STAY AWAY FROM the 8085 peripheral chips, they give you more headaches than a bottle of aspirin can cure.

Do not overlook the fact that with RS-485, you will NOT need multiple UARTs, just a master controller, search on RS-485 in the top left corner.

Philips has some wonderfuld, albeit somewhat costly, UART chips:
SC28L91 for 1
SC28L192 for 2
SC28L194 for 4
SC28L198 for 8

They all have FIFOs to allow 16 bytes to be received/transmitted before uC intervention is required. Using these chips makes life very easy.

One other possibility is to abandon the '51. The philips XA-SCC has 4 UARTs, all with DMA and a ton of other goodies.

If speed is not a concern, consider a soft UART, philips has an application note of one such that works.

If speed is a moderate concern, find a I2C driven UART, you will be able to connect several to one uC.

Have a look at www.maxim-ic.com, I tend to recall they have some external UARTS.

If the multiple uCs are on the same board, you will be much better off using dual port memories, if not use RS-485. For RS-485 interfacing you MUST (at least the first 25 times you do 4850) go to National Semiconductors and read the appnotes regarding termination, loading and such.

Have fun,

Erik