linux c сбросить последовательный буфер после записи

Простая проблема. Когда я пишу /dev/ttyS1, это не смывает это немедленно. Вероятно, это как-то связано с моей инициализацией последовательного порта … Но я не могу понять это! Мой код такой:

#define BAUDRATE B115200
#define MODEMDEVICE "/dev/ttyS1"#define _POSIX_SOURCE 1 /* POSIX compliant source */
#define FALSE 0
#define TRUE 1

volatile int STOP = FALSE;

void signal_handler_IO(int status); /* definition of signal handler */
int wait_flag = TRUE; /* TRUE while no signal received */int main()
{
int fd, c, res;
struct termios oldtio, newtio;
struct sigaction saio; /* definition of signal action */
char buf[255];

/* open the device to be non-blocking (read will return immediatly) */
fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY | O_NONBLOCK);
if (fd < 0) {
perror(MODEMDEVICE);
exit(-1);
}

/* install the signal handler before making the device asynchronous */
saio.sa_handler = signal_handler_IO;
//saio.sa_mask = 0;
saio.sa_flags = 0;
saio.sa_restorer = NULL;
sigaction(SIGIO, &saio, NULL);

/* allow the process to receive SIGIO */
fcntl(fd, F_SETOWN, getpid());
/* Make the file descriptor asynchronous (the manual page says only
O_APPEND and O_NONBLOCK, will work with F_SETFL...) */
fcntl(fd, F_SETFL, FASYNC);

tcgetattr(fd, &oldtio); /* save current port settings */
/* set new port settings for canonical input processing */
newtio.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD;
newtio.c_iflag = IGNPAR | ICRNL;
newtio.c_oflag = 0;
newtio.c_lflag = ICANON;
newtio.c_cc[VMIN] = 1;
newtio.c_cc[VTIME] = 0;
tcflush(fd, TCIFLUSH);
tcsetattr(fd, TCSANOW, &newtio);

int i = 0;

/* loop while waiting for input. normally we would do something
useful here */
while (1) {
printf(".\n");
usleep(100000);
//tcflush(fd, TCOFLUSH);
//tcflush(fd, TCIOFLUSH);
i++;
if (i == 20) {
// ------------------ write to serial ----------------------
int n = write(fd, "ATZ\n", 4);
if (n < 0)
fputs("write() of 4 bytes failed!\n", stderr);

}

/* after receiving SIGIO, wait_flag = FALSE, input is available
and can be read */
if (wait_flag == FALSE ) {
res = read(fd, buf, 255);
buf[res] = 0;
printf(":%s:%d\n", buf, res);
wait_flag = TRUE; /* wait for new input */
}
}
/* restore old port settings */
tcsetattr(fd, TCSANOW, &oldtio);
}void signal_handler_IO(int status) {
printf("received SIGIO signal.\n");
wait_flag = FALSE;
}

Любая идея?