The RS-485 Standard
RS-485 is the most versatile of the EIA standards and is an expansion of the RS-422 standard. It allows the same distance and data speed but increases the number of transmitters and receivers permitted on a line.
RS-485 permits multi-drop network communications on two wires and allows up to 32 line drivers and line receivers on the same line. An additional ground reference line is often included with the two RS-485 wires.
Each transmitter has the feature of tri-state operation with three states:
1. Logic 0
2. Logic 1
3. High impedance (or disconnected) state
A typical schematic diagram for RS-485 is shown in Figure 4.
Figure 4. RS-485 Connection