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Replace STDIN with a Serial port

  • So I'm working on a project with an "Olinuxino A20": and its supposed to replace an HMI (human machine interface). We didn't want to go for the touch screen since it will be used pretty roughly and we don't want it to get broken. So instead we went ahead for an old ATM style, with buttons on both sides.

    For the moment I've been programming on a desktop computer for simplicity and emulating the event's with the keyboard with a handler like this one:

    @void PyCGui::keyPressEvent(QKeyEvent* event){
    switch( event->key() ) {
    //~ stuff done with that key

    In the real situation, our hardware keyboard is another microcontroler that sends the key-codes (as chars) over a serial port. I found that Qt4.8 had some classes to replace directly the stdin with any device you like, but currently I can't find how to do it.

    I've even tried the 'rough way'

    @kp_fd = open("/dev/ttyS7", O_RDWR );

    But it didn't work!

  • Lifetime Qt Champion

    Hi and welcome to devnet,

    Are you thinking of something like "QSerialPort": ?

  • But how do I connect that class to the GUI events?

    I'm currently using 2 serial ports in the program for communication and I write the C code directly where it's needed since I'm more used to that. But if have to use this class to connect it with the GUI I have no problem in changing it!

  • Lifetime Qt Champion

    It will offer you an easier more integrated way to get the data from your serial ports. From there you can create and post key events to the event loop and have them processed like it would come from a standard keyboard

  • From there you can create and post key events to the event loop

    Something like this?

    QSerialPort *keypad= new QSerialPort("/dev/ttyS7", (QObject *)my_gui);

    And that's it? How do I manually connect the signals from the port to my gui?

  • Lifetime Qt Champion

    You should rather write a little class that represent that keypad and have the QSerialPort in there. You'll connect that class to your GUI to keep things cleanly separated.

    On a side note, QWidget is a QObject so there's no need to cast. Furthermore, never use c style cast with QObject derived class, use qobject_cast.

  • It' very easy to do. I know and use Olimex A20 with serial devices and graphic interaction.
    I suggest you to write a class based on QSerialPort and implement events on special data.
    If you want I can to it for you.

    Which kind of hardware do you use?

  • What i'm missing is: should I create a new thread for it to monitor the serial port and create the events? or not?

  • In this case I normally use an Object based on QThread to manage the serial device. I normally add to the object the Tcp features, because sometimes the serial device can be attached with a rs232 - tct converter.
    The object has to manage the device and can emit an event at particular situations.
    For example, if you have a usb - rfid reader, when a tag will read, the object emit an event with the read tag code.

  • So it has to be something like this:

    class Keypad : public Qthread {



    QSerialPort serial;
    void run() {
       while(1) {


  • Lifetime Qt Champion

    Technically, threads are not mandatory at all in this case unless you start heavy processing on the data you received.

  • But I don't see where and how to create a loop for the events in the GUI.

  • Lifetime Qt Champion

    You already have an event loop after you call app.exec(). Use QSerialPort in asynchronous mode

  • but how do I get in that loop!?! That's what I don't get

  • Lifetime Qt Champion

    You don't need to get in that loop, once app.exec is called, the loop runs. Events are sent, signals and slots activates when needed etc.

  • Yeah.. that's what I'm trying to do now. Since I can't use the serial port in a separated thread.

    So what now? Do I have to create a signal handler for the serial?
    I'm quite new to GUIs writing, I'm mainly a microcontroller programmer. Sorry if this are really n00b questions.

  • Lifetime Qt Champion

    I never said it won't work in a separated thread, just that it's really not needed.

    Something like this:

    class KeyPad :: public QObject {
    KeyPad(QObject *parent = 0 ) :
    connect(&serialPort, SIGNAL(readyRead()), SLOT(processData()));

    bool initialize(const QString &portName)
    // setup port

    return; // or ReadOnly if you won't write anything in it


    valueChanged(int newValue);

    private slots:
    void processData() {
    // check if enough data is there
    // parse data
    // removed processed data from buffer
    // emit signal(s)

    QByteArray data;
    QSerialPort serialPort;

    I've wrote everything in the "header" for brevity only

  • @ connect(&serialPort, SIGNAL(readyRead()), SLOT(processData()));@

    I get it now, I was doing right that as you write! SO that's what gets it in the "main event loop" right?

    Of course you didn't said it won't work in a separated thread, but I tried just that and it didn't work because of some "can't create new thread issue"


  • So I got it, now I can navigate the menues with the serial port as I did with the keyboard. But the only thing that's not working are text entries. Whenever I need to input some text in a QLineEdit widget it doesn't recognizes the input as a keystroke!

    void Keypad::SerialPortHandler (void){
    char buff;


    QKeyEvent *last_key = new QKeyEvent( QEvent::KeyPress, buff , Qt::GroupSwitchModifier, QString(buff) );


    I use Qt::GroupSwitchModifier only to differentiate the events I create from the real one so I can manually delete them. But it's not the reason the input is not working!

  • Lifetime Qt Champion

    AFAIK, text input requires a press and a release event

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