Modpoll - A New Command-line Tool for Modbus and MQTT

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Learn more about modpoll usage at documentation site.

Motivation

The idea for creating this tool originated from my need to efficiently check new devices during site surveys. These surveys are often time-constrained and space-limited, with on-site work adding to the pressure. In such situations, a portable Swiss Army knife toolkit becomes a valuable companion.

This tool can be easily deployed on a Raspberry Pi or similar embedded devices, polling data from a Modbus network or connected devices. Users can choose to log the data locally or publish it to an MQTT broker for further troubleshooting.

The MQTT broker can be set up either on the same Raspberry Pi or in the cloud. Once data is successfully published, users can subscribe to the relevant MQTT topics and conveniently view the data on their smartphones.

Moreover, you can also run this tool continuously on a server as a Modbus-MQTT gateway, i.e. polling from local Modbus devices and forwarding data to a centralized cloud service.

In fact, modpoll helps to bridge between the traditional field-bus world and the new IoT world.

💡 Tips: This tool is designed to be a standalone executable application, which works out-of-the-box on Linux/macOS/Windows. If you are looking for a Modbus python library, please consider the following great open-source projects, pymodbus or minimalmodbus

Feature

• Support Modbus RTU/TCP/UDP devices
• Show polling data for local debugging, like a typical modpoll tool
• Publish polling data to MQTT broker for remote debugging, especially on smart phone
• Export polling data to local storage for further investigation
• Provide docker solution for continuous data polling use case

Installation

This tool tested on Python 3.8+, the package is available in the Python Package Index, users can easily install it using pip or pipx.

Using PIP

Install modpoll using the following command,

pip install modpoll

Optionally, pyserial library can be installed for Modbus-RTU communication,

pip install 'modpoll[serial]'

Upgrade the tool via the following command,

pip install -U modpoll

On Windows

It is recommended to use pipx for installing modpoll on Windows, refer to here for more information about pipx.

Once pipx installed, you can run the following command in a Command Prompt terminal.

pipx install modpoll

Upgrade the tool via the following command,

pipx upgrade modpoll

Quickstart

modpoll is a python tool for communicating with Modbus devices, so ideally it makes more sense if you have a real Modbus device on hand for the following test, but it is OK if you don't, we provide a virtual Modbus TCP device deployed at modsim.topmaker.net:502 for your quick testing purpose.

Let's start exploring modpoll with modsim device, run the following command to get a first glimpse,

modpoll --once \
  --tcp modsim.topmaker.net \
  --config https://raw.githubusercontent.com/gavinying/modpoll/master/examples/modsim.csv

the modsim code is also available here

Prepare Modbus configure file

The reason we can magically poll data from the online device modsim is because we have already provided the Modbus configure file for modsim device as following,

device,modsim001,1,,
poll,coil,0,16,BE_BE
ref,coil01-08,0,bool8,rw
ref,coil09-16,1,bool8,rw
poll,discrete_input,10000,16,BE_BE
ref,di01-08,10000,bool8,rw
ref,di09-16,10001,bool8,rw
poll,input_register,30000,20,BE_BE
ref,input_reg01,30000,uint16,rw
ref,input_reg02,30001,uint16,rw
ref,input_reg03,30002,uint16,rw
ref,input_reg04,30003,uint16,rw
ref,input_reg05,30004,int16,rw
ref,input_reg06,30005,int16,rw
ref,input_reg07,30006,int16,rw
ref,input_reg08,30007,int16,rw
ref,input_reg09,30008,uint32,rw
ref,input_reg10,30010,uint32,rw
ref,input_reg11,30012,int32,rw
ref,input_reg12,30014,int32,rw
ref,input_reg13,30016,float32,rw
ref,input_reg14,30018,float32,rw
poll,holding_register,40000,20,BE_BE
ref,holding_reg01,40000,uint16,rw
ref,holding_reg02,40001,uint16,rw
ref,holding_reg03,40002,uint16,rw
ref,holding_reg04,40003,uint16,rw
ref,holding_reg05,40004,int16,rw
ref,holding_reg06,40005,int16,rw
ref,holding_reg07,40006,int16,rw
ref,holding_reg08,40007,int16,rw
ref,holding_reg09,40008,uint32,rw
ref,holding_reg10,40010,uint32,rw
ref,holding_reg11,40012,int32,rw
ref,holding_reg12,40014,int32,rw
ref,holding_reg13,40016,float32,rw
ref,holding_reg14,40018,float32,rw

This configuration tells modpoll to do the following for each poll,

• Read 16 coils from the address starting from 0 and parse the response as two 8-bits boolean values;
• Read 16 discrete inputs from the address starting from 10000 and parse the response as two 8-bits boolean values;
• Read 20 input registers from the address starting from 30000 and parse data accordingly;
• Read 20 holding registers from the address starting from 40000 and parse data accordingly;

In practical, you usually need to customize a Modbus configuration file for your own device before running modpoll tool, which defines the optimal polling patterns and register mappings according to device vendor's documents.

You can also take a look at contrib folder, which collects a few types of device configuration shared by contributors.

The configuration can be either a local file or a remote public URL resource.

Refer to the documentation site for more details.

Poll local device (modsim)

If you are blocked by company firewall for online device or prefer a local test, you can launch your own device simulator by running modsim locally,

docker run --rm -p 5020:5020 helloysd/modsim

It will create a virtual Modbus TCP device running at localhost:5020, and you can open a new terminal, poll the virtual device using modpoll tool,

modpoll \
  --tcp localhost \
  --tcp-port 5020 \
  --config https://raw.githubusercontent.com/gavinying/modpoll/master/examples/modsim.csv

Use sudo before the docker command if you want to use the standard port 502.

sudo docker run --rm -p 502:5020 helloysd/modsim

In a new terminal,

modpoll \
  --tcp localhost \
  --config https://raw.githubusercontent.com/gavinying/modpoll/master/examples/modsim.csv

Publish data to MQTT broker

This is a useful function of this new modpoll tool, which provides a simple way to publish collected Modbus data to MQTT broker, so users can view data from a smart phone via a MQTT client.

The following example uses a public MQTT broker mqtt.eclipseprojects.io for test purpose. You can also set up your own MQTT broker locally using mosquitto.

modpoll \
  --tcp modsim.topmaker.net \
  --mqtt-host mqtt.eclipseprojects.io \
  --config https://raw.githubusercontent.com/gavinying/modpoll/master/examples/modsim.csv

With successful data polling and publishing, you can subscribe the topic modpoll/modsim on the same MQTT broker mqtt.eclipseprojects.io to view the collected data.

The MQTT topic uses <mqtt_topic_prefix>/<device_name> pattern, <mqtt_topic_prefix> is provided by --mqtt-topic-prefix argument, the default value is modpoll and <device_name> is provided by the Modbus configure file.

⚠️ Note: The --mqtt-topic-prefix argument is deprecated and will be removed in the future release. Use --mqtt-publish-topic-pattern and --mqtt-subscribe-topic-pattern instead. If both are used, --mqtt-topic-prefix argument will take precedence in order to keep backward compatibility. See document for details.

Write registers via MQTT publish

The modpoll tool will subscribe to the topic modpoll/<device_name>/set by default once it successfully connected to MQTT broker, user can write device register(s) via MQTT publish,

• To write a single holding register (address at 40001)

  {
    "object_type": "holding_register",
    "address": 40001,
    "value": 12
  }

• To write multiple holding registers (address starting from 40001)

  {
    "object_type": "holding_register",
    "address": 40001,
    "value": [12, 13, 14, 15]
  }

Run with docker

A docker image has been provided for user to directly run the tool without local installation,

docker run --rm helloysd/modpoll

It shows the version of the tool by default.

Similar to the above modsim test, we can poll data with docker run, in order to avoid printing out received data, the argument --daemon or -d is recommended to use with docker.

docker run --rm helloysd/modpoll \
  modpoll -d \
    --tcp modsim.topmaker.net \
    --config https://raw.githubusercontent.com/gavinying/modpoll/master/examples/modsim.csv

If you want to load a local configure file, you need to mount a local folder onto container volume, for example, if the child folder examples contains the config file modsim.csv, we can use it via the following command,

docker run --rm -v $(pwd)/examples:/app/examples helloysd/modpoll \
  modpoll -d \
    --tcp modsim.topmaker.net \
    --config /app/examples/modsim.csv

Basic Usage

• Connect to Modbus TCP device

  modpoll \
    --tcp 192.168.1.10 \
    --config examples/modsim.csv

• Connect to Modbus RTU device

  modpoll \
    --rtu /dev/ttyUSB0 \
    --rtu-baud 9600 \
    --config contrib/eniwise/scpms6.csv

• Connect to Modbus TCP device and publish data to MQTT broker

  modpoll \
    --tcp modsim.topmaker.net \
    --tcp-port 5020 \
    --mqtt-host mqtt.eclipseprojects.io \
    --config examples/modsim.csv

• Connect to Modbus TCP device and export data to local csv file

  modpoll \
    --tcp modsim.topmaker.net \
    --tcp-port 5020 \
    --export data.csv \
    --config examples/modsim.csv

Refer to the documentation site for more details about the configuration and examples.

Credits

The implementation of this project is heavily inspired by the following two projects:

• https://github.com/owagner/modbus2mqtt (MIT license)
• https://github.com/mbs38/spicierModbus2mqtt (MIT license)

Thanks to Max Brueggemann and Oliver Wagner for their great work.

License

MIT © 2021-2024, Ying Shaodong