At the beginning of Communicating and Mobile Systems: the Pi-Calculus by Robin Milner, there is a introduction on automata and how they can simulate each other so that they cannot be distinguished : Bisimulation.
cf Bisimulation on wikipedia.
I don't remember well, I should re-read the chapter, but there was a trouble with simulation and bisimulation that made them not sufficient for computational equivalences.
Thus Robin Milner introduces his Pi-Calculus and expose it for the rest of the book.
Ultimately, in his last book The Space and Motion of Communicating Agents, you could have a look at Robin Milner's Bigraphs. They can model Automata, Petri nets, Pi-Calculus and other computational methodologies.