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TECHNICAL NOTE

Integrating Modbus TCP and Modbus RTU — Gateway Architectures and Model Selection

Field devices sit on RS-485; host systems sit on Ethernet. This article covers how the two differ, the two common gateway architectures, and how to choose a model for your site.

Compare architectures Choose a model

BASICS

RTU and TCP: same data, different transport

Both share the same data model (coils and registers); only the transport differs. Integration is about making data on the RS-485 bus reachable over Ethernet.

One at a time: the master polls slaves 1–3 in turn

In parallel: several masters read at once; CGate can be a master too

Physical layer

RS-485 (serial)

Ethernet

Addressing

Slave address

IP address + Unit ID

Error check

CRC

Handled by TCP/IP

Communication

One master polls each slave in turn

Several masters (clients) can talk to slaves (servers) at once

Settings

Baud rate, parity, etc. must match on both ends

IP address and port (502 by default)

ARCHITECTURE

Beyond conversion: CGate organizes the data first

Most gateways on the market map each RTU device straight to TCP by port definition: every read goes to the field device, and point mapping and data handling are left to the host. CGate polls first, keeps the latest values, remaps the points, then delivers to several destinations at once.

User-defined Modbus registers

Add your own devices and choose which coils and registers to read

Web monitoring

Live data
Trends
Logs

Modbus Agent

Serves the organized points as a Modbus slave and an MQTT subscriber

Illustration: CGate sending data to the cloud over MQTT and MySQL

Parallel outputs

Multiple channels for delivering data

Illustration: CGate-06 storing history in its built-in SQL database

Built-in database

History is kept in an SQL database and can be exported

Illustration: CGate alarm notification flow

Built-in alarms

Set conditions per device or per point to send an email or switch a DO

CGate-05 front-right view, Modbus gateway

Embedded Linux device

No extra PC
ARM-based

	Typical Modbus converter	CGate
How it works	Forwards each request to the device and relays the reply	Polls devices, stores the latest values, then delivers
Point mapping	Keeps each device’s own addresses; the host configures them one by one	All device points consolidated into one CGate point map
Data storage	Stores nothing; logging is up to the host	Built-in SQL database and history log
Outputs	Modbus TCP	Modbus TCP, MQTT (JSON) and MySQL in parallel
Live monitoring view	None; the host must display it	Built-in web monitoring page

Note: “typical Modbus converter” refers to gateways that only convert between Modbus RTU and Modbus TCP.

Energy Management

Environmental Monitoring

Remote Site Operations

Energy Management

Environmental Monitoring

Remote Site Operations

APPLICATION

From solar plants to remote sites, CGate collects the field data

Field Modbus devices are spread across switchboards, greenhouses and remote sites. CGate polls them on site, organizes the points, then uploads to the cloud or hands them to SCADA.

SELECTION