Garbage data from serial port

Question

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'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.

The hardware nature of the serial port makes some of it's behavior unpredictable. — zdav
At first glance, I wonder if you should copy old_opt into new_opt before overriding values. As it is you might be putting in uninitialized values from new_opt... but I would have to review the termios interface to be sure. When you say that the first doesn't work and the second is garbled, do you mean three separate runs on this program, or are you running a different version that loops? I'm wondering if you are resetting the termios setting between each run or not. — Chris Arguin
garbage on the serial port is usually indicative of incorrect serial port settings between the sender & receiver. What settings are you using for the device that is doing the sending? Also might help if you move your print of rbuff after you check the return value from read() — Tree77
Make sure that the string is still '\0'-terminated after reading. So you should either enlarge the buffer by 1 or read one less byte from the serial port. — Roland Illig

Paul Nathan Paul Nathan · Accepted Answer · 2010-06-16T17:40:34

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.

Garbage data from serial port

2 Answers