I think that in my last post i write the oposite that i want to mean:

.A master check if the line is free
.Then a mastes starts the dialogue
.At the same time the master check if it can read the same that it is writing (if not the bus or it's trasceiver is wrong)
.The slave answer

To prerent masters collision each master have a diferent delay to start the tranmision after the detection of "free bus"