The RS-485, also known as EIA-485, is a standard for serial communication that was designed to overcome the key limitations of the older RS-232C standard. Its main advantages are its support for longer distances, higher speeds, and multi-drop networking. The most significant technical difference is that RS-485 uses differential signaling. Instead of a single signal wire referenced to ground, it uses a pair of wires (twisted pair). The data is represented by the voltage difference between these two wires.
This balanced transmission method makes the communication highly resistant to common-mode noise, which is why RS-485 is extremely robust and reliable in electrically noisy industrial environments. This allows for communication over long distances--up to 1.2 kilometers (4000 feet)--and at higher speeds than RS-232C. Another major advantage is its support for "multi-drop" connections. Unlike the one-to-one limitation of RS-232C, a single RS-485 bus can support multiple devices (typically up to 32 standard devices, or more with modern transceivers). This allows a single master controller (like a PC or PLC) to communicate with many slave devices (like sensors, drives, or programmable power supplies) over a single two-wire cable, significantly simplifying wiring. Because of its robustness and multi-drop capability, the RS-485 physical layer is used as the basis for many popular industrial communication protocols, including Modbus RTU and PROFIBUS.