Serial MSR

Specifications

• Interface port: Serial (usually /dev/ttyS3)
• Baud rate: 9600
• Byte size: 7 bits
• Stop bits: 1
• Parity: even

Description

The DT515 has attached an MSR that send data through serial port, under Linux CentOS the port is recognized as /dev/ttyS3 known as COM4 on MS Windows.

The data must be converted to ascii with the function toascii() included in ctype.h library.

How to interface a serial MSR from C

The following is a piece of code that polls data from serial port to stdout

#include <assert.h>
#include <ctype.h>
#include <fcntl.h>
#include <stdio.h>
#include <termios.h>
#include <sys/ioctl.h>

char data[108];

main() {
  int fd, rc, count;

  // Open Serial Port
  fd = open ("/dev/ttyS3", O_RDONLY);
  assert (fd >= 0);

  // Read Serial Port
  while (1) {
    // Grab the buffer
    rc = read (fd, data, sizeof (data));

    // Terminate String
    data[rc] = 0;

    // Loop over the buffer and ignore the high-order bit
    // converting each char to a 7-bit ascii
    if(rc > 0) {
      for (count = 0; count < rc; count++) {
	printf("%c", toascii(data[count]));
      }

    } else if (rc == 0) {
      printf ("End of file read\\n");
      break;
    } else {
      perror ("Read error");
      break;
    }
  }
  close (fd);
}

How to allow regular users read from the serial MSR

In order to let the user have access to the device:

• Add the user to the proper group

# usermod -G $(stat -c '%G' /dev/ttyS3) <my_user>

• Re-login into that user account