Card reader program is now working using PIC24 pins RA0 for DATA, RA1 for CARD_PRESENT, and RP13/RB13 configured as INT1 for STROBE (I discovered that it's the proper name for that sort of variable, self-clocking serial clock).  These pin changes were made so that we can connect the magnetic card reader directly to the master PIC24 and not have to go through another microcontroller.