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HEy we have designed our own 8051 board using 4066 to convert single serial port to 4 port but gives lots of garbage once we power up
As I have already noted, every logic level input on the MAX232 chips must have a known state.
Your 4066 will sometimes connect the corresponding processor pin output to the MAX232 making the logic level known. But the corresponding inputs on the other MAX232 chips will then be left floating when the 4066 gates changes to high-impedance.
So add resistors on every single logic-level input of every single MAX232. Note that I am talking about logic-level input, so not the side to the outside world. And not the data direction where the MAX232 sends data to the microprocessor, but the signals where data comes (or may come) from processor and enter the MAX232.
By the way - did you intend to run your design with separate multiplexing for receive and transmit? If not, then you can manage with half the number of port pins for controlling the multiplexing.