Hi Simon,

Some clearence: RS485 is just physical level of OSI (I hope you know what it is). You can conect a driver chip of RS485 directly to 8052 UART, and use the same serial dtata format as in RS232: I meen 1 start bit, 8 data bits, 1 stop bit. As real protocol, ModBus is VERY sutable. it can adress up to 256 nodes, has CRC check and so on. More about ModBus you can find there: www.modbus.org.