The Merlin 3100 Series is an industrial cellular router providing high-speed and secure data connectivity for SCADA and IoT systems. It supports LTE Cat 4, Gigabit Ethernet, and dual SIM operation for network redundancy. Built-in cybersecurity features such as Trusted Platform Module (TPM), Secure Boot, and VPN/firewall support ensure data integrity and device protection. This guide describes how to configure the Merlin 3100 to collect Modbus data from field devices and transmit it to Ubidots via MQTT for cloud-based monitoring and visualization.

The setup involves enabling the following three key elements:

Log in to the WebUI of your Merlin PLC and then browse to the IECD configuration page, there enable IECD from the Main configuration tab:

Then hit the SAVE & APPLY button.

To set up MQTT as the Server protocol, navigate to the Slave Protocol tab and under dropdown with the same name, select MQTT:

Then click the CONFIGURE MQTT buttton, this will display the MQTT Configuration modal. Under the Main tab perform the following configurations:

At minimum, to enable TLS communication, user needs to provide a Certificate

Authority’s (CA) Public Certificate file. For setting this up, move to the TLS tab and there perform the following configurations:

Click SAVE & APPLY at the bottom of the page.

Advance to the Topic tab and there edit the MQTT Monitor Topic field:
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Ubidots expects the following topic structure:

For example:

Leave MQTT Number of Control Vars and MQTT Control Var1 Topic blank

Move to the Advanced tab and there in the MQTT Payload Format dropdown, select Ubidots/Thingsboard:

This ensures that the payload's format complies with the expcted format by Ubidots, which is:
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The RegisterHR name can be configured in the option name of config data point block.

Click SAVE & APPLY at the bottom of the page.

Back in the application's main configuration interface, go to Master Protocols tab, there select MODBUS as the Master protocol from the drop-down list, then click on CONFIGURE MODBUS button:

If you are using Modbus over serial interface, go to the Modbus #1 protocol dropdown and select Modbus Serial, then change the Serial parameters accordingly with your hardware setup:

If you are using Modbus over TCP interface, go to the Modbus #1 protocol dropdown and select Modbus TCP, then change the required parameters accordingly with your hardware setup:

For Modbus over TCP, it is required t configure a TCP route. Go to the bottom of the page and find the Modbus TCP Route section, then click the ADD button:

This will reveal the following parameters, configure them accordingly and then hit the SAVE & APPLY button.

Each data point maps one Modbus register to a corresponding MQTT variable.

Back in the application's main configuration window, go to the Points tab and click Configure Data Points

Click the ADD button to create a data point and then the EDIT button:

Here, go to the General tab and set the following parameters:

Move forward to the Master tab and there change the following parameters accordingly with the hardware setup and the previous Modbus configuration:
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Configuration example:

Save the changes