How to handle Modbus exception 0x5

Question

I'm writing a Modbus client program using Qt5 and the QModbusTcpClient class. Here the code I'm using for open a connection and read something:

QModbusClient *_modbus;

bool ModbusMaster::open(QString host, int port)
{
    // Disconnect and delete any existing instance
    if (_modbus)
    {
        _modbus->disconnectDevice();
        delete _modbus;
    }

    // Create and open the new connection
    _modbus = new QModbusTcpClient(this);
    _modbus->setConnectionParameter(QModbusDevice::NetworkPortParameter, port);
    _modbus->setConnectionParameter(QModbusDevice::NetworkAddressParameter, host);    

    _modbus->setTimeout(250);
    _modbus->setNumberOfRetries(1);

    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)
    {
        // do something
    }
    else if (reply->error() == QModbusDevice::ProtocolError)
    {
        qDebug() << QString("Read response error: %1 (Mobus exception: 0x%2)").
                                    arg(reply->errorString()).
                                    arg(reply->rawResult().exceptionCode(), -1, 16);
    } else {
        qDebug() << QString("Read response error: %1 (code: 0x%2)").
                                    arg(reply->errorString()).
                                    arg(reply->error(), -1, 16);
    }
}

Sometimes when I read something from the remote device it happens the device returns the exception 0x5. Reading the official Modbus documentation, at page 48 I read:

Specialized use in conjunction with programming commands. The server has accepted the request and is processing it, but a long duration of time will be required to do so. This response is returned to prevent a timeout error from occurring in the client. The client can next issue a Poll Program Complete message to determine if processing is completed.

[bold is mine]

I cannot find a description of this "Poll Program Complete message" that seems I must use to handle the exception 0x5.

Did I search wrong? Is there another way to handle this exception?

Answer 1

It depends on type of an equipment, you are working with. You just have to follow the logic, described in equipment mans for this particular exception.

In general there is no special 'Program Complete' event. That means, as it is written for 0x5 - "Specialized use in conjunction with programming commands.". So you just have to poll (read) some flag from your device meaning the internal process in device, which caused this exception, is complete.

Just as an example, I've met with such exception in relay protection device, which issued it when it has been in a process of writing a disturbance record. I had just to check for that record readiness in some time.