Can QSerialPort read more than 512 bytes of data? - c++

Question

I want to use QSerialPort to read data transmitted from a device. The device transmits a frame of 4000 data bytes each time. I try with the following simple code
QSerialPort *serialPort;
char receivedData[4000];
int numRead = 0;
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
/* Initialize serial port*/
serialPort = new QSerialPort(this);
QString portName = "COM6";
qint32 baudRate = 460800;
serialPort->setPortName(portName);
serialPort->setBaudRate(baudRate);
serialPort->setDataBits(QSerialPort::Data8);
serialPort->setParity(QSerialPort::NoParity);
serialPort->setStopBits(QSerialPort::OneStop);
serialPort->setFlowControl(QSerialPort::NoFlowControl);
serialPort->setReadBufferSize(4000);
if (!serialPort->open(QIODevice::ReadOnly)) {
qDebug() << "Cannot open comport";
}
connect(serialPort, SIGNAL(readyRead()), this, SLOT(serialReceived()));
}
void MainWindow::serialReceived()
{
numRead = serialPort->read(receivedData, 4000);
serialPort->flush();
}
The problem is: it always shows that only 512 data bytes are read. How can I read the whole 4000 bytes data frame? (when I'm using Matlab to read this 4000 bytes frame, it's working fine)

Answer 1

There's no limit, but you don't necessarily receive all data in single chunk.
You have to keep listening until you have the number of bytes you're waiting for (or a timeout).
void MainWindow::serialReceived()
{
receivedData.append(serialPort->readAll());
if(receivedData.size() >= 4000) {
// we're full
}
}

Answer 2

You generally have to read out the data in a loop (to ensure you get it all), here is a snippet of example code this is equivalent to your serialReceived() function, except it emits the data using emit rxDataReady(newData); to whoever is listening...
void QSerialPortReader::handleReadyRead()
{
QByteArray newData;
// Get the data
while (mp_serialPort->bytesAvailable())
{
newData.append(mp_serialPort->readAll());
}
emit rxDataReady(newData);
}
edit
Although I don't do any max size checking... but that is trivial to add if you need it (i.e. just use read(..., spaceAvail) instead of readAll and then decrement spaceAvail...