Modbus RTU Expansion Module

Modbus RTU Expansion Module: Centrally integrate up to 32 Modbus devices

The Didactum Modbus RTU expansion module is a compact slot board that lets you integrate up to 32 Modbus RTU devices, meters, or sensors directly into a Didactum monitoring system. The devices are connected in a daisy-chain wiring setup with a total length of up to 1000 meters, making it possible to centrally monitor even large installations with widely distributed Modbus components.

Each measured value from a connected Modbus device is addressed via its own input register and displayed in the Didactum system as a virtual sensor. This makes it possible to define individual threshold values, icons, and alarm rules for each value—just like for a physically connected sensor. The module is installed as a slot board in the central unit and can even be installed while the system is running, without needing to shut down the monitoring system.

A termination resistor integrated into the board is enabled by default at the factory and ensures a properly terminated bus line; if needed, for example with certain wiring topologies, it can be disabled.

Individually configurable: Select device

The Modbus RTU expansion module can be selected during the order process to match your Didactum monitoring system.

Module for the device

The reason for the two versions lies in the internal design of the respective central units: depending on the device series, the expansion slot is mounted on the main board at a different orientation, so the slot board’s power connector must either be angled by 90 degrees or designed straight to fit exactly into the respective housing. For PDU models, this expansion module is not available, as they do not have a corresponding Modbus RTU expansion slot.

From Modbus value to virtual sensor

After installation, connected Modbus RTU devices are activated and configured through the monitoring system’s web interface. Each individual measurement value of a device—such as temperature, voltage, or energy consumption—is addressed via its input register and then appears as a separate virtual sensor in the system, including a freely selectable name, icon, and threshold values for alarming.

Installing and wiring Modbus devices

The slot board is inserted into the designated expansion slot of the monitoring system—installation during operation is possible, and shutting down the system is not required.

1. Insert the expansion module into the appropriate slot of the central unit
2. Connect Modbus RTU devices in a daisy-chain wiring setup to the 3-pin terminal (B-, A+, GND); use a twisted pair for A(+) and B(-)
3. At the end of the chain, install a 120-ohm termination resistor that matches the cable’s characteristic impedance
4. If the monitoring system is at the beginning of the chain, leave the module’s internal factory-activated termination resistor enabled
5. Activate Modbus RTU in the web interface under “Preferences” and create the individual devices as elements in the system tree

Use cases

• Industrial automation: Integration of temperature sensors, flow meters, or energy meters for precise process monitoring
• Building management (BMS): Central acquisition of HVAC systems, lighting, and energy consumption via Modbus-capable devices
• Data centers: Collection of critical environmental data such as temperature, humidity, or power consumption across multiple Modbus devices
• Energy management: Monitoring of solar inverters, battery management systems, or power meters for efficient energy flow
• Remote monitoring of critical infrastructure: Integration of remote sensors, for example in pipelines or substations

What is included in the package

Please check the contents of the package for completeness upon receipt and report any missing or damaged parts to your supplier immediately.

• Modbus RTU expansion module (slot board)1 pc.
• Connection terminal, 3-pin, 3.5 mm1 pc.
• Metal cover for the expansion slot1 pc.
• Termination jumper, 2.54 mm (if required)1 pc.

Why this expansion module is the right choice

Frequently asked questions