This article introduces the hardware interface RS-485.

Overview

TIA-485-A, also known as ANSI/TIA/EIA-485, TIA/EIA-485, EIA-485 or RS-485, is a standard defining the electrical characteristics of drivers and receivers for use in balanced digital multipoint systems. The standard is published by the Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA). Digital communications networks implementing the EIA-485 standard can be used effectively over long distances and in electrically noisy environments. Multiple receivers may be connected to such a network in a linear, multi-drop configuration. These characteristics make such networks useful in industrial environments and similar applications.

The EIA once labeled all its standards with the prefix RS (Recommended Standard), but the EIA-TIA officially replaced RS with EIA/TIA to help identify the origin of its standards. The EIA has officially disbanded and the standard is now maintained by the TIA. The RS-485 standard is superseded by TIA-485, but often engineers and applications guides continue to use the RS designation.

Specifications

RS-485 only specifies electrical characteristics of the generator and the receiver. It does not specify or recommend any communications protocol, only the physical layer. Other standards define the protocols for communication over an RS-485 link.

Refer to RS-485 for details about RS-485 specification.

Data Transfer Rate

RS-485 enables the configuration of inexpensive local networks and multidrop communications links. It offers data transmission speeds of 35 Mbit/s up to 10 m and 100 kbit/s at 1200 m. Since it uses a differential balanced line over twisted pair (like RS-422), it can span relatively large distances up to 1200 m. A rule of thumb is that the speed in bit/s multiplied by the length in meters should not exceed \(10^8\). Thus a 50 meter cable should not signal faster than 2 Mbit/s.

RS-485 Connectors

Connector between RS-232 and RS-485

References

• RS-422 and RS-485 Standards Overview and System Configurations
• RS-485/RS-422 Circuit Implementation Guide
• Understanding EIA-485 Networks

• Linux RS-485 support

• RS232与RS485的功能与区别