You will not ever get technical info about the comms links on ATMs from anyone.
They use private highly secure high speed links to euthenticate each transaction and the people who manufacture them have highly restrictive agreements with the banks which mean that they would basically go out of business if they gave away any details.
The comms links probably use a highly sposhiticated challenge response protocol to ensure that its just about impossible to 'hack into' the networks in an conceivable way