Hi Erik. I know it's too much to ask you again to write pieces of codes from the charts you have suggested. But it's great help if you can do for me.

I have tried some of these, but could not get it works. May be I did make it over complicated:

RECEIVE:

char Com_getchar (void)
{	
 if (Com_rbuflen () == 0)		//If the receive buffer is empty, return an error value.
   return (-1);
 else if (Com_rbuflen () >= RBUF_SIZE)	//If the receive buffer is overflow, return an error value.
 {
	r_in = 0;	r_out = 0;				//Reset and clear the receive buffer
	return (-1);
 }
 //Return the byte pointed by r_out in rbuf, then update the pointer r_out  
 return (rbuf [(r_out++) & (RBUF_SIZE - 1)]);	
}
 

Thanks,
Ralf

TRANSMIT:

//Note: There should be a short delay after a packet transmission to avoid the transmit buffer overflow
// The short delay should be inserted after the buffer is close to be full 
char Com_putchar(unsigned char c)	//For use with the serial interrupt enabled, i.e., ES = 1; 
{
	//If the buffer is full, return an error value.
	if (Com_tbuflen () >= TBUF_SIZE)
	  return (-1);
	
	//Add the data to the transmit buffer. If the transmit interrupt is disabled, then enable it.
	tbuf [t_in & (TBUF_SIZE - 1)] = c;
	t_in++;

	return (0);
}

unsigned char Com_tbuflen (void)
{
	if (t_in - t_out >= 0)
    	return (t_in - t_out);
	else 
		return (t_in - t_out + 256);
}