Sorry for the en-masse violation of writing ethics! I was trying to give a smaller portion from the main code and so it became messy. All suggestions accepted and implemented. Here is what I figured out but which hasnt helped me solve it:
Looking at the debug code, the problem occurs when the called program (NuSMV) is taking longer time ~0.3 seconds to give the result. Somehow spurious or extra input (i.e. NuSMV prompts, 'NuSMV >') are coming back to expect in the meanwhile, and accepting that, it moves on. I cant figure out why this is happening. If it takes less time ~0.0 secs (NuSMV gives a timer), there is no problem.

Regd Open: I tried some stuff but not clear on the std I/O operations. here is the problem. A accepts input from user, passes that input to B (A' stdout -> B's stdin?), B processes, gives the result back (B's stdout -> A's stdin?), A displays the result back (A' stdout -> monitor). (Who's the reader, who is the writer? )
I suppose this should be rather straightforward for pipe gurus ;-) - can you suggest code on this.

thanks, AT