The run is about 15' end to end. I have a feeling that I know what the problem is now. While the exact codes do not work when coming from the XP-8, I think I MAY have forgotten to add a carriage return at the end of the string.

Even with that, I now think there may be a problem with the Jandy Serial Adaptor. My logic behind this is that I when using hyperterminal I am getting some very odd query responses that don't make any sense in relation to the command being sent to it. If this shit was easy we would all be out of a job!