Many Modbus Masters and Slaves or intermediate nodes declare they support multi-master setup, without indicating how they do this. Does it mean there is a certain standard way how this arrangement works? How do they co-ordinate and not conflict together/cause errors?
As such, there is no standard guideline to follow multiple master/slave setup. But, with the given infrastructure one make the Modbus Master or Slave support multi-master.
In the case of a serial network, a Modbus Master can have two RS485 physical interfaces to connect to two different Serial networks. Thereby the Modbus Master can have multi-master setup. Same applies to Modbus slave it can be controlled multiple masters if it has more than one RS485 serial connection.
In the case of an Ethernet network, we can have x number of masters connect to y number of slaves in a network. This is the advantage of having Modbus TCP.
Hope it answers your question.