Didactum Monitoring Systems
Setting up Modbus TCP & RTU

All Didactum Monitoring Systems support the widely used Modbus protocol – both in the TCP/IP variant and the RTU variant (RS-485). This allows external devices such as energy meters, air conditioning units, UPS systems, sensors and many other industrial devices to be directly integrated into the Didactum monitoring system.

Modbus is an open serial communication protocol originally developed by Modicon. It is used in industrial automation to communicate between a Master (here: the Didactum system) and one or more Slaves (external devices). Didactum systems always act as Modbus Master.

A virtual “Modbus TCP (Reading)” element is used to read data from an external device via the Modbus TCP/IP protocol and display it as a measured value in the Didactum system. The Wake-on-LAN standard is also supported to wake remote devices via a network command.

1. In the Didactum system web interface, click the “Add” button in the Group Tree or System Tree menu.
2. Select “Modbus TCP (Reading)” from the selection menu – a modal window will open.
3. Click on “Modbus TCP (Reading)” to open the configuration form.

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The selection menu appears after clicking the Add button. Here you select “Modbus TCP (Reading)” from the list of available virtual elements.

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The configuration form for the Modbus TCP Reading element. In the upper area you will find general settings for name, unit of measurement and limit values.

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In the “Additional” tab, all Modbus TCP specific parameters are configured: IP address of the slave device, port, slave address, register index (hexadecimal), function code, data type and polling interval.

A virtual “Modbus TCP (Writing)” element enables the active writing of values to Modbus registers of external devices. This allows switch commands, setpoints or control commands to be transmitted via Modbus TCP. The writing element can also be used as a THEN task in logic plans.

1. In the web interface, click the Add button in the Group Tree or System Tree menu.
2. Select “Modbus TCP (Writing)” from the selection menu.
3. The configuration form opens directly.

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The configuration form for the Modbus TCP Writing element. All relevant Modbus parameters and the value to be written are set here.

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The Modbus TCP Writing element can be used as a THEN task in Didactum logic plans – e.g. to automatically send a switch command to an external device in case of an alarm.

In this practical example, an energy meter from the manufacturer Elnet is integrated into a Didactum system via Modbus TCP. Elnet energy meters have multiple Modbus registers for voltage, current and other measured variables.

Elnet meters use decimal register indices in their documentation. However, the Didactum system expects hexadecimal register addresses. The conversion must therefore be done manually.

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The register table from the Elnet manual shows the available Modbus registers with decimal addresses. These must be converted to hexadecimal for the Didactum configuration.

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Configured element for “Current Phase L2” with register index 000F.

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Configured element for “Voltage Phase L1” with register index 0001.

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All created Elnet register elements in the Didactum system tree under Web interface → System Tree → Modbus Elements.

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All register elements have been added to the overview panel – so voltage and current of all three phases are visible at a glance.

A virtual “Modbus RTU” sensor element reads data from external devices via the Modbus RTU protocol (RS-485 bus). Typical applications are sensors, meters or control devices with an RS-485 interface.

1. Connect the RS-485 bus according to the slave device documentation.
2. In the Didactum web interface, navigate to Settings → Modbus RTU.
3. Configure the bus according to the slave device documentation (baud rate, parity, stop bits).
4. Enable Modbus RTU and save the settings.

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The Modbus RTU configuration page under Settings → Modbus RTU. Here you set the baud rate, parity and stop bits to match the slave device.

After enabling Modbus RTU, you can create virtual reading elements that read measured values from the RS-485 bus and display them in the Didactum system.

1. In the Didactum web interface, click the Add button in the Group Tree or System Tree.
2. Select “Modbus RTU (Reading)” – a modal window will appear.
3. Click on “Modbus RTU (Reading)” to open the configuration form.

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Selection menu for adding a Modbus RTU Reading element. Select “Modbus RTU (Reading)” from the list.

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The general configuration form for the Modbus RTU Reading element. The fields are largely the same as those of the Modbus TCP element.

The “Modbus RTU (Writing)” element allows active writing of values to Modbus RTU registers of external devices via the RS-485 bus. Like its TCP counterpart, it can be used as a THEN task in logic plans.

1. In the web interface, click the Add button in the Group Tree or System Tree.
2. Select “Modbus RTU (Writing)”.
3. The configuration form opens directly.

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The configuration form for the Modbus RTU Writing element with fields for slave address, register index, function code and value to be written.

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The Modbus RTU Writing element can be used as a THEN task in logic plans – e.g. for automatically switching an RS-485 control device on alarm.

The following table contains verified Modbus configurations for devices that have been successfully tested with Didactum systems. The values can be directly copied into the respective Modbus elements.

1. Check register addresses: Manufacturers often specify registers in decimal – Didactum requires hexadecimal notation. Online converters help with the conversion (e.g. Dec 60 = Hex 003C).
2. Determine data type: Check the Modbus manual of the slave device for the data format used (16-bit, 32-bit, Float ...).
3. Test byte order: If the read value appears incorrect, try a different byte order. The most common combination is high byte first, high word first.
4. Test connection: Always use the Test button before saving the element. The raw data helps with troubleshooting.
5. Polling interval: For rapidly changing values (e.g. current) choose the minimum of 10 seconds. For rarely changing values, a larger interval can reduce network load.