Serial Port : How do I raise DTR

ttumelty · 08-25-2005, 01:22 PM

How do i raise DTR for a serial port application?

I think I am confused about using and's, or's .

The Posix Serial Port How-To shows how to lower DTR :

#include <unisdt.h>
#include <termios.h>

int fd
int status;

ioctl(fd, TIOCMGET,&status );

status &= ~TIOCM_DTR;

ioctl( fd, TIOCMSET, &status );

I woud really appreciate it if anyone could help with this.

thanks in advance,
Tom

Hko · 08-25-2005, 04:08 PM

Code:

/* Parameter "level" should be zero or non-zero for
 * setting to low or high resp.
 * Assumes fd is a global open filedescriptor
 * variable to a serial device.
 */
int setDTR(unsigned short level)
{
    int status;

    if (ioctl(fd, TIOCMGET, &status) == -1) {
        perror("setDTR()");
        return 0;
    }
    if (level) {
        status |= TIOCM_DTR;
    } else {
        status &= ~TIOCM_DTR;
    }
    if (ioctl(fd, TIOCMSET, &status) == -1) {
        perror("setDTR");
        return 0;
    }
    return 1;
}

ttumelty · 08-25-2005, 05:54 PM

Thank You!!

So it looks like I can assume to lower a value (x) always use something like :

x &= ~y

and to raise a value (x) use :
x |= Y

Hko · 08-26-2005, 04:17 AM

Quote:

Originally posted by ttumelty
So it looks like I can assume to lower a value (x) always use something like :

x &= ~y

and to raise a value (x) use :
x |= Y

Well yes, sort of. Keep in mind though that this is about the bits, zeroes and ones. If y == 7, there will be 3 bits set to '1' (high). So x |= y will set 3 bits in 'x' to high. The included header that #defines TIOCM_DTR is a bit-mask, with 1 bit set to '1'.