Re (mis)information in previous postings in this tread.
OK, 485 basics.
RS-485 is a 2 wire differential communication, a "variant" RS-422 is 4 wire (2 differential full duplex), but RS-485 IS TWO WIRE DIFFERENTIAL.
RS-485 is defined in interface loads, the standatd chips such as 75176 exhibit one load, but half and quarter load chips are available. Up to 32 interface loads ate permitted, so using quarter load chips you can have 128 nodes. The UART for 485 is the same as for 232, the transciever is different, DO NOT EVEN DISCUSS RS-232 WHEN USING 485. The only place where a conversion fron RS-232 to RS-485 can come into place is a the rear of your stupid PC which has not discovered that 485 is so much better than 232 - such a converter - a good one - is available from BB (or is it B&B) electronics. The converted PC output will NOT have any protocol associated with it, it will send when "it feels like it" and thus for 485 if a PC connection is required all hell can break lose. When I have had to use a PC with a multi uC network on RS-485 I have used one of 2 solutions. 1) build an ISA interface card, run the PC under DOS (not the windows that think that Mr. Gates know better than me what I want to do at a given time) or 2) have one uC that only speak 232 to the PC (sometimes with a 9600 soft UART, sometimes a 2 UART derivative) this one then talks on the 485 network using the protocol. For 485 the "norm" is one master and umpteen slaves. The master ask in a round robin fashion the slaves if they want to speak and if yes allow them to do so before the next slave is asked. There is no "standard protocol for this, but the '51 9-bit serial mode clearly is intended for this use. If you are interested in 485 there is a great source of appnotes at National Semiconductor. If you do not understand (see NS appnotes) and apply no stubs and use proper termination do not even attempt to use RS-485.

The RS-485 is grrreeeat when properly applied, I use it in all multi uC designs I make, and absolutely horrendous when not (232 converters will make it improper).

Read the appnotes, work your protocol through before coding it and have fun with RS-485.

Erik