input/output on port ( TXD & RXD )

i am new to this Rabbit Product. I am developing a programe. for that i want to send digital signals to perticular port(TXD). same way i want to get digital signal from a perticular port(RXD) as input. can i pass some voltage current ( 1.5 v ) to that port & check it through the Dynamic C Code ? If so please can u send me the sorce code for the same procedure ?