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

📢 Announcement: Since the release of v1.3.0, our official dockerhub namespace has been changed to topmaker, you can pull the latest images from topmaker/modpoll at dockerhub.

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 3.10+ (Ubuntu 22.04+ or equivalent), the package is available in the Python Package Index, users can easily install it using pip or pipx. Ubuntu 20.04 and Python 3.8 are no longer supported.

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/main/examples/modsim.csv

the modsim code is also available here

Modbus ASCII and framers

Serial transports: use --serial PORT --framer ascii (alias: --rtu still accepted); pyserial URLs such as socket://host:port and rfc2217://host:port work for serial-over-TCP tunnels.
Serial supports framers ascii and rtu (pymodbus defaults to RTU if --framer default). Binary framer is no longer supported in pymodbus 3.9+.

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,modsim01,1,,
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,40000,44,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
ref,holding_reg15,40020,uint64,rw
ref,holding_reg16,40024,int64,rw
ref,holding_reg17,40028,float64,rw
ref,holding_reg18,40032,float64,rw
ref,holding_reg19,40036,string16,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 44 holding registers from the address starting from 40000 and parse data accordingly;

Notice the holding registers contain a 16-byte long string, which counts as 8 registers in poll size, because Modbus protocol defines a holding register as 2-byte width.

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 topmaker/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/main/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 topmaker/modsim

In a new terminal,

modpoll \
  --tcp localhost \
  --config https://raw.githubusercontent.com/gavinying/modpoll/main/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 broker.emqx.io for test purpose. You can also set up your own MQTT broker locally using mosquitto.

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

With successful data polling and publishing, you can subscribe the default data topic modpoll/modsim01/data on the same MQTT broker broker.emqx.io to view the collected data.

The MQTT topics can be customized by providing the following arguments,

mqtt-publish-topic-pattern
mqtt-subscribe-topic-pattern
mqtt-diagnostics-topic-pattern

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"
```

Modpoll

Install / Use

README