If you enter 0xFFFF it should send ASCII. To send binary data, use \xff\xff. You have to split it up by byte. In this case the order doesn't matter (since both bytes are the same) but if you try sending any other data, make sure the order matches the endian order on your instrument. There's an explanation of big-vs-little-endian
here. There's usually a setting on the instrument (accessible via a SCPI command) to set the byte order, so if it doesn't work normally then it would be easier to change the instrument's setting than to switch all your bytes before you send them.
Hope that helps.
