i have a experience of the above situation. the project was a building fire alarm system which has a master control unit at the ground and 6 to 7 floor control unit.
the communication was bidirectional. they all connected via just 2 wires.
i used rs485 and at89c52. the communication is mode 3 serial communication(interrupt driven.)
it is really easy and very flexible. all communication with just 2 wires.
May also consider using external RS232<->Rs422 converter. It depends on how many units you are building. If it is going to be just a few units, it might not worth to take RS485 in chip level. Instead, using a simple MAX232 for full-duplex operation and use an external RS232<->RS422 converter could retain full duplex operation and make software more simple.