I had a look at the programming manual and it says you must append a CR to each write. You are not doing that. Without the CR, the write command is not getting processed correctly and you will get a timeout on the read because there is nothing there. It is a possibility that as soon as you open Hyperterminal, the commands that are queued up in the instrument's receive buffer are getting flushed with a CR and then the instrument will process them.
