View Full Version : Serial port

October 1st, 2009, 11:43 AM

Hey all,

The task here is to interface to a device over serial, the only way to get it to turn on and respond is to play with the RTS and DTR pins the right way on connect.

After much confusion and rearanging I got the chirp from the device, and it matched. The key to not getting a wierd first bit was to set the attributes after I toggled the pins.

Here is the code. I'm sorry I havn't programmed in a long time so the code is probably nasty, hints on improvements are greatly appreciated.

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

int open_port(void) {
int fd;

fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
if (fd == -1) {
fprintf(stderr, "Unable to open /dev/ttyS0 - %s\n", strerror(errno));
else {
fcntl(fd, F_SETFL, 0);

return (fd);

int main (void) {

int fd, res, status = 0;
struct termios options;
char buf[3];

printf("Attempting to open port\n");
fd = open_port();
if (fd != -1) {

printf("Port opened\n");

tcgetattr(fd, &options);
// Get attributes

cfsetispeed(&options, B9600);
cfsetospeed(&options, B9600);

options.c_cflag |= (CLOCAL | CREAD);
options.c_cflag &= ~PARENB;
options.c_cflag &= ~CSTOPB;
options.c_cflag &= ~CSIZE;
options.c_cflag |= CS8;
options.c_cflag &= ~CRTSCTS;

options.c_lflag &= ~(ICANON | ECHO | ISIG);

options.c_iflag = 0;
options.c_oflag = 0;

options.c_cc[VTIME] = 0;
options.c_cc[VMIN] = 3;
// Generated Attributes

ioctl(fd, TIOCMGET, &status);
// Get Status

status &= ~TIOCM_RTS;
status &= ~TIOCM_DTR;
ioctl(fd, TIOCMSET, &status);
// Set RTS and DTR low


status |= TIOCM_RTS;
status &= ~TIOCM_DTR;
ioctl(fd, TIOCMSET, &status);
// Set RTS high and DTR low

tcflush(fd, TCIFLUSH);
tcsetattr(fd, TCSANOW, &options);
// Flush things and set the attribs

while (1) {
res = read(fd,&buf,sizeof(buf));
printf("Read 3 bytes\n");

printf("Got %02X:%02X:%02X::%d\n", buf[0], buf[1], buf[2], res);

printf("Port closed\n");
return 1;

I hope this helps someone down the line.