RS-485: The Essential Industrial Bus
The RS-485 bus (EIA-485 standard) is the most widely used serial interface in industrial installations: PLCs, variable-speed drives, energy meters, gas analyzers, temperature sensors, controllers, and inverters. Its robustness (long transmission distances, immunity to electromagnetic interference) ensures that it remains a relevant standard even 40 years after its introduction.
Problem: RS-485 is a local bus designed for industrial use, not for the Internet. Connecting RS-485 devices to the cloud requires a conversion gateway.
RS-485-to-Cloud Architecture
RS-485 Technical Specifications
| Parameter | Standard RS-485 Value |
|---|---|
| Maximum number of devices | 32 (standard) / 127 (high impedance) |
| Maximum distance | 1,200 m (at 100 kbps) |
| Data rate | 300 bps to 10 Mbps (typical: 9,600 to 115,200 bps) |
| Mode | Half-duplex (one direction at a time) |
| Recommended cable | Shielded twisted pair (AWG 24 or 26) |
| Termination | 120 Ω resistor at the end of the bus |
| Differential level | +/-200 mV to +/-6 V |
Configuring Modbus RTU Polling on RS-485
The gateway configures:
- Baud rate: Must be the same for all devices on the bus (9,600, 19,200, 115,200 bps).
- Parity: None, Even, or Odd—must be the same across the entire bus.
- Modbus addresses: Each device has a unique address (1 to 247).
- Registers to read: Selection of registers per device (hold registers, inputs, coils).
- Polling frequency: from once per second to once per hour, depending on the criticality of the data.
The Eziwan gateway allows you to configure multiple groups of registers with different update frequencies: alarms every 5 seconds, process measurements every minute, and energy meters every 15 minutes.
Common RS-485 Devices and Their Modbus Registers
| Equipment | Manufacturer | Typical Data Points |
|---|---|---|
| Energy Meter | Carlo Gavazzi EM340 | Active power, energy, voltage, current |
| Power Analyzer | Schneider PM800 | 250+ measurement data points |
| Variable-speed drive | Schneider ATV312 | Motor frequency, current, faults |
| Temperature controller | Eurotherm 3504 | Measured value, setpoint, output |
| CO₂ Sensor | Senseair S8 | CO₂ (ppm), Temperature, Humidity |
| Electromagnetic Flowmeter | Endress+Hauser | Instantaneous Flow, Cumulative Flow |
The Eziwan Approach
The Eziwan Gateway features a native RS-485 port (DIN-rail terminal blocks) with Modbus RTU master firmware. It polls up to 32 devices on the bus, converts the registers to JSON, and uploads the data to the Eziwan Cloud via MQTT and a 4G VPN tunnel.
Learn more: Modbus cloud, Modbus RTU/TCP gateway, and industrial IoT gateway.