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Asked: 2026-05-21T06:44:14+00:00

Hi I have a problem with a modem and win32 api. When I open

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Hi
I have a problem with a modem and win32 api.

When I open the comm port from my source code the port is ready to receive commands but the modem does not write output. All at commands I write are processed by the modem but I don´t receive the output from the modem.

If I connect the modem using hyperterminal, disconect and then use my software, then the modem works succesfully.

My code is as follows:

bool open() {
    unsigned long confSize = sizeof(COMMCONFIG);
    winCommConfig.dwSize = confSize;
    DWORD dwFlagsAndAttributes = 0;

    if (!isOpen()) {
        winHandle = CreateFileA(port.toAscii(), GENERIC_READ | GENERIC_WRITE,
                FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING,
                dwFlagsAndAttributes, NULL);
        if (winHandle != INVALID_HANDLE_VALUE) {
            /*
             * Configure the port settings.
             */
            GetCommConfig(winHandle, &winCommConfig, &confSize);
            GetCommState(winHandle, &(winCommConfig.dcb));

            /*
             * Configure port parameters.
             */
            winCommConfig.dcb.fBinary = TRUE;
            winCommConfig.dcb.fInX = FALSE;
            winCommConfig.dcb.fOutX = FALSE;
            winCommConfig.dcb.fAbortOnError = FALSE;
            winCommConfig.dcb.fNull = FALSE;

            setBaudRate(settings.BaudRate);
            setDataBits(settings.DataBits);
            setStopBits(settings.StopBits);
            setParity(settings.Parity);
            setFlowControl(settings.FlowControl);
            setTimeout(settings.TimeoutMillisec);

            /*
             * Set the final parameters.
             */
            SetCommConfig(winHandle, &winCommConfig, sizeof(COMMCONFIG));
            SetCommState(winHandle, &(winCommConfig.dcb));
        }
    }
    return isOpen();
}

Where:
port represents the comm port
baudrate: 115200
databits: 8
stopbits: 1
parity: none
flowcontrol: off

What am I doing wrong?

This happens if I use jablocom gdp-04. When I use other modems (huawei, novatel, …) it works perfectly.

Thanks in advance.
Regards.

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1 Answer

  1. Editorial Team

    flowcontrol: off

    The device won’t send anything until it sees the RTS and DTR signals active, indicating that you are “online” and ready to receive. You will need to set fRtsControl = RTS_CONTROL_ENABLE and fDtrEnable = DTR_CONTROL_ENABLE. Enabling hardware handshaking never hurts btw, assuming the device implements it properly, especially handy while debugging. Also do a basic check with HyperTerminal or Putty to ensure that the wiring is okay.

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