Modbus permission errors

rcraig114 · March 6, 2026, 7:33pm

I’m trying to get data from my solar controller, which supports RS485. I have a USB to RJ11 cable connected to a Raspberry Pi4. I can see the USB device under hardware.

Device path:/dev/ttyUSB0

ID:/dev/serial/by-id/usb-FTDI_FT232R_USB_UART_BG00ZBE2-if00-port0

However, the Modbus2MQTT addon keeps complaining about permissions. Anyone ever run across this?

Permission denied for read serial device /dev/ttyUSB0 Error: EACCES: permission denied, access ‘/dev/ttyUSB0’

InitialConnect /dev/serial/by-id/usb-FTDI_FT232R_USB_UART_BG00ZBE2-if00-port0: Error: Permission denied, cannot open /dev/serial/by-id/usb-FTDI_FT232R_USB_UART_BG00ZBE2-if00-port0

IOT7712 · March 6, 2026, 7:37pm

How?
[Please quote any code using the forum </> formatting options.]
Have you tried the protocol wizard yet?

rcraig114 · March 6, 2026, 7:43pm

Sorry, I’m using an addon called Modbus <=> MQTT.

IOT7712 · March 6, 2026, 7:49pm

Are you running this in a VM type environment?

I see you raised an issue in GitHub too.

github.com/modbus2mqtt/modbus2mqtt

Device permission errors

opened 03:52PM - 06 Mar 26 UTC

rcraig114

I'm trying to get data from my solar controller, which supports RS485. I have a …USB to RJ11 cable connected to a Raspberry Pi4. I can see the USB device under hardware. Device path:/dev/ttyUSB0 ID:/dev/serial/by-id/usb-FTDI_FT232R_USB_UART_BG00ZBE2-if00-port0 However, the Modbus2MQTT addon keeps complaining about permissions. Anyone ever run across this? Permission denied for read serial device /dev/ttyUSB0 Error: EACCES: permission denied, access '/dev/ttyUSB0' InitialConnect /dev/serial/by-id/usb-FTDI_FT232R_USB_UART_BG00ZBE2-if00-port0: Error: Permission denied, cannot open /dev/serial/by-id/usb-FTDI_FT232R_USB_UART_BG00ZBE2-if00-port0

rcraig114 · March 6, 2026, 7:50pm

HAOS on a RaspberryPi4.

IOT7712 · March 6, 2026, 7:53pm

Latest updates, cold restart? Any other integrations using your USB/RS485 adapter?
What Solar Controller are you attempting to access?

rcraig114 · March 6, 2026, 7:58pm

Yup, no pending updates. Full reboot, power off/power on. I have a brand new Dyness 40A MPPT connected.

IOT7712 · March 6, 2026, 8:15pm

Seems like the issue has been encountered before.

IOT7712 · March 6, 2026, 8:23pm

github.com/modbus2mqtt/modbus2mqtt

docs/getting-started.md

6a2c86f85

# Getting Started with modbus2mqtt

This guide will help you quickly set up and use modbus2mqtt.

## Prerequisites

Before you begin, make sure you have:

- A Modbus device (RTU or TCP)
- MQTT broker (e.g., Mosquitto)
- For Modbus RTU: USB-to-RS485 adapter

## Quick Start

### 1. Access the Web Interface

Open your browser and navigate to:

```
http://localhost:3000

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See troubleshooting step in the instructions that describes your scenario with Linux access issues.

Let us know if you succeed to connect, and don’t forget to update the GitHub report on your progress as well.

rcraig114 · March 6, 2026, 8:41pm

Yeah, I’ve seen several articles out there about changing permissions. However, I am using HAOS and it doesn’t like the typical Linux commands. Below is what I get.

[core-ssh ~]$ sudo usermod -a -G dialout $USER
-bash: sudo: command not found

IOT7712 · March 6, 2026, 8:43pm

How are you issuing the commands?

rcraig114 · March 6, 2026, 8:45pm

Putty into HAOS (Terminal & SSH) app running to expose port 22). Connected to HAOS as “root” and used the password I specified in the Addon.

IOT7712 · March 6, 2026, 8:54pm

The ‘standard’ SSH Addon (cut down to prevent neophytes from hurting themselves), or one with extended features turned on?

github.com/home-assistant/addons

ssh/DOCS.md

5fc1881fd

# Home Assistant App: Terminal & SSH

## Installation

Follow these steps to get the app installed on your system:

1. This app is only visible to "Advanced Mode" users. To enable advanced mode, go to **Profile** -> and turn on **Advanced Mode**.
2. In Home Assistant, go to **Settings** > **Apps** > **Install app**.
3. Find the "Terminal & SSH" app and click it.
4. Click on the "INSTALL" button.

## How to use

This app adds two main features to your Home Assistant installation:

- a web terminal that you can use from your browser, and
- enable connecting to your system using an SSH client.

Regardless of how you connect (using the web terminal or using an SSH client), you end up in this app's container. The Home Assistant configuration
directory is located on the path `/config`.

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Quote:
Known issues and limitations

This app will not enable you to install packages or do anything as root. This is not working with Home Assistant.

(Yeah, I know, it shouldn’t be this hard) - put that in your GitHub update to say the workaround doesn’t work.

rcraig114 · March 6, 2026, 9:01pm

Yup, thats the one.

IOT7712 · March 6, 2026, 9:07pm

Whst steps did you take to install the RS485 USB Device?

Maybe play with the Wizard instead?

rcraig114 · March 7, 2026, 12:20am

I just plugged the USB device into my RPI4. RJ11 end in Solar Charge Controller. I know the charge controller detected it because as soon as you connect the cable, the screen goes blank.
I tested with wizard and I see “modbus hub connected”, but adding any register/entities for this charge controller reveals no data. I’m starting to wonder if this USB device is junk.