Hmmm… Just taking a guess at this. Look at the help patch for the serial object and you’ll see that the ints sent to serial are AT and return: "65 84 13". The only thing different that what you are doing is that you aren’t sending the return.
I’m currently trying to do exactly the same thing,
I have a keyspan adapter and I want to switch on/off my projector (epson emp 8300).
Could you tell me a little bit more about the process you use?
I have tried without results.