Multimaster RS485 presents the problem of multiple masters trying to send at one time as we all know. If we look at ethernet which implements carrier sense and collision detection - we can detect transitions on the bus to see if someone is sending (remember two of more masters might do this at the same time and even having a bus busy signal doesn't avoid collisions) and we can receive the data we send to detect a collision. Physically, a collision isn't much of an issue - the RS485 bus drivers should have current limiting - in fact I've seen schemes where the current is sensed to detect a collision. Then the issue is what to do in the event of a collision - ethernet implements a binary backoff algorithm so the colliding nodes back off for a random amount of time the idea being that when they retry, that they don't retry at the same time. To implement this adds a lot of complexity.

The other method is use token passing where a token packet is passed from master to master. Upon receiving the token, the master either passes the token on or sends its data.

Failing that, use ethernet instead of RS485 - its getting cheaper !