Thanks ALL

I have taken care of all the design parameters like LED driving current and fail-safe termination of RS485 network. As Michael sugessted its seems that noise is creating the prob in actual environment(railway station). As i told you that the boards are already under prod. and few have been installed in one of the site where i am facing this prob. I have tried all the possible combination, 75176B seems to be more stable in this application. I also think that noise is creating the problem - altough i have taken care of all the points mentioned in " ten ways to bullet proof RS485". I will have to address the noise issue now

Thanks for all your help.

Santanu