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Old 02-02-2005, 10:25 PM   #1
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Lightbulb serial port programming!!!

Hi! I dont know if this is the right place to ask this question.

I am writing a serial communications program(C). This program will open port and initialize modem. However, looks like my modem do not support auto answer. I tried to initialized it wth ATS0=1 so that it will answer automatically after first ring but it is not working so I have to is ATA command to manually answer the connection. My problem is how will I know if remote pc is trying to connect to my server. The environment is linux. I tried ioctl(fd, TIOCMGET...) to get the modem status but not successful. Any idea will be greatly appreciated...

code snippet;

int fd=open("/dev/ttyS0"...)

write(fd, "ATS0=1\r",7) --> modem should be set to auto answer...but not

since the remote pc already initiated the connection but nobody answered then it gets a NO CARRIER response from the modem...

Thanks in advance.... any ideas will be appreciated!!!
Old 02-03-2005, 12:25 AM   #2
Registered: Sep 2004
Location: New Zealand
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Last time I used a modem, it would send the string "RING" when it detected an incoming call. Does your modem not do this?
Old 02-03-2005, 07:09 AM   #3
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Thanks for your reply... I am not getting any response from my modem until I issue a "ATA".
Old 02-03-2005, 07:38 AM   #4
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I don't have a direct answer to your question, but I hope this may be of some help.

A nice document about serial programming:

Some code I used once to wait (sleeping) for one (or more) serial control lines to change state (from hi->low, or low->hi). IRCC this isn't mentioned in the document above, I gues because its pobably Linux-specific.
#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <linux/serial.h>

/* ... */

int fd;
unsigned int status, oldstate, newstate, changed, mask;

/* Assuming fd is an open file descriptor for (one of) the
 * serial port(s) (/dev/ttyS[0-9]).

/* To wait for *any* of the 4 control lines to change state,
 * use this instead of the line below:
mask = TIOCM_RNG;

/* Get current status of the control lines in mask */
if (ioctl(fd, TIOCMGET, &status) == -1) {
oldstate = status & mask; /* Save current status of control lines */

/* The ioctl() call below sleeps, and does not return until one 
 * of the  serial control lines specified in mask changes state,
 * or it was interrupted by some signal.
rewait:  /* Goto-label for when we get a signal. */
if (ioctl(fd, TIOCMIWAIT, mask) == -1) {
    if (errno == EINTR) {
        /* Signal received. Handle this case. Here we're
         * just restarting the ioctl-wait call.
        goto rewait;
    } else {
        /* Error condition */

/* Get new status of the control lines in mask */
if (ioctl(fd, TIOCMGET, &status) == -1) {
newstate = status & mask;

/* XOR old with new status to find the one(s) that changed */
changed  = newstate ^ oldstate;

/* Do something with the result of al this */
if (changed & TIOCM_RNG) {   /* RNG line has changed */
    if (newstate  & TIOCM_RNG) {
        printf("RNG went from: 0 --> 1\n");
    } else {
        printf("RNG went from: 1 --> 0\n");

Last edited by Hko; 02-03-2005 at 07:40 AM.
Old 02-03-2005, 08:50 PM   #5
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Looks like the problem is with the device driver that is controlling the serial port. I have tried this code before but I the modem status is not changing... i have seen similar problem on the web and they patched the device driver. How can I check the version of the device driver? I will try first if I can access the port/irq directly. I am still looking for the equivalent of inp() /*win32 */ or inputb() /*turboc */. Any idea? Thanks again and I appreciate all your inputs...


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