Hi Maurizio
you have to use a Casio FA-3 Tape interface:
[
pocket.free.fr]
Here you'll find the circuit diagram of the a Casio FA-3 Tape interface
[
pisi.com.pl]
And here some further informations about the FX-730P
[
pisi.com.pl]
Ciao,
Manfred
[
manib.funpic.de]