QModbusTcpClient - How to read input registers of device?

[robro]

Hello,

I am trying to use modbus the first time.

I got one modbus device delivering sensor values and this can be used with tcp.

In python I already managed to query the device.
All I needed for that were the ip-address, port, input register and number of bytes to read.

However, in Qt I somehow cannot manage to get this running.

I would like to read one byte from register 1002.
I am also wondering what to enter in "sendReadRequest" as second argument. I tried different values without success.

The following code delivers the following output:

    QVector<quint16> data(1); // One Byte
    QModbusDataUnit adu(QModbusDataUnit::InputRegisters, 1002, data); // Read one byte of register 1002
    QModbusReply *reply = device->sendReadRequest(adu, 3); // What is the second argument for? What do I need there?
    if(reply != nullptr) {
        qDebug() << "Sent read request, state is: " << device->state();
                qDebug() << reply;
                qDebug() << "Value 0: " << reply->result().value(0);
                qDebug() << "Value count: " << reply->result().valueCount();
                qDebug() << "Input registers:" << reply->result().InputRegisters;
    } else {
        qDebug() << "Send of ready request failed! Error is: " << device->state();
    }

Output:
Sent read request, state is: QModbusDevice::ConnectedState
QModbusReply(0x17cce5d00c0)
Value 0: 0
Value count: 0
Input registers: 3

What am I doing wrong?

Thank you very much!

[jsulm]

@robro said in QModbusTcpClient - How to read input registers of device?:

What am I doing wrong?

You're reading the reply just after sending request.
You should do the reading in a slot connected to https://doc.qt.io/qt-5/qmodbusreply.html#finished

[robro]

Thank you very much!
Using the "finished"-signal everything works fine :-)