Serial Port problems

Hello,
i wrote a small program which sends one byte data over serial port 0 and receives one as answer.
As the conencted device reported serial overflow we connected another pc to the serial port. The Problem is:
The other PC gets the correct byte, but after one or two seconds it receives multiple other bytes.
The code i used for serial communication is (mostly taken out of the serial programming guide):

Code:

int CheckAmp(int b)
{
	struct termios newtio, oldtio;
	char *port="/dev/ttyS0";
	int portfd;
	sigset_t block_mask;
	struct sigaction saio;

	portfd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NONBLOCK);
	if (portfd <0)
	{
		close(portfd);
		return 0;
	}
          
        fcntl(portfd, F_SETOWN, getpid());
        fcntl(portfd, F_SETFL, FASYNC);
	
	sigemptyset (&block_mask);

	saio.sa_handler = signal_handler_IO;
	saio.sa_mask = block_mask;
        saio.sa_flags = 0;
        saio.sa_restorer = NULL;
        sigaction(SIGIO,&saio,NULL);
        
	
	newtio.c_cflag = B9600 | CRTSCTS | CS8 | CLOCAL | CREAD;
	newtio.c_iflag = IGNPAR;
	newtio.c_oflag = 0;
	newtio.c_lflag = 0;
	newtio.c_cc[VMIN]= 0;
	newtio.c_cc[VTIME]= 5;
	tcflush(portfd, TCIFLUSH);
	tcsetattr(portfd,TCSANOW,&newtio);
	write(portfd, &b, 1);
	char rd;
	int STOP=FALSE;
	char buff[255];
	int res;
	wait_flag=TRUE;
	while (STOP==FALSE) 
	{
		if (wait_flag==FALSE)
		{
			printf("Reading...\n");
			res = read(portfd,buf,1);
			buf[res]=0;
			printf("Got %i \n", buf[0]);
			STOP=TRUE;
		}
		
	}
	rd = buf[0];
	checkamp = rd;
	printf("CheckAmp: 0x%hx \n", rd);
	tcflush(portfd, TCIFLUSH);
	close(portfd);
	return 0;
}
void signal_handler_IO (int status)
{
	printf("received SIGIO signal.\n");
	wait_flag = FALSE;
}

Thanks for your help

Well actually it showed that the PC System i used had an broken COM Port.

On a new machine the code just runs fine. An admin oculd delete this, please.