I bet those 2 dots are the termination characters. Probably CR/LF.
Try this... In Basic Serial vi right click on "String to write" and make sure "\" codes display is on. Then try typing your command followed by \n (a line feed character), or \r (a carriage return character), or \r\n (carriage return / line feed).
I think the last one will work, because Hyperterminal sends CR/LF.
