Modbus device access and protocol conversion
The SimtoLink Modbus gateway is a protocol converter for Modbus RTU on an RS-485 serial port and Modbus TCP > on Ethernet (Modbus TCP/IP). It supports one-master multi-slave and multi-master multi-slave layouts, and reports Modbus data as MQTT, HTTP, OPC UA. It is used where power, water, and industrial IIoT (Industrial Internet of Things) sites pack many devices onto one bus.
Several RS485 masters can poll at the same time, and one unit can take 64 slaves. The gateway is already in hundreds of power-distribution, water, environmental, and industrial projects, with more than 100,000 Modbus devices connected.
Protocols and function codes
Configurations follow the southbound method and the northbound protocol, for SCADA, energy management, and cloud platforms. Typical protocols and capacity:
| Southbound protocol | Typical setup | Northbound protocol |
|---|---|---|
| Modbus RTU | 1～4 RS485 ports, master mode | MQTT, HTTP, OPC UA |
| Modbus TCP | Ethernet Modbus TCP/IP, client or server | MQTT, HTTP, database |
| Function codes | 01/02/03/04/05/06/15/16 | Coil and register read/write |
| Poll interval | 1s～24h, configurable per slave or register | Custom JSON mapping |
Byte order can be set to big-endian, little-endian, or byte swap. MQTT supports QoS 0/1/2, and the topic and payload format can be set. A Modbus point scanner from SimtoLink can discover device registers (register mapping).
Typical applications and results
From distribution rooms to wastewater plants and PV stations, the gateway covers industrial collection and cloud upload. Typical deployments and results:
Meter collection in a distribution room
One distribution room connected 20 smart meters through a Modbus gateway and reported them over MQTT to the energy platform for sub-metering and demand management.
Instrument collection at a wastewater plant
A wastewater plant brings 80 instruments (flow, pH, and dissolved oxygen) through a Modbus gateway into the control-room SCADA so process values can be watched live.
Generation monitoring at a PV plant
A PV plant uses a Modbus gateway for inverters, meters, and the weather station, for generation monitoring and day-to-day operations.
Plant PLC and instrument data to the cloud
A manufacturer converts PLCs, temperature controllers, and flow meters on Modbus to MQTT and sends them to an industrial internet platform.
Configuration and selection notes
- Point setup: Set the slave address, function code, and register address, and check byte order (big-endian or little-endian). Use the device manual or SimtoLink templates for common devices.
- Polling: With several slaves, set the poll interval so the bus does not collide or stall. For MQTT, QoS 1 is the usual choice when delivery matters.
- Expansion and operations: A new slave needs an address that does not collide. If communications fail, check wiring, baud rate, address, and the termination resistor. The Modbus point scanner helps with that check.
- Implementation support: SimtoLink can configure the gateway, troubleshoot the bus, and help with protocol integration, and can supply templates for common devices.
Implementation and maintenance
A practical sequence for putting a Modbus gateway into service:
The point scanner and configuration service help get devices onto the platform without a long mapping exercise.
Services and support
SimtoLink helps select and configure the Modbus gateway, including point scanning and mapping, and can troubleshoot the bus and integrate protocols. Product manuals and solution quotations are available.