sWORDs:

Your SerialCode will return a String containing '\', 'x', '0', '2', ...

What you want is something that returns a String containing '\x02', '\x40', ...

A possible way to do this is like:

function SerialCode(command, parameter0, parameter1, parameter2, parameter3)
{
        var stx = 0x02;
        var etx = 0x03;
        var calc = command + parameter0 + parameter1 + parameter2 + 
                         parameter3 + etx;
        return String.fromCharCode( stx, command, parameter0, parameter1, 
                         parameter2, parameter3, etx, (calc >> 8)&0xff, calc & 0xff);
}

and use it like:

mycode = SerialCode(0x40, 0x00, 0x00, 0x00, 0x00);
rs232port2.send(mycode);

Note that you cannot set such a String containing arbitrary binary values as a label (you can, but the result will be garbage - the binary content will be interpreted as ASCII codes)

You could use something like:

function BinaryDump(s)
{
        var result = "";
        for (var i=0; i<s.length; i++) {
                result += "\x" + s.charCodeAt(i).toString(16);
        }
        return result;
}

mycode = SerialCode(0x40, 0, 0, 0, 0);
panel2.label = BinaryDump(mycode);
rs232port2.send(mycode);

Enjoy.