EL
BG
DE
ES
FR
ITGlossary · Communication and monitoring
RS-485
Differential serial interface over a wire pair that links many devices on one bus over long distances, usually with the Modbus RTU protocol.
Technical level:Advanced
Unlike RS-232, RS-485 sends each signal as a voltage difference between two conductors, A and B. This makes it resistant to electrical noise, lets it reach about 1,200 metres and allows many devices on the same cable.
On UPS units, solar inverters and EV chargers, the RS-485 port is used to connect to a building automation system, SCADA or a data logger via Modbus RTU. All devices need the same speed (baud rate) and settings, but different addresses.
Use shielded twisted pair, wire the devices in a chain rather than a star, and fit 120 Ω termination resistors at both ends of the bus. If there is no communication, try swapping A and B; manufacturers do not always label them the same way.
Related terms
- ModbusA communication protocol used for connecting industrial electronic devices, including UPS systems.
- RS-232Point-to-point serial communication standard, usually on a DB9 connector, for linking a UPS to one computer over a distance of a few metres.
- NUT (Network UPS Tools)Open-source software that reads the UPS status and shares it over the network, so that several servers and NAS devices can shut down in a coordinated way.
- SNMPA standard network protocol used to monitor and control devices like UPS systems over a network.
- Alarm ContactA relay output on a UPS that changes state on an alarm, such as battery mode, low battery or a fault, to alert an external system.
- Line Interactive UPSA UPS that corrects voltage sags and swells with an automatic voltage regulator and switches to battery only when the voltage is lost or out of range.