IoT Guide

RS485 Modbus 4G — Field Connection Without a Fixed Network

Connect your RS485 Modbus devices to the cloud via 4G LTE without Ethernet or a local network. 32 slaves per bus, multi-carrier SIM, secure VPN. Eziwan Guide.

Quick answer
A footbridge RS485 Modbus 4G connects directly to the RS485 bus on your equipment, reads Modbus registers without Ethernet or a local network, and transmits the data via 4G LTE with multi-carrier SIM cards connected to the cloud. It is the ideal solution for remote sites: pumping stations, substations, solar power plants, and field equipment.

RS485 and Modbus RTU: Ubiquity in Industrial Settings​

The RS485 bus with the Modbus RTU protocol has been the de facto standard for industrial instrumentation and automation since the 1980s. It is estimated to be present in 70 to 80 percent of industrial equipment worldwide: variable-frequency drives, energy meters, compact PLCs, process sensors, temperature controllers, and inverters.

Its robustness (immunity to electrical noise), simplicity, and low cost explain its longevity. Today’s challenge: connecting this equipment to the cloud without modifying the existing setup or deploying an Ethernet network where none exists.

The Solution: 4G RS485 Modbus Gateway​

The RS485 Modbus 4G gateway combines:

  • RS485 interface (terminals A/B) for direct connection to the existing bus
  • Built-in Modbus RTU master for querying slaves without additional equipment
  • 4G LTE modem with SIM slot for cloud connectivity without a local network
  • MQTT/VPN client for secure data transmission
  • Flash buffer to store measurements in the event of a network outage
Existing RS485 Bus Eziwan Cloud Gateway
──────────────────         ─────────────────           ──────────────
Variateur  ──┐             ┌── RS485 port              Dashboard
Compteur   ──┤─── RS485 ──►│   Modbus master     4G   Alertes
Sensor    ──┤ │   FC03 Polling ──LTE►REST API
Controller   ──┘ │   MQTT TLS Reporting
(2 fils A/B)               └── SIM multi-op

RS485 Wiring: The Basic Rules​

Proper RS485 wiring is essential for a reliable bus. Wiring errors are the leading cause of communication problems in industrial settings.

Essential Rules:

  1. Linear bus topology — RS485 cable running from one device to the next, with no star-shaped branches. Branches cause signal reflections.
  2. Termination resistors — 120 Ω at both ends of the bus (in the first and last devices). Do not place a terminator in the middle of the bus.
  3. Shielded twisted-pair cable — Belden 9842, Lapp UNITRONIC BUS, or equivalent; impedance 120 Ω; cross-sectional area 0.25 to 0.5 mm².
  4. Common ground — Connect the shields and grounds of the devices to the same ground point to prevent potential differences.
  5. Cable Separation — Keep the RS485 cable at least 30 cm away from power cables (drives, motors) or use a dedicated cable tray.

Modbus RTU Configuration on the Gateway​

The gateway is configured through the Eziwan cloud interface, without the need for physical access:

ParameterTypical ValueDescription
Baud rate9,600 / 19,200 bpsMust match all slaves
ParityNone 8N1Most common in industry
Response timeout500 msDelay before retransmission
Retry count3Number of attempts before marking as an error
Polling interval10 sFrequency of register reads
Slave addresses1 to 247Configured per slave

Use Cases: Remote Locations Without a Landline Network​

The 4G RS485 Modbus connection is particularly well-suited for sites without network infrastructure:

Pumping Stations and Water Towers

  • Pressure, flow, and level monitoring
  • Leak and pump failure detection
  • Remote control (if Modbus communication is enabled)

HV/LV Substations

  • Measurement of power, energy, and power quality
  • Transformer monitoring (temperature, load)
  • Circuit breaker fault alerts

Open-Field Photovoltaic Systems

  • Monitoring of string inverters (SMA, Fronius, ABB, Huawei)
  • Measurement of power output, irradiance, and cell temperature
  • Detection of underperformance by string

Agricultural Silos and Tanks

  • Level measurement (hydrostatic pressure, ultrasound)
  • Storage temperature
  • Fill and empty alerts

Optimizing Mobile Data Usage​

In areas with limited 4G coverage or with a restricted M2M plan, several optimization measures can reduce data usage:

  • Deadband: Send a value only if it varies by more than X% from the previous value
  • Aggregation: Calculate the 5-minute average and send only the result
  • Compression: MQTT with compressed (gzip) JSON payload reduces data volume by 60 to 70%
  • QoS 0 for non-critical data (no MQTT acknowledgment = fewer messages)
  • Periodic reporting: one measurement per hour for slow-changing variables (ambient temperatures)

Frequently Asked Questions

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