Serial.read

String inputString = "";

  while (Serial.available()) {
    char inChar = (char)Serial.read();
    inputString += inChar;

    if (inChar == '\n') {
       if(inputString.substring(0, 3) == "F1:")
          Freq1 = inputString.substring(3, inputString.length()).toInt();
        if(inputString.substring(0, 3) == "D1:")
          duty1 = inputString.substring(3, inputString.length()).toInt();
       if(inputString.substring(0, 3) == "F2:")
          Freq2 = inputString.substring(3, inputString.length()).toInt();
       Serial.println(inputString);
       inputString = "";
    }
  }