How to open COM port using USB to RS232 converter?

Question

So I have USB port and converter cable on one side, and RS232 cable on other side. I am trying to open COM port using CreateFile() API, and so far I managed to do this:

HANDLE dev = CreateFile(devicePath, (GENERIC_READ | GENERIC_WRITE), 0,  NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
    ShowError("connectCOM()");
    if (dev == INVALID_HANDLE_VALUE)
    {
        return false;
    }

But I get AccessDenied error. I am now stuck because I am new to C programming and device communication.

What is the necessary step before calling CreateFile API in case of virtual COM port?

How do you know the USB com port is "COM1" and not "COM3", etc.? — chux - Reinstate Monica, Jun 10 '14 at 11:53
I looked it up in device manager. I didn't figure it out how to detect it programatically. Maybe you have an idea? — user3139293, Jun 10 '14 at 12:57

Ruslan Gerasimov · Answer 1 · 2014-06-10T10:48:32.360

The first way could be like this, assuming your Serial USB convertor creates /dev/ttyUSB0, it also involves system call to set up the port :

int usbdev;
char command[10]; 
char response[10];
system("stty -F /dev/ttyUSB0 115200 cs8 -cstopb -parity -icanon min 1 time 1");
usbdev = open("/dev/ttyUSB0", O_RDWR);
write(usbdev, command, 2);
read(usbdev, response, 1);
close(usbdev);

Another way to implement it,

#include <stdio.h>      // standard input / output functions
#include <stdlib.h>
#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 definitions

int main()
{
    /* Open File Descriptor */
    int USB = open( "/dev/ttyUSB0", O_RDWR | O_NDELAY );

    /* Error Handling */
    if ( USB < 0 )
    {
        //cout << "Error " << errno << " opening " << "/dev/ttyUSB0" << ": " << strerror (errno) << endl;
        perror("USB ");
    }

    /* *** Configure Port *** */
    struct termios tty;
    memset (&tty, 0, sizeof tty);

    /* Error Handling */
    if ( tcgetattr ( USB, &tty ) != 0 )
    {
        //cout << "Error " << errno << " from tcgetattr: " << strerror(errno) << endl;
        perror("tcgerattr ");
    }

    /* Set Baud Rate */
    cfsetospeed (&tty, B9600);
    cfsetispeed (&tty, B9600);

    /* Setting other Port Stuff */
    tty.c_cflag     &=  ~PARENB;        // Make 8n1
    tty.c_cflag     &=  ~CSTOPB;
    tty.c_cflag     &=  ~CSIZE;
    tty.c_cflag     |=  CS8;
    tty.c_cflag     &=  ~CRTSCTS;       // no flow control
    tty.c_lflag     =   0;          // no signaling chars, no echo, no canonical processing
    tty.c_oflag     =   0;                  // no remapping, no delays
    tty.c_cc[VMIN]      =   0;                  // read doesn't block
    tty.c_cc[VTIME]     =   5;                  // 0.5 seconds read timeout

    tty.c_cflag     |=  CREAD | CLOCAL;     // turn on READ & ignore ctrl lines
    tty.c_iflag     &=  ~(IXON | IXOFF | IXANY);// turn off s/w flow ctrl
    tty.c_lflag     &=  ~(ICANON | ECHO | ECHOE | ISIG); // make raw
    tty.c_oflag     &=  ~OPOST;              // make raw

    /* Flush Port, then applies attributes */
    tcflush( USB, TCIFLUSH );

    if ( tcsetattr ( USB, TCSANOW, &tty ) != 0)
    {
        //cout << "Error " << errno << " from tcsetattr" << endl;
    }

    /* *** WRITE *** */

    unsigned char cmd[] = {'I', 'N', 'I', 'T', ' ', '\r', '\0'};
    //int n_written = write( USB, cmd, sizeof(cmd) -1 );

    /* Allocate memory for read buffer */
    char buf [256];
    memset (&buf, '\0', sizeof buf);

    /* *** READ *** */
    int n = read( USB, &buf , sizeof buf );

    /* Error Handling */
    if (n < 0)
    {
        //cout << "Error reading: " << strerror(errno) << endl;
        perror("read error ");
    }

    /* Print what I read... */
    //cout << "Read: " << buf << endl;
    printf("%s",buf);;

    close(USB);
}

My USB serial converter is in my office, not with me at the moment, so I will able to check it tomorrow,

"assuming your Serial USB convertor creates `/dev/ttyUSB0`" This assumption is likely to be wrong, as the OP uses `CreateFile()`, which seems to be a part of Windows. — glglgl, Jun 10 '14 at 11:00
@glglgl oh, really? I did not marked that, I am not windows developer, would you suggest me to delete my answer? — Ruslan Gerasimov, Jun 10 '14 at 11:05

How to open COM port using USB to RS232 converter?

1 Answers1