How to open COM port using USB to RS232 converter? - c

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?

Answer 1

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,