who to make the serial port Connection?

Are you using the SerialPort class for the connection.The error may be due to the invalid port name.The following code works fine here.Try this,
To Create the connection,

SerialPort port = new SerialPort("COM1");

To Opens the port

if (false == port.IsOpen)<br />
      {<br />
        port.Open();<br />
      }

To Write the data
port.Write("test");

Regards,
Vineeth