Serial Port Programming Part 10 - Reading/Writing Status of Control Lines: DTR/RTS/CTS/DSR

If the user is interested in finding out the status of Control Lines:DTR/DSR/RTS/CTS. He can use 'TIOCMGET' control code in the ioctl call.

'TIOCMSET' control code in  ioctl allows you to set/clear DTR and RTS lines as they are output.

Example Code:

	#include <stdio.h>
	#include <sys/types.h>
	#include <termios.h>
	#include <sys/ioctl.h>
	#include <fcntl.h>
	#include <stdlib.h>
	#include <string.h>
	#include <time.h>
	#define SERIAL_DEVICE "/dev/ttyUSB0"
	int set_DTR(int fd, unsigned short level)
	{
	int status;
	if (fd < 0) {
	perror("Set_DTR(): Invalid File descriptor");
	return -1;
	}
	if (ioctl(fd, TIOCMGET, &status) == -1) {
	perror("set_DTR(): TIOCMGET");
	return -1;
	}
	if (level)
	status |= TIOCM_DTR;
	else
	status &= ~TIOCM_DTR;
	if (ioctl(fd, TIOCMSET, &status) == -1) {
	perror("set_DTR(): TIOCMSET");
	return -1;
	}
	return 0;
	}
	int set_RTS(int fd, unsigned short level)
	{
	int status;
	if (fd < 0) {
	perror("Invalid File descriptor");
	return -1;
	}
	if (ioctl(fd, TIOCMGET, &status) == -1) {
	perror("set_RTS(): TIOCMGET");
	return -1;
	}
	if (level)
	status |= TIOCM_RTS;
	else
	status &= ~TIOCM_RTS;
	if (ioctl(fd, TIOCMSET, &status) == -1) {
	perror("set_RTS(): TIOCMSET");
	return -1;
	}
	return 0;
	}
	int main()
	{
	int fd;
	int retval;
	int serial;
	fd = open(SERIAL_DEVICE, O_RDWR);
	if (fd < 0) {
	perror("Failed to open SERIAL_DEVICE");
	exit(1);
	}
	retval = ioctl(fd, TIOCMGET, &serial);
	if (retval < 0) {
	perror("ioctl() failed");
	exit(0);
	}
	if (serial & TIOCM_DTR)
	printf("%s:DTR is set\n", SERIAL_DEVICE);
	else
	printf("%s:DTR is not set\n", SERIAL_DEVICE);
	if (serial & TIOCM_LE)
	printf("%s:DSR is set\n", SERIAL_DEVICE);
	else
	printf("%s:DSR is not set\n", SERIAL_DEVICE);
	if (serial & TIOCM_RTS)
	printf("%s:RTS is set\n", SERIAL_DEVICE);
	else
	printf("%s:RTS is not set\n", SERIAL_DEVICE);
	if (serial & TIOCM_CTS)
	printf("%s:CTS is set\n", SERIAL_DEVICE);
	else
	printf("%s:CTS is not set\n", SERIAL_DEVICE);
	if (set_RTS(fd, 0)) {
	printf("%s: Failed to set RTS\n", SERIAL_DEVICE);
	exit(1);
	}
	if (set_DTR(fd, 0)) {
	printf("%s: Failed to set DTR\n", SERIAL_DEVICE);
	exit(1);
	}
	retval = ioctl(fd, TIOCMGET, &serial);
	if (retval < 0) {
	perror("ioctl() failed");
	exit(0);
	}
	if (serial & TIOCM_RTS)
	printf("%s:RTS is set\n", SERIAL_DEVICE);
	else
	printf("%s:RTS is not set\n", SERIAL_DEVICE);
	if (serial & TIOCM_DTR)
	printf("%s:DTR is set\n", SERIAL_DEVICE);
	else
	printf("%s:DTR is not set\n", SERIAL_DEVICE);
	return 0;
	}

view raw set_dtr_rts.c hosted with ❤ by GitHub

Output: