I won't comment on what you're trying to do so much as using HyperTerminal to do it.
I do a LOT of work that involves talking to serial devices, both from Windows (DOS mode and Win32 apps) and from Linux (both C and Perl).
Regardless of how you feel about Windows, HyperTerminal is the biggest piece of crap to have ever been included on a CD. It is a complete and total waste of disk space, CPU time, and your time. HyperDooDoo (as we're fond of calling it), has been responsible for more wasted time with clients than ANY OTHER SINGLE ITEM. We've had several clients that rather than trying to teach them to use HyperDooDoo, we Fedex them a copy of Procomm. And trying to teach someone to use HyperDooDoo over the phone can be next to impossible.
HyperDooDoo hangs when hardware flow control is missing, it has bugs in the file transfer protocols, in the terminal emulation, and in a word, sucks. It is inflexible and a general pain in the ass to use.
I absolutely cannot believe that Microsoft includes this garbage on the CD. Or (despite how you may feel about the inherent quality of Windows) that it ever got past Quality Assurance at Microsoft. I've seen better apps written by 14 year olds as their first program.
So unless you have the rest of your life to waste trying to debug their code for them, don't use it. Procomm, Kermit, or any of the other several dozen public domain programs will serve you far better. And when you're debugging, at least you can be 99% assured that the code that's flawed will be yours.