-bit reload 
PCON |= 0x80 ; //SMOD=1; 
TH1 = 0xF4 ; //Baud:4800 fosc=11.0592MHz 
IE |= 0x90 ; //Enable Serial Interrupt 
TR1 = 1 ; // timer 1 run 
// TI=1; 
}
 void send_char_com( unsigned char ch) 
{ 
SBUF=ch;
 while (TI== 0 );
 TI= 0 ;
 } 
 void send_string_com( unsigned char *str, unsigned int strlen)
 { 
unsigned int k= 0 ; 
do 
{ 
send_char_com(*(str + k));
 k++; 
} 
while (k < strlen); 
}  
void serial () interrupt 4 using 3 
{ 
if (RI) 
{ 
unsigned char ch; RI = 0 ; ch=SBUF; 
if (ch> 127 ) 
{ 
count3= 0 ;
 inbuf1[count3]=ch; 
checksum= ch- 128 ; 
} 
else 
{
 count3++; 
inbuf1[count3]=ch;
 checksum ^= ch; 
if ( (count3==(INBUF_LEN- 1 )) && (!checksum) ) 
{ 
read_flag= 1 ;

 }
 } 
}
 }
 main() 
{ 
init_serialcomm(); 
while ( 1 ) 
{ 
if (read_flag) 
 { 
read_flag= 0 ; 
send_string_com(inbuf1,INBUF_LEN); 
} 
} 
}