You would generally need to take the following actions to transmit data from an RS485 interface to Mendix:

1. Setting up RS485 communication requires connecting your energy monitor device to either an RS485-to-USB or RS485-to-Ethernet converter, depending on whether interface is supported by the server or gateway for your Mendix application. Check to ensure that the converter is set up for RS485 connectivity.

2. Create a module for communication: Establish a Mendix module or component to manage communication with the RS485 interface. This module should have the ability to connect to the RS485 converter, communicate with the energy monitor device, and receive feedback.

3. Implement the RS485 protocol: The RS485 physical layer protocol does not specify a particular message structure or data format. To communicate over RS485, you must ascertain the protocol your energy monitor equipment employs. A common protocol like Modbus or a bespoke protocol might be used for this. To follow the protocol and accurately understand the data received from the device, implement the essential logic in your communication module.

4. Data parsing and processing are required in your Mendix application once you have received data from the RS485 port. This might entail pulling pertinent data from the messages you get, transforming the data if required, and storing it in your application's data model or causing particular actions to be triggered in response to the data.

5. Update the user interface: Display the received data in the dashboards, charts, or reports of your Mendix application. Based on the information obtained from the RS485 interface, the user interface components may be designed and updated using Mendix's visual modelling capabilities.

It's crucial to remember that the particular implementation details may change based on the RS485 converter, the protocol of the energy monitor device, and the capabilities of your Mendix application. To guarantee a successful connection, be sure to refer to the documentation and resources offered by the makers of the RS485 converter, the energy monitor device, and the Mendix platform.

I would look at using a microcontroller such as an Arduino or Rasperberry Pi Pico to talk to the RS485 and expose the data over REST or MQTT. You can then get your Mendix application to talk to that.

Good luck!