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Can you be a bit more clear? Is the idea to have a single 8051 with its UART connected to several other RS232 devices at once? Is the difficulty that each of your slave devices has a different protocol?
I think you need to post a lot more info, such as - how many serial conections do you need?, what are the speeds and protocols?, are they just slave devices with all communication intiated by the processor?.
As a minimum I would imagine you will need a MAX232 for each serial connection, and two pins on the processor, and write multiple software USART's - or at least one, which you can set to use different pins.
If you use RS485 which supports multidrop configurations, this seems relatively straightforward. I've built a similar device in the past that interfaced with amongst other things a toledo weigh scale. I believe there were 6 slaves on the bus. The processor would interpret macro commands coming to it from the host controller. Then it would convert those into a series of commands to read/write the proper sequence from the devices. RS485 is very similar to RS232 so the conversion is straightforward.