inside MainWindow:
Qt Code: 
  1. plcTimer = new PlcTimer();
  2.     connect(plcTimer, SIGNAL(jack_sendToRS(char*,int)), rs_plc, SLOT(rs_plcDataAqustn(char*,int)));
  3.     plcTimer->start();
  4.     rs_plc->rs_plcOpenPort((char *)"/dev/ttyS0"); /*/dev/ttyS3*/
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QThread signal or QTimer signal connects to serial SLOT which write n reads packet to and from serial port:
Qt Code: 
  1. bool RS::rs_plcDataAqustn(char* data, int len)
  2. {    
  3.     QByteArray rd15Bytes;
  4.  
  5.     //    QByteArray built((char*)data, 6) ;
  6.     //    qDebug() << built.toHex();
  7.  
  8.  
  9.     if(!rs_serialWrite(data, len))
  10.     {
  11.         qDebug() << "Failure:( rs_dataqustn: rs_plcWrite(data, len)";
  12.         emit plc_port_dscntd();
  13.         return false;
  14.     }
  15.  
  16.     if(len == 3)
  17.     {
  18.         qDebug() << "len = 3";
  19.  
  20.         rs_delay();
  21.  
  22.         if(rs_plcRead(&rd15Bytes))
  23.         {
  24.             qDebug() << rd15Bytes.length();
  25.             //qDebug() << "->" << rd15Bytes.toHex();
  26.  
  27.             if(rd15Bytes.isEmpty())
  28.             {
  29.                 emit logMessage("Failure:( rs_dataAqustn, if(rd15Bytes.isEmpty())");
  30.             }
  31.             else
  32.             {
  33.                 if(!rs_plcCheckChecksum(&rd15Bytes))
  34.                     emit logMessage("Failure:( rs_dataAqustn, if(!rs_plcCheckChecksum(&rd15Bytes))");
  35.                 else
  36.                 {
  37.                     emit rs_plcSendRdPacket(rd15Bytes);
  38.  
  39.                     if(!rs_serialWrite((char* )"0x06", 1))
  40.                     {
  41.                         qDebug() << "Failure:( rs_dataqustn: rs_plcWrite(PLC_ACK, 1)";
  42.                     }
  43.                 }
  44.             }
  45.         }
  46.         else
  47.         {
  48.             emit plc_hmiBoard_dscntd();
  49.         }
  50.     }
  51.  
  52.     return true;
  53. }
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Read from Plc:
Qt Code: 
  1. bool RS::rs_plcRead(QByteArray *data)
  2. {
  3.     unsigned char buff[128];
  4.     int len, tries;
  5.  
  6.     tries = 0;
  7.  
  8.     while(tries < 13)
  9.     {
  10.         len = read(fd, buff, PLC_READ_BYTE);
  11.  
  12.         if((buff[0] == PLC_HEADER) && (len == PLC_READ_BYTE))
  13.         {
  14.             data->append((char *)buff, PLC_READ_BYTE);
  15.             //qDebug() << data->toHex();
  16.             return true;
  17.         }
  18.         else
  19.         {
  20.             tries++;
  21.             qDebug() << "retry";
  22.         }
  23.     }
  24.  
  25.     return false;
  26. }
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should you need more info, please note it.