hi,

Well, you may use RS485 as Physical layer and UART as Data Link layer.

It means, that you should connect microcontrollers with RS485 line drives IC - for example, http://para.maxim-ic.com/compare.a...ce.cfm&ln=
and use UART for transfer/receive data. As you have only one master on-line and some slaves so they all should be configured to use UART mode 3. It allows you to use multiprocessor communication.
Then at each time slot, master sends data packet to a slave including slave number and data (read more about multiprocessor communication and automatic address recognition).

As about faulure detector then some ways may be used. It may be additional line connected, for example, to /INTx pin of master. As well, you may use "destroy stream" technique. This way you need not with additional line. Any slave indicates its failure by putting TX line low for some time (longer than two or three bytes transfer period). Master detects such condition as start bit but then it should check for pulse duration time and if it is longer than defined limit then it means a slave failure. By the way, modern 8051 derivatives have "frame error" detector which may be used as failure detector (because slave destroys stop bit).

Regards,
Oleg