Hi,
You have asked for the simplest solution and for 8051 family, the 10 bit data mode of the serial port is the simplest solution, which is not supported in 89C2051. You can always implement Master slave configuration in software by sending down the slave address to a group of controllers connected over an RS485 network. In this case, the command respose protocol has to done in software. The slave address could be programmed using DIP switches or stored in an EEPROM on each slave. Refer to MODBUS protocols for a standard followed for such applications.
Regards, Laktronics