so the buffer you posted a link to there is a transceiver, meaning it will pass current in either direction.
it will solve your over voltage problem but not the problem of you sending an output pin a signal.
most of us on the forum here are used to dealing with microcontrollers with fairly robust i/o pins but i believe the correct way to connect digital i/o pins is to put a resistor between them.
for example in your diagram, there should be a resistor between ping 16 of your buffer and the DSP.
to find the value of the resistor, look up the DSP datasheet and see if you can find it's I/O pins maximum output current then use I=V/R (from your high school physics class) to find the value of the resistor for that voltage.
this calculation will be presuming that the buffer pin is outputting 0Volts and the DSP pin is at logic 1.
the resistor will dissipate the power between the two pins but should still allow you to make readings when one of the pins is set as an input.
let me know if i'm not making sense.