About QModbus usage(How to read coils at a fixed frequecy?)

MartinChan

Hi everyone.

I used to ask a question about how to read coil in forum before,and I got a good answer.

However,I came about another question:I don't just want to read the coils a 'Singel' time,I want to read these coils with a fixed interval(like 10ms or less).

I tried the code like following to solve these quesition,but I didn't think it's a good way to solve this question:

#include "modclient.h"
#include <QVariant>
#include <QModbusDataUnit>
#include <QDebug>
#include <QTime>
#include <QCoreApplication>
modclient::modclient()
{
    client=new QModbusTcpClient();
    client->setConnectionParameter(QModbusDevice::NetworkAddressParameter,"192.168.0.201");
    client->setConnectionParameter(QModbusDevice::NetworkPortParameter,502);
    client->setTimeout(200);
    client->setNumberOfRetries(3);
    first[0]=false;
    first[1]=false;
    control=true;
}

void modclient::connecttoModbusServer(){
    control=true;
    if(client->state()==QModbusClient::UnconnectedState){
        if(!client->connectDevice()){
            qDebug()<<QString("There's something wrong with the device");
        }else {
            qDebug()<<QString("Right connection");
            connect(client,&QModbusTcpClient::stateChanged,this,&modclient::query);
/****************Here is the first which puzzles me***************/ 
        }
    }
}

void modclient::readready(){
    if(control){
        QModbusReply *reply=qobject_cast<QModbusReply *>(sender());
        const QModbusDataUnit result=reply->result();
        bool temp0=false,temp1=false;
        qDebug()<<QString("The value of %1 is %2").arg(1).arg(result.value(1));
        qDebug()<<QString("The value of %1 is %2").arg(3).arg(result.value(3));
        if (result.value(1)==1) temp0=true;
        if (result.value(3)==1) temp1=true;

        if (temp1==true && first[1]==false){
            //delay(Time2);
            emit finishedreading2();
        }
        else if(temp0 ==true && first[0]==false){
            //delay(Time1);
            emit finishedreading();
        }
        first[0]=temp0;
        first[1]=temp1;
    }
}


void modclient::query(){
    QModbusDataUnit readUnit(QModbusDataUnit::Coils,00000,4);
/****************Here is the second which puzzles me***************/ 
    while(1){
            if (auto *reply = client->sendReadRequest(readUnit,1))
            {
                if (!reply->isFinished())
                {
                    // connect the finished signal of the request to your read slot
                    connect(reply, &QModbusReply::finished,this,&modclient::readready);
                }
                else
                {
                    delete reply; // broadcast replies return immediately
                }

            }
            else
            {
                // request error
            }
            delay(10);
        }

}

void modclient::Quit_Query(){
    control=false;
    //disconnect(client,&QModbusTcpClient::stateChanged,this,&modclient::query);
}

void modclient::delay(int a){
    QTime t;
    t.start();
    while(t.elapsed()<a)
    QCoreApplication::processEvents();
}

I marked two places which puzzles me:
1)When the signal 'StateChanged' is emitted,it means the device state changed(like from unconnceted to connect,or reversed),or the coil state changed(I know its impossible but I still want to ask)?
2)I just want to use a single QThread to do the reading(coils) job,however,it seems not recommended to use a 'infinite' while loop in the class which will be added to a QThread.So,what should I do is a better way to construct a right way to 'continously' read the coil?

MartinChan

My previous topic was here:link

And I used to refer to the Qt ModbusClient example,nevertheless,I didn't find too much about how to read coils synchronously,but when I test the program it did execute ‘synchronously’

beecksche

Hi @MartinChan,

I think you want to poll to your slave like http://www.modbusdriver.com/modpoll.html

If you want to have a synchronous API i would start a local QEventLoop after sending your request and stop this loop when the reply has finished. Sth like that:

void modclient::readCoil()
{
 QModbusDataUnit readUnit(QModbusDataUnit::Coils,00000,4);
 auto *reply = client->sendReadRequest(readUnit,1);

 QEventLoop loop;

 auto connection = QObject::connect(reply, &QModbusReply::finished, ()[&loop] { loop.quit(); }

 loop.start();
 QObject::disconnect(connection);

 // now you can get the result of the query and do your stuff

 const QModbusDataUnit result=reply->result();
 bool temp0=false,temp1=false;
 qDebug()<<QString("The value of %1 is %2").arg(1).arg(result.value(1));
 qDebug()<<QString("The value of %1 is %2").arg(3).arg(result.value(3));
 if (result.value(1)==1) temp0=true;
 ...

 if (polling)
   QTimer::singleShot(interval, &modclient::readCoil)

}

Haven't testet the code, may you have to add some security mechanism in the event loop.

MartinChan

@beecksche Thx~！And I will try it soon~

MartinChan

@beecksche Hi,Thx first and I have another little quesition .

The code u provided,there is a line：

 auto connection = QObject::connect(reply, &QModbusReply::finished, ()[&loop] { loop.quit(); }

It seems u use the lambda expression in '()[&loop] { loop.quit(); }' (I am sorry for I didn't know too much about Lambda expression...)However,it didn't work,can I just use

auto connection=QObject::connect(reply,&QModbusReply::finished,loop.quit());

to replace it?If not,why??

beecksche

@MartinChan
Oh, yes of course. Forgot that quit is a slot of the QEventLoop. Then you have to change the function:

auto connection = QObject::connect(reply, &QModbusReply::finished, &loop, &QEventLoop::quit);

Edit:
Because of the lambda function, i mixed up the correct writing. It should be:

auto connection = QObject::connect(reply, &QModbusReply::finished, [&loop]()
{ 
loop.quit();
}) ;

jsulm

@MartinChan You cannot do it like this:

auto connection=QObject::connect(reply,&QModbusReply::finished,loop.quit());

In this case you call the method loop.quit() instead of passing a pointer to it to connect. So, do it like @beecksche suggested.

Yunus

@beecksche @MartinChan When I tried the code I get an error in the line:

loop.start();

as

error: no member named 'start' in 'QEventLoop'

What can cause that ?

jsulm

@Yunus said in About QModbus usage(How to read coils at a fixed frequecy?):

What can cause that ?

It's not start but https://doc.qt.io/qt-6/qeventloop.html#exec ...