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

// from http://www.control.com/thread/1026231695
//
// To compile:
//  gcc -o manual-set-rs232-signals.o manual-set-rs232-signals.c

main() {
int fd, sercmd, serstat;

fd = open("/dev/ttyUSB9", O_RDONLY); // Open the serial port.

//TIOCM_LE        DSR (data set ready/line enable)
//TIOCM_DTR       DTR (data terminal ready)
//TIOCM_RTS       RTS (request to send)
//TIOCM_ST        Secondary TXD (transmit)
//TIOCM_SR        Secondary RXD (receive)
//TIOCM_CTS       CTS (clear to send)
//TIOCM_CAR       DCD (data carrier detect)
//TIOCM_CD         see TIOCM_CAR
//TIOCM_RNG       RNG (ring)
//TIOCM_RI         see TIOCM_RNG
//TIOCM_DSR       DSR (data set ready)
sercmd = TIOCM_RTS;
printf("Setting the RTS pin.\n");
ioctl(fd, TIOCMBIS, &sercmd); // Set the RTS pin.

// Read the RTS pin status.
ioctl(fd, TIOCMGET, &serstat);
if (serstat & TIOCM_RTS)
printf("RTS pin is set.\n");
else
printf("RTS pin is reset.\n");

getchar(); // Wait for the return key before continuing.

printf("Resetting the RTS pin.\n");
ioctl(fd, TIOCMBIC, &sercmd); // Reset the RTS pin.

// Read the RTS pin status.
ioctl(fd, TIOCMGET, &serstat);
if (serstat & TIOCM_RTS)
printf("RTS pin is set.\n");
else
printf("RTS pin is reset.\n");

close(fd);
}