how to i synchronize serial read and write without using thread?

[pooch]

here are some portion of my code:

//MaiWindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    timer_for_read=new QTimer(this); //this timer is used for calling continuous_monitor()
    QObject::connect(timer_for_read,SIGNAL(timeout()),this,SLOT(continuous_monitor()));
    QObject::connect(timer_for_read,SIGNAL(timeout()),timer_for_read,SLOT(start()));//didnt know ways to restart timer again.
    start_check[0]=0;
 
}
void MainWindow::on_Start1_clicked()
{
    if(!modbus_master.port_init)
        modbus_master.init_port();
    modbus_master.data_buffer.Reset();//these are for data 
    modbus_master.data_buffer.Append(50);
    while (modbus_master.CheckBusy());
    modbus_master.WriteOutputReg(10,1000);
    start_check[0]=1;
    qDebug()<<timer_for_read->isActive();
    if(!timer_for_read->isActive())
        timer_for_read->start(5000);
}
void MainWindow::continuous_monitor()
{
    QApplication::processEvents(QEventLoop::ExcludeUserInputEvents);//dint know where to do this
    if(start_check[0]){
        while(modbus_master.CheckBusy());// this checks whether the code is listening to response, returns 1 when listening is done
        modbus_master.ReadInputReg(10,1000,4);// reads input register// this writes a message to serial port and listens to response
    }
    if(start_check[1]){
        while(modbus_master.CheckBusy());
         modbus_master.ReadInputReg(10,1000,4);
    }
}

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    timer_for_read=new QTimer(this); //this timer is used for calling continuous_monitor()
    QObject::connect(timer_for_read,SIGNAL(timeout()),this,SLOT(continuous_monitor()));
    QObject::connect(timer_for_read,SIGNAL(timeout()),timer_for_read,SLOT(start()));//didnt know ways to restart timer again.
    start_check[0]=0;
    
}
void MainWindow::on_Start1_clicked()
{
    if(!modbus_master.port_init)
        modbus_master.init_port();
    modbus_master.data_buffer.Reset();//these are for data 
    modbus_master.data_buffer.Append(50);
    while (modbus_master.CheckBusy());
    modbus_master.WriteOutputReg(10,1000);
    start_check[0]=1;
    qDebug()<<timer_for_read->isActive();
    if(!timer_for_read->isActive())
        timer_for_read->start(5000);
}
void MainWindow::continuous_monitor()
{
    QApplication::processEvents(QEventLoop::ExcludeUserInputEvents);//dint know where to do this
    if(start_check[0]){
        while(modbus_master.CheckBusy());// this checks whether the code is listening to response, returns 1 when listening is done
        modbus_master.ReadInputReg(10,1000,4);// reads input register// this writes a message to serial port and listens to response
    }
    if(start_check[1]){
        while(modbus_master.CheckBusy());
         modbus_master.ReadInputReg(10,1000,4);
    }
}

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//ModbuMaster.cpp

ModbusMaster::ModbusMaster()
{
    timer=new QTimer(this);
    QObject::connect(timer,SIGNAL(timeout()),this,SLOT(onTimeOut()));// onTimeout asserts that the response is invalid
    QObject::connect(this,SIGNAL(WriteEmit(char*)),this,SLOT(writeData(char*)));
    port=new QextSerialPort();
    QObject::connect(port,SIGNAL(readyRead()),this,SLOT(readData()));
}
 
void ModbusMaster::readData()
{
    qDebug("read data");
    QApplication::processEvents(QEventLoop::ExcludeUserInputEvents);
    port->flush();
    //    char buff[1024];
    //    qDebug("read data");
    QByteArray response=port->readAll();
    for(int i=0;i<response.length();i++)
    {
        ReadCharacterCB((uint8_t)response[i]); //this function has intrepretation of data
    }
}
void ModbusMaster::writeData(char* msg)
{
    //qDebug("Writing data");
    port->write(msg,1);
    port->flush();
 
}

ModbusMaster::ModbusMaster()
{
    timer=new QTimer(this);
    QObject::connect(timer,SIGNAL(timeout()),this,SLOT(onTimeOut()));// onTimeout asserts that the response is invalid
    QObject::connect(this,SIGNAL(WriteEmit(char*)),this,SLOT(writeData(char*)));
    port=new QextSerialPort();
    QObject::connect(port,SIGNAL(readyRead()),this,SLOT(readData()));
}

void ModbusMaster::readData()
{
    qDebug("read data");
    QApplication::processEvents(QEventLoop::ExcludeUserInputEvents);
    port->flush();
    //    char buff[1024];
    //    qDebug("read data");
    QByteArray response=port->readAll();
    for(int i=0;i<response.length();i++)
    {
        ReadCharacterCB((uint8_t)response[i]); //this function has intrepretation of data
    }
}
void ModbusMaster::writeData(char* msg)
{
    //qDebug("Writing data");
    port->write(msg,1);
    port->flush();
    
}

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