As Klaus states, there is no rule about how to do this. The method I use is to assign one master device which polls each slave in turn asking if it want to send data. The slaves have an address which is transmitted by the master, when they recognize their own address, they momentarily enable their own driver to say if they have anything to send then disconnect again. Something like this:
Master sends "slave 3 do you have anything to send?"
Slave 3 sends "I do not have anything at the moment"
Master sends "slave 4 do you have anything to send?"
Slave 2 sends " I have some data for you, here it is..."
Master sends "slave 5 do you have anything to send?" - or it might be further communication with slave 4.
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In reality, it doesn't send text of course, in my case just a single byte address followed by any data as needed.
Brian.