@requires_firmware(<NUM_LIT>)<EOL><INDENT>def sn(self):<DEDENT>
string = []<EOL>self.cnxn.xfer([<NUM_LIT>])<EOL>sleep(<NUM_LIT>)<EOL>for i in range(<NUM_LIT>):<EOL><INDENT>resp = self.cnxn.xfer([<NUM_LIT>])[<NUM_LIT:0>]<EOL>string.append(chr(resp))<EOL><DEDENT>sleep(<NUM_LIT:0.1>)<EOL>return '<STR_LIT>'.join(string)<EOL>
Read the Serial Number string. This method is only available on OPC-N2
        firmware versions 18+.

        :rtype: string

        :Example:

        >>> alpha.sn()
        'OPC-N2 123456789'