The RS485 serial bus interface standard transmits differential balanced signals. This has strong anti-interference ability in the common mode, allows a twisted pair transmitter driver on a number of connected devices. The communication distance is up to 1200 meters, the rate is up to 20Mbps, and it can be used in high noise environments, such as industrial automation.
In an RS485 network at any time only one device is in the sending state, and all other equipment should be in the receiving state. Each Arduino controls its RE / DE pin control to specify the device which is sending data to the RS485 network, or receiving data from the RS485 network.
See the example RS485 Network Diagram (scroll down).
NOTE: The network has 120 ohm termination resistors at each END. If you use more modules along the network you need to remove the 120 ohm resistors (R7) on those intermediate modules. The 20K resistors hold a known signal when no one is transmitting.