I don't think these Black Box modules have anything fancy - they've been available since the 70's methinks. At a guess, I'd say they're some kind of current loop device and probably have a load of discrete devices in them. Half the components will probably be used for protection. This is pure speculation as I have not touched or used these devices but as the others have mentioned, for a small amount of money, you can find out what is involved and maybe save yourself a lot of time. Then you can tell us how they work.