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
– zdav, Commented Jun 7, 2010 at 4:00
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
– Chris Arguin, Commented Jun 7, 2010 at 5:12
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
– Tree77, Commented Jun 9, 2010 at 21:22
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
– Roland Illig, Commented Jun 13, 2010 at 9:53

Paul Nathan · Accepted Answer · 2010-06-16 17:40:34Z

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.

mjh2007 · Accepted Answer · 2010-06-16 17:29:16Z

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.

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