Communicating with a Modem on a Serial Port

Overview

This sample project creates a command-line program that uses IOKit to discover a modem on a serial port and to send and receive data. Run the program within Xcode and use the console to monitor the progress of the communication.

Discover Attached Modems

IOKit provides a matching service for various types of I/O devices, including serial ports and modems. The sample code’s findModems() function uses kIOSerialBSDServiceValue to locate a serial device. Once located, the code identifies the type of serial device as a modem with kIOSerialBSDModemType.

classesToMatch = IOServiceMatching(kIOSerialBSDServiceValue);
if (classesToMatch == NULL) {
    printf("IOServiceMatching returned a NULL dictionary.\n");
}
else {
    // Look for devices that claim to be modems.
    CFDictionarySetValue(classesToMatch,
                         CFSTR(kIOSerialBSDTypeKey),
                         CFSTR(kIOSerialBSDModemType));
}

// Get an iterator across all matching devices.
kernResult = IOServiceGetMatchingServices(kIOMasterPortDefault, classesToMatch, matchingServices);

The findModems() function returns an IOKit iterator that getModemPath() uses to locate the first available modem and obtain its formal BSD device path.

If the function fails to find a modem or can’t obtain the device, the program prematurely quits.

Open the Serial Port and Initialize the Modem

After the findModems function finds the modem, openSerialPort() uses the low-level open() call to connect to the BSD file path discovered earlier. This function also captures the state of the serial port and saves it for later restoration.

fileDescriptor = open(bsdPath, O_RDWR | O_NOCTTY | O_NONBLOCK);
if (fileDescriptor == -1) {
    printf("Error opening serial port %s - %s(%d).\n",
           bsdPath, strerror(errno), errno);
    goto error;
}

This code allows the function to set up communication parameters such as baud rate, blocking characteristics, and handshaking lines.

If the call succeeds, it returns a BSD file descriptor. The initalizeModem() function then takes the file descriptor and sends the attention (AT) command to the modem using the low-level write() function. It then calls the read() function to obtain a response that confirms the modem is present.

Close the Serial Port

The closeSerialPort() function takes the passed file descriptor and waits for all output to flush, then restores the serial port to its original state and closes the file descriptor.

if (tcdrain(fileDescriptor) == -1) {
    printf("Error waiting for drain - %s(%d).\n",
           strerror(errno), errno);
}

// Traditionally it is good practice to reset a serial port back to
// the state in which you found it. This is why the original termios struct
// was saved.
if (tcsetattr(fileDescriptor, TCSANOW, &gOriginalTTYAttrs) == -1) {
    printf("Error resetting tty attributes - %s(%d).\n",
           strerror(errno), errno);
}

close(fileDescriptor);

This action terminates communication to the modem and concludes the use of the serial port.