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Details

Type: Bug
Resolution: Done
Priority: P1: Critical
Fix Version/s: 5.13.0 RC 1
Affects Version/s: 5.6.0, 5.7.0 Alpha, 5.7.0 Beta
Component/s: SerialBus: MOD Bus
Labels:
None
Environment:
windows 7

Commits:
a40fcae40fdc4d7cd658c4b8ab38aad7572f2ecf

Description

Calculation of send timeout is incorrect.

Here is the calculation code:

http://code.qt.io/cgit/qt/qtserialbus.git/tree/src/serialbus/qmodbusrtuserialmaster_p.h#n294

            // Example: 9600 baud, 11 bit per packet -> 872 char/sec
            // so: 1000 ms / 872 char = 1.147 ms/char * 3.5 character
            m_sendTimer.start((1000. / (qreal(m_baudRate) / 11.)) * m_current.adu.size());

Which in my case (115200 and 8 bytes of data) gives 0.7 ms. And after conversion to int, it becomes zero.
So m_sendTimer is fired up immediately, which brings us to to this code:

http://code.qt.io/cgit/qt/qtserialbus.git/tree/src/serialbus/qmodbusrtuserialmaster_p.h#n323

            } else if (m_current.bytesWritten < m_current.adu.size()) {
                qCDebug(QT_MODBUS) << "(RTU client) Send failed:" << m_current.requestPdu;

                if (m_current.numberOfRetries <= 0) {
                    if (m_current.reply) {
                        m_current.reply->setError(QModbusDevice::TimeoutError,
                            QModbusClient::tr("Request timeout."));
                    }

So there comes a premature timeout.

I experimented with a timeout and found that even a value of 15-20ms does not give stable results. only the value of around 50ms gives a stable result in my case.
I suppose that timeout calculation must include some additional minimum time constant.

It is only related to windows. In linux everything works perfectly.