serial input buffer

I have looked at the assembler serial input buffer sample provided by melabs. My question is that I cannot figure out to handle MODBUS RTU input. The message frame here is such and the end of the message is signalled by at least 3.5mS of silence. Not having a terminating character is a pain but maybe someone in the forum has ideas how to buffer incoming MODBUS messages.

I am using a 876 with a MAX233. I can communicate fine with a MODBUS master but the moment my program does other things I can get data overruns. Has anybody used external UARTs ? I heard that Maxim makes some but they are SMT. I would have the end of message detection problem.

- Vijay