Home automation on Cat5e again: is this a reasonable multidrop protocol?
RS-485 is a poor choice of protocol with multiple masters. Unlike CAN, it does not have 'dominant' and 'recessive' states - 1s and 0s both involve driving the bus. Thus, a collision results in both devices driving the bus to opposite states, sourcing and sinking as much current as they can handle. The resulting voltage will depend on the drivers' relative strengths, and won't have a well defined value; indeed, each device may read a different value.

Have you considered using ethernet? You can define your own protocol on top of it, as well as carrying IPv4 or IPv6 if you wish.