Andy,
Just like Richard says the key is to make sure that the slaves never ever starts to speak without being spoken to. If you have one master and multiple slaves you need to come up with a scheme where the master interrogates the slaves, one by one, and the slaves only speak when asked to speak.

Most of my experience with this type of thing comes from writing code for MODBUS. With MODBUS there's one master and one or more slaves (or servers as they're sometimes called). The master sends out messages on the bus containing a slave adress, a command and some data like "Slave 2, give me the value of your register number 5 and a checksum). All the slaves gets the message, verifies the chcksum and then determines if the message was for them or not. Only the slave with the adress matching that of the message is allowes to speak and must do so within a certain amount of time or a timeout error occurs. A slave can never start talking unless asked to by the master.

There are of course other alternatives, like each slave has a "slave select line". The master asserts the slave select line of one slave at the time and the slave sends whatever it needs to send.

If you need to have multimaster capabillity then perhaps CAN is an alternative.

/Henrik.