"2) TX of slaves is connected together, and you are wondering why the slaves don't get damaged? The reason (i am not sure, but an idea only) is that when you use serial port, you don't declare TX as output. Do you?????"

Yes, you do; however in the '51 port scheme you can pull a high output low (that is how the ports work as input).

"So the hardware, declares it o/p when it has to transmit some characters, and since at one time, only one slave transmits so the TX of other 3 slaves is not O/P pin at that time."
Horsefeathers, see above.

"I may be wrong at the answers, but this communication scheme does work."

This is a typical "if it works, it must be OK" a very dangerous way to do things. Some day this scheme is going to be implemented with a slightly different interconnection distance/connector and everybody is going to wonder why it does not work (not to mention the time hunting in software for a hardware bug).

I would not have any qualms using this scheme if both processors were on the same board, going off the board (even when staying in the same cage) would be asking for 'unexplainable' problems.

Erik