As I understand, according to this FAA booklet on TCAS, transponders will engage in cycles of interrogations and responses to establish if a collision may occur.
The next step is for both TCAS systems to negotiate respective resolution advisories. My question is how the negotiation takes place in the TCAS system internally, and in particular what communication protocol takes place for the negotiation.
To elaborate, as an example, a browser can make an HTTP request to a server (like this site) which follows protocols as to what information should be provided and how it should be encoded.
Along the same vein, how is information encoded for TCAS systems to negotiate? Since I don't know of any open source TCAS used in commercial aviation, I can't inspect code.