Garbage data from serial port

I wrote a code in Linux platform that read the data in serial port, my code below:

int fd;
char *rbuff=NULL;
struct termios new_opt, old_opt;
int ret;

fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY);
if( fd == -1 )
{
   printf("Can't open file: %s\n", strerror(errno));
   return -1;
}
tcgetattr(fd, &old_opt);
new_opt.c_cflag = B115200 | CS8 | CLOCAL | CREAD;
new_opt.c_iflag = IGNPAR /*| ICRNL*/;
new_opt.c_oflag = 0;
new_opt.c_lflag = ICANON;

tcsetattr(fd, TCSANOW, &new_opt);
rbuff = malloc(NBUFF);
printf("reading..\n");
memset(rbuff,0x00,NBUFF);
ret = read(fd, rbuff, NBUFF);
printf("value:%s",rbuff);
if(ret == -1)
{
   printf("Read error:%s\n",strerror(errno));
   return -1;
}
tcsetattr(fd, TCSANOW, &old_opt);
close(fd);

My problem is the code above doesn开发者_StackOverflow't read the first data that was transmitted, then the second transmission the data is garbage, then the third is the normal data.

Did I missed a setting in the serial port?

Thanks.

Sounds like your serial port settings are off - at a guess, you are reading in 8 bits instead of 7. You have to have both sides transmitting with the same settings.

What I would do is have a table of "bytes expected, bytes gotten", and run it for some 5-6 trials.

Next, if that doesn't help you, crank the baud on both sides down to 2400 or so. Yes, I'm serious. That can fix some oddball errors.

You should investigate getting your hands on an oscilloscope. If you anticipate this to be something you're maintaining in the long-term, an o-scope can be quite handy.

If you are talking about a hardware serial port (RS-232) I would recommend using a serial port analyzer like a BusBee to see what is actually being sent to the serial port. If you use a bus bee remember to also place a transceiver before the BusBee to adjust the voltage levels from RS-232 to TTL. Alternatively if you have access to an Oscilloscope you could use that to read the signals on the RS-232 lines and decode the bytes yourself.