USB parameters for ACM

Posted on 2001-09-02
Last Modified: 2011-04-14
Hello all,

I'm writing a little program which communicates with the modem using AT commands.
I'm using a proprietery USB modem (CDC - ACM).
Under windows(2000) everything works fine, but when I tried doing the same under linux things stopped working...
Here's the code:

int retval;
int fd = open("/dev/input/ttyACM0",  O_RDWR | O_NOCTTY);
if ( fd == -1 )
while( true) {
  retval = write(fd,"atz\r",4);
  if ( retval == -1 )
  retval = read(fd,respBuff,255);
  if ( retval == -1 )
  respBuff[retval] = '\0';
} /* while */

The response which should be (and is under windows):
<end of response>
Under linux sometimes there is no reponse (especially in the 1st time I'm sending the string) although I can see that the modem got the command and returned OK.
And usually I get weired responses like:
and sometime more then 100 A's!!!
The modem is perfectly fine (works perfect under windows).

I found out that if I add the following to my code (after the open) the problem is solved:

struct termios newtio;
bzero(&newtio, sizeof(newtio));
newtio.c_cflag = CRTSCTS | CS8 | CLOCAL | CREAD;
newtio.c_iflag = 0;
newtio.c_cc[VTIME]    = 0;
newtio.c_cc[VMIN]     = 1;
tcflush(m_fd, TCIFLUSH);

What was the original problem?
Still sometimes there are problems with the response, is there any better settings?
Any more tips on this issue?


Question by:giligen

Accepted Solution

bryanh earned 100 total points
Comment Utility
Nothing jumps out at me, but TTYs are complicated, and I know what I'd do to try to reach an answer:  Since you have a working and non-working cases, just narrow the difference until you figure out the crucial variable.  Instead of just setting the tty attributes to the settings that work, use tcgettr(), then change some settings and leave some the same, then tcsetattr().

By the way, perhaps an easier way to experiment is to use the stty command.  Stty's job is to issue a tcsetattr (or tcgetattr).  You can set the settings before running your program.

The only annoying thing about stty is that, on every version I've seen, it does the settings on Standard Input, and you have to use the shell to open your modem as Standard Input, and the shell does a blocking open.  That means that if certain things are set wrong on the tty, you can't open it as Standard Input, so you can't change the setting!  I wrote a little program to open Standard Input nonblocking for just this reason, though I find it hard to believe someone didn't have another solution for it already.
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