readAll() from QSerialPort doesn't include the last response sent

Question

I'm using Qt to control a serial device. If I send a command to my serial device, I do something like serial->write("command \r\n"). I made a push button which changes the text inside a plain text widget to the response of the serial port. To get the response of the serial port, I'm using serial->readAll(). The problem is it shows the 2nd to last response rather than the one I was expecting. Does Qt have some sort of buffer which is keeping hold of this response?

EDIT I botched it by using recursion and compared the strings recieved

score 3 · Accepted Answer · answered Jun 29 '16 at 15:38

You might be calling readAll before the response is available. You should hook your code to the readyRead signal to be notified each time new chunk of data is ready to be read. Keep in mind that readyRead can be emitted with any number of bytes available to read - at a minimum, it'll be just one byte. You can't expect the data to be chunked/blocked in any particular way, since the serial port doesn't act as a message-based communication device. Your receiver code must be able to piece the data together from small chunks and act accordingly when it got all the data it needs.

For example, suppose that the device responses have a fixed, known length. You'd only want to react when a complete response has arrived. E.g.:

class Protocol : public QObject {
   Q_OBJECT
   QBasicTimer m_timer;
   QPointer<QIODevice> m_port;
   int m_responseLength = 0;
   int m_read = 0;
   void timerEvent(QTimerEvent * ev) override {
      if (ev->timerId() != m_timer.timerId()) return;
      m_timer.stop();
      emit timedOut();
   }
   void onData() {
      m_read += m_port->bytesAvailable();
      if (m_read < m_responseLength)
         return;
      m_timer.stop();
      emit gotResponse(m_port->read(m_responseLength));
      m_read -= m_responseLength;
      m_responseLength = 0;
   }
public:
   Q_SIGNAL void gotResponse(const QByteArray &);
   Q_SIGNAL void timedOut();
   Q_SLOT void sendCommand(const QByteArray & cmd, int responseLength, int cmdTimeout) {
      m_responseLength = responseLength;
      m_port->write(cmd);
      m_timer.start(cmdTimeout, this);
   }
   explicit Protocol(QIODevice * port, QObject * parent = nullptr) :
      QObject(parent), m_port(port) {
      connect(m_port, &QIODevice::readyRead, this, &Protocol::onData);
   }
};

...
Protocol protocol(0,0);
protocol.sendCommand({"foo"}, 10, 500);
QMetaObject::Connection cmd1;
cmd1 = QObject::connect(&protocol, &Protocol::gotResponse, [&]{
   QObject::disconnect(cmd1);
   qDebug() << "got response to foo";
});
QObject::connect(&protocol, &Protocol::timedOut, []{ qDebug() << "timed out :("; });

Thanks this was a really helpful response! – jxb458 Jul 01 '16 at 10:13 — jxb458, Jul 01 '16 at 10:13

readAll() from QSerialPort doesn't include the last response sent

1 Answers1

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