in case any controller fails the master should know immediately how can i do this

I need first to deviate from the question to get to the answer.

The "time slice" method, while possible, is not the 'normal' way.

Normally you would do the following
in a round robin fashion the master ask the slaves 'do you have data', and the addressed slave MUST respond either 'no', or with data. If neither response is received within the assigned time, the master goes into a "is this node dead" routine. (there might have been just a glitch)

Since the 'no' response is a short record, the polling cycle can be much faster than your timeslice method.

On another note: make sure your 485 is properly terminated.

Read 10 ways to bulletproof 485 found at http://www.national.com/apnotes/A...rface.html

Erik