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.