Not sure of the other methods used, but I know the HP48 series is half duplex
serial com. It is software timed, the acutal IR led, and IR phototransistor
are memory mapped. There is no modulation or demodulation to worry about but
range is limited to about 12". Just use a phototransistor to read and a IR led
to write to the Hp48 using normal (8,N,1) serial com. (2400baud max). Matt