After further research, I learned more about the CAN bus (controller area network). It only requires three wires (high, low, ground) and terminating resistors (typically 110 ohms for CAT5 cable) and a CAN transceiver. Unlike RS485, the CAN bus specifications specify the physical layer as well as the protocol layer. The bus allows for multi master communication over long distances (1000 m) and sufficient data rates (1 Mbit/s). The details such as collision avoidance/detection error checking and so on are all taken care of at the chip level.
Maxim IC has some CAN transceivers and I just sampled them. I’m planning to use them over CAT3 or CAT5 cable (twisted pair). Let’s see how this goes.
More on this once I get the system up and running.