RX LO is just the local oscillator frequency fed into the mixer on a superhet receiver. When you change the TX frequency, it also has to change the receive frequency.

Check http://en.wikipedia.org/wiki/Superheterodyne_receiver and scroll down to the Overview section for a brief explanation, or lookup superhet receiver design online for the nitty-gritty details.

Also - the HP3 series serial communications will not work with DEBUG or any other asynchronous (means no clock) since it is synchronous (means it requires clock & data) serial.

Check out http://linxtechnologies.com/Documents/AN-00155.pdf for a code example of serial communications with an HP3 series.