Check out my digital circuit simulator at kretsim.se .
This episode explains the expansion board interface, the memory and interrupt models and the way the CPU and the boards communicate.
This is a part of the video series for the Alt6502 project. This project takes the Altair 8800 design, changes some historic circumstances and design decisions, and tries to construct the ideal 70's hobbyist 8-bit machine, based on the 6502.