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Old 10-29-2013, 03:08 PM   #1
Registered: Nov 2004
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Distribution: Ubunty, CentOS ,Mandriva, Gentoo, RedHat, Fedora, Knoppix
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serial port without tty?

I am a newbie to linux and drivers and I'm hoping I could get some help with a serial port question I have.

I have noticed that Linux installs TTY on all serial ports and USB ports, by default. This means evreything has to go through a TTY layer before it is passed through to the serial port. I have read a few tutorials and articles on that, and I think I understand that.

My question is, is there any possible way to not use TTY for serial port access? To send data in "RAW" mode?

The reason for this question is, if I understand the concept of TTY correctly, TTY requires a carriage return newline (\r\n) to send the data through to the serial port. Without the newline the data won't be transmitted.

This poses a major headache for my existing programs I am trying to port to Linux from other OS's. I have external devices that write to serial port without issuing newline. If the data is not actually sent out some of my existing codes will not function properly.

Can anybody help me out with understanding this and how I can actually work around this please?

Old 10-29-2013, 03:55 PM   #2
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No reason you need a carriage return to send data. Just use cfmakeraw and set the flags for raw I/O in your program (assuming it's C?)

Here's a program that reads raw binary from the serial port and dumps it to a file, no carriage returns or newlines anywhere in what's being sent:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <termios.h>
#include <fcntl.h>
#include <sys/ioctl.h>

//#define BREAK_SIZE 0xA00000 // Split to a new file every 10 MiB
#define BREAK_SIZE 0x100000 // Split to a new file every 1 MiB

int main ()
   int sfd, ofd;
   int n,n2;
   int bytes;
   int incr = 0;
   int bytesum = 0;
   char filename[255];

   struct termios options;
   char buff[10000];

   sfd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
   if (sfd == -1)
      fprintf(stderr, "unable to open serial port\n");
      fcntl(sfd, F_SETFL, FNDELAY);

   tcgetattr(sfd, &options);

   cfsetispeed(&options, B115200);
   cfsetospeed(&options, B115200);


   options.c_cflag &= ~(PARENB | CSTOPB | CSIZE | CRTSCTS);
   options.c_cflag |= (CLOCAL | CREAD | CS8);

   options.c_oflag &= ~OPOST; // select raw output
   options.c_cc[VMIN] = 1;
   options.c_cc[VTIME] = 0;

   tcsetattr(sfd, TCSANOW, &options);

   n2 = sprintf(filename, "/data/data_%06d.dat", incr);

   ofd = open(filename, O_WRONLY|O_CREAT, S_IRUSR|S_IWUSR);
   if (ofd == -1)
      fprintf(stderr, "unable to open output file\n");

   while (1)
      ioctl(sfd, FIONREAD, &bytes);
      if (bytes > 0)
         n = read(sfd, buff, 10000);
         //fprintf(stderr, "available/read: %d/%d\n",bytes,n);
         if (n < 0)
            fprintf(stderr, "read failed\n");
         if (n > 0)
            if (bytesum > BREAK_SIZE)
               n2 = sprintf(filename, "/data/data_%06d.dat", incr);

               ofd = open(filename, O_WRONLY|O_CREAT, S_IRUSR|S_IWUSR);
               if (ofd == -1)
                  fprintf(stderr, "unable to open output file\n");
               bytesum = 0;

            write(ofd, buff, n);
            bytesum += n;

Last edited by suicidaleggroll; 10-29-2013 at 03:56 PM.
1 members found this post helpful.
Old 10-29-2013, 04:04 PM   #3
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sfd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
When the O_NOCTTY flag is used it will not become the process's controlling terminal even if the process does not have one. As stated using CR and/or LF is not required to send serial data.

You didn't say how you were porting your application but I would not use bash script.

Last edited by michaelk; 10-29-2013 at 04:22 PM.


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