You can easily achieve this condition if you apply the MODBUS protocol. The time between messages (master inquiry - slave answer) is normally 3,5 bytes of silence. In case of 9600 Bd it means about 3,5 ms. You can then set up the time out condition to 5 - 10 ms. If the answer didn't come in that period - something is wrong with the slave.

Regards, Slobodan