Write ASCII Character and read response using serialport

mrjj

Hi
Its asynchronous so i think the issue lies here

QByteArray MotorPort::getresponse()
{
    QByteArray buffer = "?";
    write(buffer); // send 
    getData();// you just call it so might be nothing to read yet.  its not called by serialport due to incoming data
    return Output;
}

So you cant really do it this way. it has to be like this

You send something
Serialport issues one or more readyRead signals
in getData you read and append to a buffer

Output += QIODevice::readAll(); // note the plus

then when data contains a CR, we know you got full set.
Then you can send signal to mainWindow that data is ready.
and it that slot in main window, you can then use the data.

I know it seems a bit complicated but thats how async programming is.

If you really find it hard to get work, you can use the blocking api of QSrialport
https://doc.qt.io/qt-5/qtserialport-creadersync-example.html

But do note that this LAGS the GUI. gui will not redraw etc while code is running and that is
why the async way with signal and slot is preferred.

H.dragon

@mrjj
Hi. I read your answer but It is a little difficult for me to understand the concept of asynchronous, I googled it and some functions, I changed MotorPort::getresponse() like below. then it works

QByteArray MotorPort::getresponse()
{
    QByteArray buffer = "?";
    write(buffer);
    waitForReadyRead(500);
    return Output;
}

Is this the solution of your answer? Actually I don't think I fully understood your answer.
Anyway, Thank you mrjj!

mrjj

@H-dragon
Hi
asynchronous means it will happen at another point in time (than right now) and yes it can be
difficult to handle.

You used the blocking call
waitForReadyRead(500);
so yes that was what i meant in the second part of the answer.
Even now you kinda mix sync and async but it seems that it fires in right order
so it works :)

H.dragon

@mrjj
HI.
Can I ask you one last question?

QByteArray MotorPort::getresponse()
{
    QByteArray buffer = "?";
    write(buffer);
    waitForReadyRead(500);
    return Output;
}

How the private member Output gets some data from serialport when I call the function getresponse(), although there isnt the function getData()? As I know, the waitForReadyRead(500) blocks the cell until new data is available.
Is that because I connected signal to slots? (connect(this, SIGNAL(readyRead()), this, SLOT(getData()));)
Thank you for your continued kindness :)

mrjj

@H-dragon
Hi
Yes its due to the readyRead signal calling getData() and fill Output.
So we kinda mixed async and sync calls.
But docs says
"This function blocks until new data is available for reading and the readyRead() signal has been emitted. "
https://doc.qt.io/qt-5/qserialport.html#waitForReadyRead

so it does come in right order and then it worked.

H.dragon

@mrjj
Hi. Thanks to you, I completed my GUI work. I really appreciate it.
But while writing codes, I came up with a question.
In your second answer, you changed Output = QIODevice::readAll() to Output += QIODevice::readAll().
Can you explain why the former doesn't work(my gui terminate)? is it related with asynchronous?

mrjj

Hi
Good to hear
Well there is only a small difference

This overwrites all that already is in Output
Output = QIODevice::readAll()

This Adds to what is already is in Output
Output += QIODevice::readAll().

The last is the best due to the following reason.

When you sent something big enough over serial - it will result in multiple pieces of data
so the readyRead() will trigger more than one time.

So in such cases,
one has to add the incoming data to a buffer as to collect it all

before using it.
and that is what
Output += QIODevice::readAll().
does. Add data to the Output buffer.

So that is something to keep in mind.

H.dragon

@mrjj
Do I understand well?
for example, let's say something big data (123456789) is divided into multiple pieces(123, 456, 789).

If I use Output = QIODevice::readAll(), it can cause follow situation
readyRead()
Output is 123
readyRead()
Output is 456
readyRead()
Output is 789

but when I use Output+=QIODevice::readAll(),
readyRead()
Output is 123
readyRead()
Output is 123456
readyRead()
Output is 123456789

The reason += is the best is It can prevent the former situation. right?

mrjj

@H-dragon
Hi
Yes that is exactly like that.

When it will be broken up depends on the hardware. some have bigger buffers than others and
so on but it does happen.

H.dragon

Thanks!!

mrjj

@H-dragon

Np.

In your concrete use case, all feedback from the hardware ends with CR
so its easy to spot when you have read all data.

In other cases one has to look at size to know if
all has been read.