sevral questions: is there electrical power to be transferred or is it
only signals? uni- or bidirctional?
if its unidirectional signals and the frequency is not tto high (<
100kHz), you should consider optocouplers, they come in lots of
varieties and packages, try googling for IL74 or 4N26, there are duals
and quads too (ILD74 and ILQ 74 respectively) the datasheets you find
show how they should be connected.
greetings
Frank