See the FX-700P system bus description.
See the HD61914 chip description. The PB-700 uses 4 of them.
|1||CE1||chip enable of the printer interface, active low|
|2||GND||ground (positive supply voltage rail)|
|3||CE2||chip enable of the cassette interface, active low|
|5||INIT||output for the printer control signal INIT|
|7||VDD3||-5V when operational, 0V when stand-by|
|9||BUSY||input for the printer status signal BUSY|
|11||FAULT||input for the printer status signal FAULT|
|15||GND||ground (positive supply voltage rail)|
The cassette subsystem in the FA-4 interface is almost identical with the FA-3 one, so the tape interface protocol used by the FX-700P applies to the PB-700 as well.
However, there's an interesting quirk in the PB-700 protocol. The calculator writes to the tape more data than reads back. For example the last 3 bytes of the file header segment remain unread. To skip these data the calculator inserts a close/open sequence between the segments which causes loss of a few bytes read from the tape. This effectively places the tape head to the lead-in of the next file segment. Seems like a kludge to me...
The printer communication protocol can be easily figured out from the circuit diagram of the FA-4 parallel port interface.