I am Argentinian and de original code is in Spanish, therefore I translate it so you can understand.
"" was eaten by this forum it is a Null character that I send it as final data to compare and change of state.

...insert code here[/

void serial_int () interrupt 4 
{
	static char index;
	static int State; 
	
	if (RI == 1)
	{

	RI= 0;

	switch (Estado)
		{
		case WAITING:

				if ( SBUF == 'X')
				{
					index = 0;
					State = RECEPTION;
				}
		 
		break;

		case RECEPTION:
							 
				if ( SBUF != 'Null')
				{
					Buff_RX[index] = SBUF;
					index ++;
				}
				else 
				{
					
					Buff_RX [index] = '';
					FlagCommandProccesor = 1;
					State = WAITING;
					
				}
		break;
	  	}

   	}
}]