Modbus RTU client: send failed error

Mark81

Here the output of my program:

State changed -> QModbusDevice::State(ConnectingState)
State changed -> QModbusDevice::State(ConnectedState)

(RTU client) Sent Serial PDU: 0x0303fe0001 ((null):0, (null))
>> "Tx: 0x010303fe0001e5be"
 (RTU client) Sent Serial ADU: 0x010303fe0001e5be ((null):0, (null))
 (RTU client) Send failed: 0x0303fe0001 ((null):0, (null))
Error "Request timeout." 
<< "Rx: 0x0103020000b844"
(RTU client) Response buffer: "0103020000b844" ((null):0, (null))
(RTU client) Received ADU: "0103020000b844" ((null):0, (null))
(RTU client) Cannot match response with open request, ignoring ((null):0, (null))

Here my code:

    QModbusClient *_modbus;

    bool ModbusMaster::open()
    {
        _modbus = new QModbusRtuSerialMaster(this);

        connect(_modbus, &QModbusClient::stateChanged, this, &ModbusMaster::state_changed);
        connect(_modbus, &QModbusClient::errorOccurred, this, &ModbusMaster::error_occurred);
        _modbus->setConnectionParameter(QModbusDevice::SerialPortNameParameter, _port);

        _retry = RTU_RETRY; // 0
        _timeout = RTU_TIMEOUT;  // 1000

        _modbus->setTimeout(_timeout);
        _modbus->setNumberOfRetries(_retry);

        return _modbus->connectDevice();
}

    bool ModbusMaster::read(QModbusDataUnit::RegisterType type, int startAddress, quint16 count)
    {
        if (!_modbus) return false;
        if (_modbus->state() != QModbusDevice::ConnectedState) return false;

        QModbusDataUnit req(type, startAddress, count);    
        if (auto *reply = _modbus->sendReadRequest(req, _id))
        {
            if (!reply->isFinished()) connect(reply, &QModbusReply::finished, this, &ModbusMaster::readReady);
            else delete reply;
            return true;
        }
        return false;
    }


    void ModbusMaster::readReady()
    {
        auto reply = qobject_cast<QModbusReply *>(sender());
        if (!reply) return;
        reply->deleteLater();

        if (reply->error() == QModbusDevice::NoError)
        {
            const QModbusDataUnit unit = reply->result();
            // do something
        }
        else if (reply->error() == QModbusDevice::ProtocolError)
        {
            QStringList msg = QStringList() << "Error protocol" << reply->errorString() << QString::number(reply->rawResult().exceptionCode()) << QString::number(reply->result().startAddress());
            emit errorOccurred(msg.join(", "));
        }
        else emit errorOccurred(reply->errorString());
    }

What does may cause a failure during send?
Of course the tty port is available and I can write to it.