@vishbynature said in Qt Program start on bootup and start GUI interaction automatically:

As soon as MainWindow GUI appears I should start SPI data transfer.

Sure. You can use QTimer to trigger the transfer.

If your Raspi have a 'generic' SPI driver (aka spidev), than I don't see any problems. Just google about 'spidev' (how to configure, how to open/close, how to read/write).

Root permission to the program
2.Spi Begin function

After these modifications it started working. beginners mistake.

Getting back to document conclusions of my research on best practice of using RaspiComm RS-485 under Qt for anybody, who finds this thread useful (I expect many of it is valid for RS-232 on same board as well, did not test myself):

Do not try to implement communication with MAX3140 UART chip yourself (maybe except of asm based implementation, what is however most probably unnecessary exertion). I succeeded to implement it based on kernel /dev/spidev0.0 only to find out that even this partially system driver supported solution is too slow for even 19 200 Bd speed (loosing about 5 characters on one received).
I myself feared about RaspiComm RS-485 driver /dev/ttyRPC0 based on some forums complaints and not clear versioning, however found out, that the forum thread was heavy outdated. These official installation instructions worked like a charm and the resulting driver worked out-of-box for my latest Raspbian Wheezy (4.1.7+ #817). So I strongly recommend to take this approach.

QSerialPort class has constructor with signature QSerialPort(const QString &, QObject *), where QString may contain even device which is not included in the QList that QSerialPortInfo::availablePorts() returns. This works with no surprise. If one does not set anything than (eventually) baud speed, one gets standard 8 bit, 1 stop bit, no parity without any character translations, good old plain raw binary. So QSerialPort may be used with /dev/ttyRPC0 directly and easily.

QSerialPort rpc0("/dev/ttyRPC0");
rpc0.write("Binary request\0even\xffcontaining weird\x03characters", length_of_binary);
if(rpc0.waitForReadyRead(100)) // Enough long time in miliseconds
QByteArray data = rpc0.readAll(); // You may have to wait/read repetitively in loop
// and merge data on higher link speeds

Hardware of RaspiComm uses RTS pin to select in/out direction of data. CTS is kept active permanently by wiring, so using default use RTS/CTS mode works properly with no inappropriate blocking.

That's all you should need, so enjoy your communication.

@Dan90 You are assigning a value to a pointer, but data is actually an array of "unsigned char":

unsigned char *data = 0b10000000;

Instead you should do (C++11):

unsigned char data[] = { (unsigned char)0b10000000 };