C#

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    if (serialPort1.IsOpen) serialPort1.Close();
}

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{

    if(!serialPort1.IsOpen) return;

    char[] buff = new char[1];

    buff[0] = e.KeyChar;

    serialPort1.Write(buff, 0, 1);

    e.Handled = true;
}

private void DisplayText(object sender, EventArgs e)
{
    textBox1.AppendText(RxString);

}

private void serialPort1_DataReceived
  (object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
{
    RxString = serialPort1.ReadExisting();
    this.Invoke(new EventHandler(DisplayText));
}

SerialPort port=new SerialPort();
// take care of opening and port settings
...
// do whatever is needed to have the peripheral send something
...
// wait for enough data to arrive
Thread.Sleep(5000);
// read all of it now
int DIM=300;
byte[] bytes=new byte[DIM];
int n=port.Read(bytes,0,DIM);
for(int i=0; i<n; i++) {
    byte b=bytes[i];
    char c=b;
    log("bytes["+i+"]=0x"+b.ToString("X2")+"="+c);
}

public static void log(string s) {
    Console.WriteLine(s);
}