POSIX:termios

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>

#define BAUD_RATE    B9600
#define DEVICE_PATH  "/dev/ttyUSB0"
#define CMD_ON       "RY 1 1\r"
#define CMD_OFF      "RY 1 0\r"

int main()
{
    int fd = open(DEVICE_PATH, O_RDWR);

    if (fd < 0) {
        perror("Failed to open serial port");
        return 1;
    }

    struct termios tty;
    memset(&tty, 0, sizeof tty);

    if (tcgetattr(fd, &tty) != 0) {
        perror("Error from tcgetattr");
        return 1;
    }

    tty.c_cflag = BAUD_RATE | CS8 | CLOCAL | CREAD;
    tty.c_iflag = IGNPAR;
    tty.c_oflag = 0;
    tty.c_lflag = 0;

    tcflush(fd, TCIFLUSH);

    if (tcsetattr(fd, TCSANOW, &tty) != 0) {
        perror("Error from tcsetattr");
        return 1;
    }

    write(fd, CMD_ON, strlen(CMD_ON));
    printf("ON command sent\n");

    sleep(2);

    write(fd, CMD_OFF, strlen(CMD_OFF));
    printf("OFF command sent\n");

    close(fd);

    return 0;
}