Yeh, I'll be the 'poller', the other end likely has the resistors and such already in place.
To be the client, it sounds like all I need to do is add a transformer, I'm not yet sure if it's AC or DC, and what potential.
To find that out, I suppose I need to know the resistance on the other end...
Then, to talk to it, the opto isolator should be all that is needed?