Hi Chan.
There is a several of ways to handle master/slave communication.
The most common is polling system there you from master
send and receive an request to each slave-unit, a sort of
hand-shaking.
The other way is what they called slottky alloha, (never mind how it
spells).
In that case you are sending a general request from master,
and the slaves answer after each other in real-time, and
you win time comparing to the polling mode.
In this mode you have to know the time it takes to answer,
and implement that in software.

Usally, this two types of master/slave-mode are common in
radiocommunication systems.

Magnus Munthe