RUT200 Modbus TCP Script

NETWORKING SOLUTIONS
1

Hi Guys,
I want to write a script that uses a Modbus TCP client to periodically read data from Modbus devices and publish it to an MQTT broker, while also receiving messages from an MQTT topic (to which it is subscribed) and write to Modbus registers based on those messages. But I couldn’t find the packages documentations and alike.

I have read about the UBUS lib, but I could not find a script that uses Modbus TCP to read and write data.

What packages or libraries should I use? Does someone have an example of something similar? How can I find the documentation of the packages installed in the RUTOS?
Thank you,
Miguel Chagas

3

Hello,

Thank you for reaching out. Your request is currently under analysis, and we are reviewing the appropriate libraries, available documentation, and practical implementation steps.
Once the review is done, I will follow up here with a concise summary and any available examples.

In the meantime, thank you for your patience and understanding.

Additionally, since you have a related topic already created, feel free to leave a comment or reminder there as well to help consolidate the discussion.

Best regards,

1 Like
4

After reviewing relevant information found on the forum and other sources and analyzing it, in summary:

To read data from a Modbus TCP server, you can utilize the ubus call interface available in RutOS. All available ubus parameters and endpoints can be listed using the following command:

ubus -v list

Here is an example ubus call to read holding registers (Function Code 3) from a Modbus TCP server:

ubus call modbus_client.rpc tcp.test '{
  "id":1,
  "timeout":1,
  "function":3,
  "first_reg":1,
  "reg_count":"1",
  "data_type":"16bit_int_hi_first",
  "no_brackets":1,
  "ip":"192.168.1.138",
  "port":"502",
  "delay":0
}'

In this example:

  • It reads 1 register starting at address 1
  • Data type is 16bit_int_hi_first
  • Target Modbus server IP is 192.168.1.138, port 502

To publish or subscribe to an MQTT broker, you can use the Mosquitto client, which can be installed on the device with the following commands:

opkg -e /etc/opkg/openwrt/distfeeds.conf update
opkg -e /etc/opkg/openwrt/distfeeds.conf install mosquitto-client-ssl

This will allow you to:

  • Use mosquitto_pub to publish data
  • Use mosquitto_sub to subscribe and react to messages

You can then script Modbus reads and MQTT publishes in a loop, and simultaneously listen for MQTT messages and write back to Modbus registers.

Furthermore, there were similar topics/requests discussed on these threads:

In addition:

  • For writing a complete automation script, you can leverage community-shared examples and further details in related forum threads.
  • You can also use AI tools like chatgpt/deepseek to help draft and troubleshoot your script logic using ubus and mosquitto commands.

I hope this can bring you any useful insights or hints for writing the required script.

Best regards,

1 Like
5

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