Qt Serial Port - Reading data consistently

Question

I am sending (writing) bytes to a device via my serial port. I am using the QSerialPort (http://qt-project.org/wiki/QtSerialPort) module to instantiate device IO support. When I send messages to my INSTEON modem (serial), upon reading my message the device sends back a copy of my message + 0x06 (ACK Byte) followed by a status message.

I have tested my message using DockLight (http://www.docklight.de/). I send the following message to query the state of the device:

    02 62 1D E9 4B 05 19 00

Using Docklight, I receive the response:

    02 62 1D E9 4B 05 19 00 06 02 50 20 CB CF 1E DA F7 21 00 FF

The returned message indicates exactly what I would expect, that the device is on. If off, the modem would send back 0x00 in the last byte position if the device was off. Now, my problem - I must not have my function setup properly to send and then receive the response bytes. I have tried many different examples and configurations, currently I am using the following:

Setup signal-slot connections:

QObject::connect(&thread, SIGNAL(sendResponse(QByteArray)), 
    this, SLOT(handleResponse(QByteArray)));
QObject::connect(&thread, SIGNAL(error(QString)), 
    this, SLOT(processError(QString)));
QObject::connect(&thread, SIGNAL(timeout(QString)), 
    this, SLOT(processTimeout(QString)));

Function used to iterate through QList of devices. If device is desired type ("Light"), then we format the device ID to the intended QByteArray message structure. Pass message to thread for sending. (Thread modified from QSerialPort BlockingMaster example.

void Device::currentStatus(QList<Device *> * deviceList){
    QString devID, updateQry;
    int devStatus, updateStatus;
    updateStatus=0;
    QSqlQuery query;
    for(int i=0; i<deviceList->size(); i++){
        if(deviceList->at(i)->type == "Light"){
            devStatus = deviceList->at(i)->status;
            devID = deviceList->at(i)->deviceID;
            QByteArray msg;
            bool msgStatus;
            msg.resize(8);

            msg[0] = 0x02;
            msg[1] = 0x62;
            msg[2] = 0x00;
            msg[3] = 0x00;
            msg[4] = 0x00;
            msg[5] = 0x05;
            msg[6] = 0x19;
            msg[7] = 0x00;
            msg.replace(2, 3, QByteArray::fromHex( devID.toLocal8Bit() ) );
            qDebug() << "Has device " << deviceList->at(i)->name << "Changed?";
            //send(msg,&msgStatus, &updateStatus);
            //msg.clear();
            thread.setupPort("COM3",500,msg);
            if(devStatus!=updateStatus){
                qDebug() << deviceList->at(i)->name << " is now: " << updateStatus;
                updateStatus = !updateStatus;
            }
        }
    }
}

SetupThread function used to set local thread variables and executes (runs) thread.

void serialThread::setupPort(const QString &portName, int waitTimeout, const QByteArray &msg){
    qDebug() << "Send Message " << msg.toHex();
    QMutexLocker locker(&mutex);
    this->portName = portName;
    this->waitTimeout = waitTimeout;
    this->msg = msg;
    if(!isRunning())
        start();
    else
        cond.wakeOne();
}

Run Function - Handled sending and receiving

void serialThread::run(){
    bool currentPortNameChanged = false;
    qDebug() << "Thread executed";
    mutex.lock();
    QString currentPortName;
    if(currentPortName != portName){
        currentPortName = portName;
        currentPortNameChanged = true;
    }

    int currentWaitTimeout = waitTimeout;
    QByteArray sendMsg = msg;
    mutex.unlock();
    QSerialPort serial;

    while(!quit){
        if(currentPortNameChanged){
            serial.close();
            serial.setPortName("COM3");

            if (!serial.open(QIODevice::ReadWrite)) {
                emit error(tr("Can't open %1, error code %2")
                           .arg(portName).arg(serial.error()));
                return;
            }

            if (!serial.setBaudRate(QSerialPort::Baud19200)) {
                emit error(tr("Can't set baud rate 9600 baud to port %1, error code %2")
                           .arg(portName).arg(serial.error()));
                return;
            }

            if (!serial.setDataBits(QSerialPort::Data8)) {
                emit error(tr("Can't set 8 data bits to port %1, error code %2")
                           .arg(portName).arg(serial.error()));
                return;
            }

            if (!serial.setParity(QSerialPort::NoParity)) {
                emit error(tr("Can't set no patity to port %1, error code %2")
                           .arg(portName).arg(serial.error()));
                return;
            }

            if (!serial.setStopBits(QSerialPort::OneStop)) {
                emit error(tr("Can't set 1 stop bit to port %1, error code %2")
                           .arg(portName).arg(serial.error()));
                return;
            }

            if (!serial.setFlowControl(QSerialPort::NoFlowControl)) {
                emit error(tr("Can't set no flow control to port %1, error code %2")
                           .arg(portName).arg(serial.error()));
                return;
            }
        }

        //write request
        serial.write(msg);
        if (serial.waitForBytesWritten(waitTimeout)) {
            //! [8] //! [10]
            // read response
            if (serial.waitForReadyRead(currentWaitTimeout)) {
                QByteArray responseData = serial.readAll();
                while (serial.waitForReadyRead(10)){
                    responseData += serial.readAll();
                }

                QByteArray response = responseData;
                //! [12]
                emit this->sendResponse(response);
                //! [10] //! [11] //! [12]
            } else {
                emit this->timeout(tr("Wait read response timeout %1")
                             .arg(QTime::currentTime().toString()));
            }
            //! [9] //! [11]
        } else {
            emit timeout(tr("Wait write request timeout %1")
                         .arg(QTime::currentTime().toString()));
        }
        mutex.lock();
        cond.wait(&mutex);
        if (currentPortName != portName) {
            currentPortName = portName;
            currentPortNameChanged = true;
        } else {
            currentPortNameChanged = false;
        }
        currentWaitTimeout = waitTimeout;
        sendMsg = msg;
        mutex.unlock();
    }
    serial.close();
}

handleResponse function, SLOT which receives response signal

void Device::handleResponse(const QByteArray &msg){
    qDebug() << "Read: " << msg.toHex();
}

I receive the following output:

Has device  "Living Room Light" Changed? 
Send Message  "02621de94b051900" 
Has device  "Bedroom Light" Changed? 
Send Message  "026220cbcf051900" 
Thread executed 
Read:  "026220cbcf05190006" 
Polling for changes... 
Has device  "Living Room Light" Changed? 
Send Message  "02621de94b051900" 
Has device  "Bedroom Light" Changed? 
Send Message  "026220cbcf051900" 
Read:  "025020cbcf1edaf721000002621de94b05190006" 
Polling for changes... 
Has device  "Living Room Light" Changed? 
Send Message  "02621de94b051900" 
Has device  "Bedroom Light" Changed? 
Send Message  "026220cbcf051900" 
Read:  "02501de94b1edaf72100ff02621de94b05190006" 

Two issues here.

1. I never receive any response regarding the second device (Bedroom Light), this is the message that is sent second. It seems that the send is being blocked, how would you recommend I format my sending so that I send after the response is received for the first send? There is only 1 COM port that can be used to send/receive. I believe I should Send message to Device 1, receive Device 1 response, Send to Device 2, receive Device 2. Could I end up seeing a huge traffic jam with a lot of devices and using wait conditions, ie. wait for device 1 communication process to finish before executing comm process for device 2?

2. The very first read contains the appropriate 1st half of the receive. Read: "026220cbcf05190006" The second receive contains the 2nd half of the 1st response followed by the 1st half of the second response: Read 2 - Read: "025020cbcf1edaf721000002621de94b05190006" The appropriate full response is 02621DE94B05190006 025020CBCF1EDAF72100FF (note 20CBCF is Device 2's ID in the full response example)

What corrections should be made to the way I am receiving data from the serial port? Thank you!

Answer 1

I believe my problems have shifted from the scope of this question. With help from Kuzulis I have implemented Write/Read functions to successfully send and read serial messages consistently. Kuzulis recommended using the Synchronous blocking communication pattern, however it was later decided that the Asynchronous Non-Blocking method would be best fit for my application.

My implementation closely follows the "Master" example provided with the QSerialPort source files.

I use CurrentStatus to iterate through a QList of Device objects. For each Light in the Device list, I format an 8 Byte message to query the current status of the device (ON/OFF).

void Device::currentStatus(QList<Device *> * deviceList){
    QString devID, updateQry;
    int devStatus, updateStatus;
    updateStatus=0;
    QSqlQuery query;
    for(int i=0; i<deviceList->size(); i++){
        if(deviceList->at(i)->type == "Light"){
            devStatus = deviceList->at(i)->status;
            devID = deviceList->at(i)->deviceID;
            QByteArray msg;
            msg.resize(8);

            msg[0] = 0x02;
            msg[1] = 0x62;
            msg[2] = 0x00;
            msg[3] = 0x00;
            msg[4] = 0x00;
            msg[5] = 0x05;
            msg[6] = 0x19;
            msg[7] = 0x00;
            msg.replace(2, 3, QByteArray::fromHex( devID.toLocal8Bit() ) );
            qDebug() << "Has device " << deviceList->at(i)->name << "Changed?";

            emit writeRequest(msg);

            if(devStatus!=updateStatus){
                qDebug() << deviceList->at(i)->name << " is now: " << updateStatus;
                updateStatus = !updateStatus;
            }
        }
    }
}

In the Device class constructor, I connect the signals and slots:

Device::Device(){

    serialTimer.setSingleShot(true);
    QObject::connect(&serial, SIGNAL(readyRead()),
                     this, SLOT(handleResponse()));
    QObject::connect(&serialTimer, SIGNAL(timeout()),
                     this, SLOT(processTimeout()));
    QObject::connect(this, SIGNAL(writeRequest(QByteArray)),
                     this, SLOT(writeSerial(QByteArray)));
}

After the message to send in currentStatus has been prepared, emit writeRequest(msg); is called. This dispatches a signal that is connected to the slot writeRequest. writeRequest is used to setup and actually write the message to the serial port.

void Device::writeSerial(const QByteArray &msg){
    if (serial.portName() != "COM3") {
        serial.close();
        serial.setPortName("COM3");

        if (!serial.open(QIODevice::ReadWrite)) {
            processError(tr("Can't open %1, error code %2")
                         .arg(serial.portName()).arg(serial.error()));
            return;
        }

        if (!serial.setBaudRate(QSerialPort::Baud19200)) {
            processError(tr("Can't set rate 19200 baud to port %1, error code %2")
                         .arg(serial.portName()).arg(serial.error()));
            return;
        }

        if (!serial.setDataBits(QSerialPort::Data8)) {
            processError(tr("Can't set 8 data bits to port %1, error code %2")
                         .arg(serial.portName()).arg(serial.error()));
            return;
        }

        if (!serial.setParity(QSerialPort::NoParity)) {
            processError(tr("Can't set no patity to port %1, error code %2")
                         .arg(serial.portName()).arg(serial.error()));
            return;
        }

        if (!serial.setStopBits(QSerialPort::OneStop)) {
            processError(tr("Can't set 1 stop bit to port %1, error code %2")
                         .arg(serial.portName()).arg(serial.error()));
            return;
        }

        if (!serial.setFlowControl(QSerialPort::NoFlowControl)) {
            processError(tr("Can't set no flow control to port %1, error code %2")
                         .arg(serial.portName()).arg(serial.error()));
            return;
        }
    }
    qDebug() << "Message written";
    this->msgRequest = msg;
    serial.write(msgRequest);
    serialTimer.start(400);
}

After setting up the serial port, I save the current message to msgRequest. This may have to be used to resend the message if there is an error. After serial.write() is called, I setup a timer for 400ms. Once this timer expires, I check what was read from the serial port.

handleResponse() is a slot that is called everytime QSerialPort emits the readyRead() signal. readyRead() appends any available data to the QByteArray response.

void Device::handleResponse(){
    response.append(serial.readAll());  
}

After 400ms, serialTimer (one shot Timer) will emit a timeout() signal. serialTimer was started right after writing our requested message to the serial port. processTimeout() is where we finally check the response received from the PowerLinc Modem after sending our message. When messages are sent to the INSTEON PowerLinc Modem (PLM), the PLM echoes back the message and appends either 0x06 (Positive ACK) or 0x15 (NACK). In processTimeout() I check to make sure the last byte received is the ACK byte, if not - resend our originally requested message.

void Device::processTimeout(){
    qDebug() << "Read: " << response.toHex();
    int msgLength = this->msgRequest.length();
    if(response.at(msgLength)!=0x06){
        qDebug() << "Error, resend.";
        emit writeRequest(msgRequest);
    }
    response.clear();
}

I used the Serial Port Monitor 4.0 (Eltima Software) to verify the write and read transactions on the serial port. Below, you can see the log printout for 1 sample transaction.

20:44:30:666 STATUS_SUCCESS 02 62 1d e9 4b 05 19 00   <--- Send
20:44:30:669 STATUS_SUCCESS 02 62 1d e9 4b 05 19 00 06   <--- Receive
20:44:30:875 STATUS_SUCCESS 02  <--- Receive
20:44:30:881 STATUS_SUCCESS 50 1d e9 4b 1e da f7 21 00 ff   <--- Receive

For 20 sends, I received the same response. Thus, I can safely say my issues with inconsistent data arrival have been resolved. Now I am struggling with multiple write requests, but I believe that is a separate question to be investigated. I appreciate everyone's support.

Answer 2

1. See the BlockingMaster example in the repository and read the documentation about the blocking I/O. Also, do not use blocking I/O unnecessarily.

2. Use bytesAvailable() to get the number of available data for reading, because not the fact that you immediately receive a complete response package.