QSerialPort read delay

[M4chin3]

Hi,

i'm using QSerialPort to read a constant datastream from my comport.
Therefore i'm using a seperate thread that calls itself with a QTimer within a 1ms interval with signal and slots.

Basically like this:

connect(ReceiveTimer, SIGNAL(timeout()), this, SLOT(SlotTimer()));
 
void SlotTimer(void){
 QByteArray ReceivedDat = SerialPort->readAll();
qDebug() << "Received bytes: " << ReceivedDat.size() << " Timestamp: " << QDateTime::currentMSecsSinceEpoch();
}

connect(ReceiveTimer, SIGNAL(timeout()), this, SLOT(SlotTimer()));

void SlotTimer(void){
 QByteArray ReceivedDat = SerialPort->readAll();
qDebug() << "Received bytes: " << ReceivedDat.size() << " Timestamp: " << QDateTime::currentMSecsSinceEpoch();
}

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So far this works fine for me. The outputs of qDebug() show me that the thread is called every millisecond like it should be.
The problem is that ReceivedDat does not hold data at some calls of the slot depending on the baudrate.

I use two baudrates 38400 and 57600 so therefore within 1ms about 3-4 Bytes and 5-6 Bytes should be stored in the buffer.
When running my program i got new data in an interval of 16ms (38400) and around 800!ms (57600).

Is there anything i'm missing?


Win7 Prof 64-bit
Qt 5.2.0 (MSVC 2010, 32 bit)
Revision 27d10d8dcd

[Lesiok]

Why You don't use readyRead() signal ?

[kuzulis]

@M4chin3,

why you are surprised? It is normal because asynchronous I/O. Besides these intervals depend on load of Qt-event loop of thread (main GUI thread or other thread) where is your QSerialPort instance.

[M4chin3]

@Lesiok

You are right i could and i tried but it does not change the update interval. 800ms is way too much for my application.

@kuzulis

I'm surprised because my QSerialPort instance is in the separte thread with the QTimer. Also the load of the program/whole pc is < 5% and therefore i dont understand delays in the size of ~800ms.
Next point is why should this time increase from 16ms to 800ms only from changing the baudrate from 38400 to 57600?

edit: btw the QThread does nothing more than creating the Timer interval and reading the data from the serialport.

[Lesiok]

Windows is not RT system.

[M4chin3]

Windows is not RT system.

Absolutely!
But i think with 2x 2,6GHz it should be possible to receive data from a comport under 800ms (0,8s).

Maybe it is not quite clear i try it with an example:

void SlotTimer(void){ // called with a 1ms interval
QByteArray ReceivedDat = SerialPort->readAll();
if(0 < ReceivedDat.size()){
qDebug() << "Received bytes: " << ReceivedDat.size() << " Timestamp: " << QDateTime::currentMSecsSinceEpoch();
}else{
qDebug() << "No data." <<  " Timestamp: " << QDateTime::currentMSecsSinceEpoch();
}
}

void SlotTimer(void){ // called with a 1ms interval
QByteArray ReceivedDat = SerialPort->readAll();
if(0 < ReceivedDat.size()){
qDebug() << "Received bytes: " << ReceivedDat.size() << " Timestamp: " << QDateTime::currentMSecsSinceEpoch();
}else{
qDebug() << "No data." <<  " Timestamp: " << QDateTime::currentMSecsSinceEpoch();
}
}

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Using a baudrate of 38400 results in this output pattern (16ms between every chunk of data):
"Received bytes: 62 Timestamp: 1389632940000"
"No data. Timestamp: 1389632940001"
... ~13 times more
"No data. Timestamp: 13896329400015"
"Received bytes: 61 Timestamp: 1389632940016"
"No data. Timestamp: 1389632940017"
... ~13 times more
"No data. Timestamp: 1389632940031""
"Received bytes: 62 Timestamp: 1389632940032"
... and so on for minutes

Using a baudrate of 57600 results in this output pattern (800ms between every chunk of data):
"Received bytes: 4608 Timestamp: 1389632940000"
"No data. Timestamp: 1389632940001"
... ~800 times more
"No data. Timestamp: 1389632940799"
"Received bytes: 4608 Timestamp: 1389632940800"
"No data. Timestamp: 1389632940801"
... ~800 times more
"No data. Timestamp: 1389632941599"
"Received bytes: 4608 Timestamp: 1389632941600"
... and so on for minutes

So i'm talking not about an sporadic error.

[kuzulis]

Ok, now please:

1. What type of serial device you use?
2. Please re-implement to readyRead() usage and show your timestamps, e.g.:

connect(SerialPort, SIGNAL(readyRead()), this, SLOT(SlotReadyRead()));
 
    void SlotReadyRead() {
        QByteArray ReceivedDat = SerialPort->readAll();
        if(0 < ReceivedDat.size()){
            qDebug() << "Received bytes: " << ReceivedDat.size() << " Timestamp: " << QDateTime::currentMSecsSinceEpoch();
        }else{
            qDebug() << "No data." << " Timestamp: " << QDateTime::currentMSecsSinceEpoch();
        }
    }

connect(SerialPort, SIGNAL(readyRead()), this, SLOT(SlotReadyRead()));

    void SlotReadyRead() {
        QByteArray ReceivedDat = SerialPort->readAll();
        if(0 < ReceivedDat.size()){
            qDebug() << "Received bytes: " << ReceivedDat.size() << " Timestamp: " << QDateTime::currentMSecsSinceEpoch();
        }else{
            qDebug() << "No data." << " Timestamp: " << QDateTime::currentMSecsSinceEpoch();
        }
    }

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[M4chin3]

@kuzulis

1. I'm using a virtual comport device with a controller from FTDI (FT 232 RL)
2. I reimplemented it with readyRead() usage and it seems to work. I will take a closer look tomorrow and reply in detail. (@Lesiok: sorry i misread your reply, i thought you ment readAll and readData)

Thanks in advance.

[ChrisW67]

I'm surprised because my QSerialPort instance is in the separte thread with the QTimer.

You do not need a separate thread to read from a serial port. Unless the thread has another reason for existing it only complicates what should be a straightforward problem.

[kuzulis]

@M4chin3,

I reimplemented it with readyRead() usage and it seems to work.

Hmm.. It is very interest and strange.. Can you please also, in additional, provide a your project's source code?

@ChrisW67,

if to be honest, in Windows use of a additional thread is desirable, because still there is this bug with a freezing: https://bugreports.qt-project.org/browse/QTBUG-34946