Question

I want to send a certain string on my serial port and read the answer into a buffer for further analysis. I have come up with some code but am not able to read any answer even tho screen /dev/ttyUSB0 19200 on the shell works just fine for me. The decive expects 8 data bits, 1 start bit, 1 stop bit, and no parity. at 19200 baud.
Now my code looks like this and it keeps timning out: 🙁

/////////////////////////////////////////////////
// Serial port interface program               //
/////////////////////////////////////////////////
#include <stdio.h> // standard input / output functions
#include <string.h> // string function definitions
#include <unistd.h> // UNIX standard function definitions
#include <fcntl.h> // File control definitions
#include <errno.h> // Error number definitions
#include <termios.h> // POSIX terminal control definitionss
#include <time.h>   // time calls


int open_port(void)
{
int fd; // file description for the serial port

fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY);

if(fd == -1) // if open is unsucessful
{
perror("open_port: Unable to open /dev/ttyUSB0");
}
else
{
fcntl(fd, F_SETFL, 0);
}

return(fd);
}

int configure_port(int fd)      // configure the port
{
struct termios port_settings;      // structure to store the port settings in

cfsetispeed(&port_settings, B19200);    // set baud rates
cfsetospeed(&port_settings, B19200);

port_settings.c_cflag &= ~PARENB;    // set no parity, stop bits, data bits
port_settings.c_cflag &= ~CSTOPB;
port_settings.c_cflag &= ~CSIZE;
port_settings.c_cflag |= CS8;

cfmakeraw(&port_settings);
tcsetattr(fd, TCSANOW, &port_settings);    // apply the settings to the port
return(fd);

}

int query_modem(int fd)   // query modem with an AT command
{
int n;
fd_set rdfs;
struct timeval timeout;
ssize_t retval;
char bufptr[100];
char chr;
int cnt = 0;
int i = 0;

// initialise the timeout structure
timeout.tv_sec = 2; // ten second timeout
timeout.tv_usec = 0;

if (FD_ISSET(fd, &rdfs)){
  FD_ZERO(&rdfs);
  FD_CLR(fd,&rdfs);
}

retval = write(fd, "TEST\r", 5);  // send an AT command followed by a CR
/*usleep(50);
while (read(fd, &chr, 1))
{
printf("0x%x\n",chr);
usleep(10);
}*/

// do the select
n = select(fd + 1, &rdfs, NULL, NULL, &timeout);

// check if an error has occured
if(n < 0)
{
perror("select failed\n");
}
else if (n == 0)
{
printf("Timeout\n");
}
else
{
printf("\nBytes detected on the port!\n");
}

}

int main(void)
{
int fd = open_port();
configure_port(fd);
query_modem(fd);
return(0);
}

What I would expect back is a string saying
“TEST” the same as in screen upon I hit enter.
Any help would be appreciated!
Thank you very much!

Ron

Answer 1

My suggestion is that you haven’t added fd to rfds. Check the FD_* macros to clear a set, add a fd and check if a fd has input.

Update

You really need to add the fd to the fdset:

write(...
FD_ZERO(&rdfs);
FD_SET(fd, &rfds);
n = select(...
if (n > 0) {
    if (FD_ISSET(fd, &rfds)) {
        // this fd has input waiting to be read

n can become > 1 when you are selecting multiple fd.