Graham,
It is the only method of comms but it is driven by software routines.
I.E. The SCI is only a byte sending and receiving hardware device.
Routines within the chip, monitor the input, recognise data strings and transmits the reply data (fixed and live readings) using a combination of normal and interupt routines.
If you like, I will post up the basic structure code here with some parts removed to protect agasint chip copying . ???