Glossary · Automation fundamentals, platforms and components
RS-485
Also known as: EIA-485, TIA/EIA-485
German: RS-485
RS-485 (TIA/EIA-485) is an electrical interface standard for balanced, differential serial transmission over twisted-pair cable, supporting multiple drivers and receivers on one bus line. It defines only the physical layer; protocols such as Modbus RTU and PROFIBUS DP use it.
- Automation components
- Standards
In one sentence
RS-485 is a differential, multidrop serial physical layer on twisted pair, used by Modbus RTU, PROFIBUS DP and many device buses.
Example
Twelve energy meters are daisy-chained on one RS-485 line with termination resistors at both ends and polled by the PLC via Modbus RTU.
How it applies
- Engineering: Use a line Topology with short stubs, terminate both ends with the cable's characteristic impedance, and provide biasing. Cable length and data rate trade off against each other.
- Commissioning: Typical faults are swapped A/B lines, missing termination, star wiring and grounding problems. Communication settings (Parity, baud rate) must match on all nodes.
- Documentation: Wiring diagrams should show the bus order, termination points and the naming convention for the data lines, because vendors label A and B inconsistently.
RS-485 vs. protocol
RS-485 defines voltage levels and electrical behavior, not message formats. A statement like "the device supports RS-485" says nothing about the protocol; the manual must also name it, for example Modbus RTU, with its settings.