Processing of data received by the C#Serial Port

Question

When I read the data from the Serial port in C# and add in a String RxString i receive the following strings:

"??\u0002???? u0002???? u0002???? u0002???? u0002???? u0002????? u0002????..."

or

"???? ??? ??? ??? ???..".

The correct sequence would be integer numbers separated by space:

Example: "123 456 789 1023 0 12 02..."

Code where I receive the data from the Serial port:

string RxString;

private void Form1_Load(object sender, EventArgs e)
{
    SerialCOM.PortName = "COM6"; //comboBoxCOM.Text;
    SerialCOM.BaudRate = 9600; // Int32.Parse(comboBoxBR.Text);
    SerialCOM.Parity = Parity.None;
    SerialCOM.StopBits = StopBits.One;
    SerialCOM.DataBits = 8;
    SerialCOM.Open();        
    SerialCOM.DiscardOutBuffer();
    SerialCOM.DiscardInBuffer();
    timerDRAW.Enabled = true; //interval = 1ms
}

private void timerDRAW_Tick(object sender, EventArgs e)
{
    SerialCOM.DataReceived += new SerialDataReceivedEventHandler(SerialCOM_DataReceived);
}

private void SerialCOM_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
    SerialPort sData = (SerialPort)sender;
    {
        if(sData.BytesToRead >= 1) RxString += sData.ReadExisting();
        /...
    }
}

When I transmit the data of a Arduous for Serial port has some specific way of receiving the data?