I'm doing a grad-school software engineering project and I'm looking for the protocol that governs communications between ATMs and bank networks.

I've been googling for quite a while now, and though I'm finding all sorts of interesting information about ATMs, I'm surprised to find that there seems to be no industry standard for high-level communications.

I'm not talking about 3DES or low-level transmission protocols, but something along the lines of an Interface Control Document; something that governs the sequence of events for various transactions: verify credentials, withdrawal, check balance, etc.

Any ideas? Does anything like this even exist?

I can't believe that after all this time the banks and ATM manufacturers are still just making this up as they go.

A shorter question: if I wanted to go into the ATM software manufacturing business, where would I start looking for standards?