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Can any 1 explain me about "How the UART is connected b/w system bus and rs232 ?
This has nothing to do with Keil!
This sounds very much like a homework question - so your teacher expects that you should be able to find the explanation in your course notes, textbook(s), and further study in your school/college library.
If you have some specific question about a particular point, people will be happy to help - but you need to do the basic groundwork yourself
Don't blindly beleive everything you read on Wikipedia, but it should be a good starting point - and give you plenty of links to other stuff...