Hi everyone, I’m trying to communicate between my Arduino Opta (set as Modbus RTU master) and an HMI display (set as Modbus RTU slave) using RS-485. My goal is to display an analog sensor value (connected to the Opta) on the HMI screen. Here’s my setup: Master: Arduino Opta (using Arduino PLC I…

Post what you have and maybe we can figure out what is happening. Since there is a possibility of a hardware error post your annotated schematic that shows all connections, power sources and grounds.

Hi, There are 2x AC power supplies and no common reference. Therefore, connect GND power supplier together in order to have the same reference.

[image] manucast: There are 2x AC power supplies Where you see an AC power on the diagram?

@dhinakaran_08 What is the "12-24v power supply" on your diagram that has a "+""-" and GND? Could you show a photo? As mentioned @manucast , you need a common GND connection between OPTA and HMI display module to make a Rs485 work.

Sorry, 2x DC and not AC. I did a mistake w/ my fingers / keyboard :sweat_smile:

[WhatsApp Image 2025-10-23 at 14.28.16_7514a51e] This is my power supply for the sensor, and I have also connected a common GND between the Opta and the HMI. I think the issue might be in the Modbus configuration in the PLC IDE.

[image] dhinakaran_08: This is my power supply for the sensor, Does the sensor needs a bipolar power source? If not, consult power supply manual should you use the negative or GND terminal in unipolar mode? But in any case, you should use the same terminal for both sensor power and ground fo…

Add a RS485 snifer in your technical chain in order to check the data.I use this dongle https://www.waveshare.com/usb-to-rs485.htm It is more easy to debug and check if there is a communication between master and slave(s). Futhermore, it is possible to send request to the slave and check if respons…

Important update: According to the power supply description, the "GND" terminal on the front panel is connected to chassis of the supply and not intended to use for DC output. You must use the "minus" terminal for GND connection on your diagram.. [image]

Arduino Opta (Modbus Master) to HMI Display (Modbus Slave) via RS-485

Official Hardware Opta

dhinakaran_08 October 23, 2025, 5:39am 3

Topic		Replies	Views
Arduino Opta not communicating with EM4M energy meter (Modbus RTU) Opta	9	122	November 7, 2025
Arduino Opta Modbus RTU issue Opta	0	101	January 6, 2026
Opta WiFi RS485: Modbus-Request funktioniert, Reply kommt nicht an Deutsch	7	46	March 28, 2026
OPTA MODBUS RTU problem Opta	1	120	February 10, 2026
OPTA & Modbus RS485 Opta	2	277	December 20, 2025

Arduino Opta (Modbus Master) to HMI Display (Modbus Slave) via RS-485

Related topics