ModbusTcpClient not working

[SPlatten]

@Gelvis , I have no idea what QModbusClient is doing, but in order for a slave to talk to a master it must send a request, modbus is a very simple protocol, there is no connection except when a request is sent to the master then the master responds. In your first post I cannot see where you set the slave address of the client.

In your new post, you connect then issue a request to get the coils, this is correct and a connection is only made when the request is issued.

Move:

if (!modbusClient.state() == QModbusDevice::ConnectedState) {
qDebug() << "Modbus connection failed";
}

to after:

modbusClient.sendReadRequest(readRequest, 1);

[Gelvis]

I do not understand QModbusTcpClient myself

I guessed the 1 in "modbusClient.sendReadRequest(readRequest, 1);" is the slave address

Tried modbusClient.setSlaveAddress(1), but that method does not exist

Moving the check made no difference

[SPlatten]

@Gelvis , also there are various inconsistencies in the protocol, for example in the protocol, all addresses start from 0, but 0 actually translates in the real world to 1. This is a serious gotcha.

[Gelvis]

@SPlatten

I guess. However setting it to 0 made no difference

[SPlatten]

@Gelvis , please explain? I suggest downloading the Modicon Modbus Protocol Specification. It’s very easy to follow and freely available.

[J.Hilk]

@Gelvis I would suggest looking at the return value of modbusClient.connectDevice(); it hopefully returns true .

this one modbusClient.sendReadRequest(readRequest, 1); returns a QModbusReply instance, you completely ignore that one, that is super wrong.

Because you have to listen to the finished() signal of that reply! Only once that signal is emitted can you actually read the received data.

[Gelvis]

I did test for the modbusClient.state() == QModbusDevice::ConnectedState which gives true (connectDevice also returns true)

I also have more code reading the reply, but my point was that it will never get to that after sendReadRequest gives this error: qt.modbus: (Client) Device is not connected

(fails on first line, never gets to qDebug messages or errorString)

edit: gets to the last one obviously "Read request failed, check connection status"

// Wait for the response
if (auto *reply = modbusClient.sendReadRequest(readRequest, 1)) {
    if (!reply->isFinished())
        connect(reply, &QModbusReply::finished, this, [this, reply]() {
            if (reply->error() == QModbusDevice::NoError) {
                const QModbusDataUnit unit = reply->result();
                qDebug() << "Read register value:" << unit.value(0);
            } else {
                qDebug() << "Error:" << reply->errorString();
            }
            reply->deleteLater();
        });
    else
        reply->deleteLater();
} else {
    qDebug() << "Read request failed, check connection status";
}

[Gelvis]

@SPlatten

I might have to. I was hoping I did not have to implement the whole thing myself for this simple task

I got this working in 5 minutes on Linux with a header file I found on github, however I have to make this for Windows for the assignment

[SPlatten]

@Gelvis , you only need to implement the messages you intend to provide services for , coils, status inputs, input registers and holding registers.

[Gelvis]

Turned out an event loop was needed to make QModbus work. I had no idea.

Connecting a timer to a method to read registers worked
connect(&m_timer, &QTimer::timeout, this, &ModbusController::readInputRegisters);