There are problems with this scheme. One is that it is possible for two bus initiators to assert the busy wire (set it to 1) at the exact same moment, or so close to the same time that it is simultaneous for all purposes. The two bus initiators need to be able to figure out that something strange has happened and arrange to stop and retry again later.

This brings up the fact that there must be rules of behavior in order for all the bus devices to get along without clobbering each other's data. These rules are called a bus protocol. Every bus architecture has a published protocol attendant to it and any device manufacturer must conform to the protocol or else the device cannot be attached to the bus without problems.