patrik said"I think (as I stated in previous post) that you should consider the centronics I/f (or its successor the IEEE 1284). "

I attached many instrument to computer parallel port bidirectionaly, so I do know what centronics is! Centronics is synchronous or should I say very synchronous! It mans that two 8051s should stop their jobs and start talking to each other which I said before that's not what I’m looking for. I want an Async (or whatever you call it) communication so that 8051s do not interrupt each other for communication. I especially like to know your opinion about cross checking feature of protocol which nobody addressed in detail yet.
Thank you.

Regards,
Ebrahim Vakilpour