Modbus485::Modbus485(QObject *parent) : QThread(parent)
{
    _device = nullptr;
}

bool Modbus485::open(QString serialPort, QString DE_bank, int DE_pin, QSerialPort::BaudRate baudRate, QSerialPort::Parity parity, QSerialPort::DataBits dataBits, QSerialPort::StopBits stopBits)
{
    _gpio->set(PIN_DE, false);

    if (_device) delete _device;
    _device = new QModbusRtuSerialMaster();
    QObject::connect(_device, &QModbusClient::errorOccurred, [this](QModbusDevice::Error)
    {
        qInfo().noquote() << Utils::timestamp() << "[RS485] Error occurred:" << _device->errorString();
        return false;
    });

    _device->setConnectionParameter(QModbusDevice::SerialPortNameParameter, serialPort);
    _device->setConnectionParameter(QModbusDevice::SerialBaudRateParameter, baudRate);
    _device->setConnectionParameter(QModbusDevice::SerialParityParameter, parity);
    _device->setConnectionParameter(QModbusDevice::SerialDataBitsParameter, dataBits);
    _device->setConnectionParameter(QModbusDevice::SerialStopBitsParameter, stopBits);
    _device->setTimeout(250);
    _device->setNumberOfRetries(0);

    if (!_device->connectDevice())
    {
        qInfo().noquote() << Utils::timestamp() << "[RS485] Connection failed:" << _device->errorString();
        return false;
    }

    return true;
}

bool Modbus485::read(int startAddress, quint16 count)
{
    _gpio->set(PIN_DE, true);

    QModbusDataUnit request = QModbusDataUnit(QModbusDataUnit::HoldingRegisters, startAddress, count);
    if (auto *reply = _device->sendReadRequest(request, _serverAddress))
    {
        QTimer::singleShot(30, [this]() { _gpio->set(PIN_DE, false); });
        if (!reply->isFinished()) QObject::connect(reply, &QModbusReply::finished, [reply]()
        {
            if (reply->error() == QModbusDevice::NoError)
            {
                const QModbusDataUnit unit = reply->result();
                qDebug() << unit.values(); // read ok!!!
                return true;
            }
            else if (reply->error() == QModbusDevice::ProtocolError)
            {
                qInfo().noquote() << Utils::timestamp() << "[RS485] Protocol error:" << reply->errorString() << reply->rawResult().exceptionCode();
                return false;
            }
            else
            {
                qInfo().noquote() << Utils::timestamp() << "[RS485] Error occurred:" << reply->errorString();
                return false;
            }
            reply->deleteLater();
        });
        else
        {
            qDebug() << "REPLY FINISHED!";
            delete reply;
            return false;
        }
    }
    else
    {
        qInfo().noquote() << Utils::timestamp() << "[RS485] Generic error:" << _device->errorString();
        _gpio->set(PIN_DE, false);
        return false;
    }

    return false;
}

But when I try to read something I get this debug output:

>> "Tx: 0x01030031000295c4"
Debug: (RTU client) Sent Serial ADU: 0x01030031000295c4 ((null):0, (null))
Debug: (RTU client) Send failed: 0x0300310002 ((null):0, (null))
[RS485] Error occurred: Request timeout
Debug: (RTU client) Sent Serial PDU: 0x0300310002 ((null):0, (null))
>> "Tx: 0x01030031000295c4"
Debug: (RTU client) Sent Serial ADU: 0x01030031000295c4 ((null):0, (null))
Debug: (RTU client) Send successful: 0x0300310002 ((null):0, (null))
<< 37 "Rx: 0x0103040000138d37"
Debug: (RTU client) Response buffer: "0103040000138d37" ((null):0, (null))
Debug: (RTU client) Incomplete ADU received, ignoring ((null):0, (null))
<< 39 "Rx: 0x0103040000138d3766"
Debug: (RTU client) Response buffer: "0103040000138d3766" ((null):0, (null))
Debug: (RTU client) Received ADU: "0103040000138d3766" ((null):0, (null))
Debug: QVector(0, 5003) 

Questions:

1. how can the sending fail?
2. what does mean Sent Serial PDU after Send failed?
3. why there are two receiving events, where the first one is incomplete? Is there anything I can set to avoid this?