PyModbus - A Python Modbus Stack Documentation Status Downloads

Supported versions

Version 2.5.3 is the last 2.x release (Supports python 2.7.x - 3.7).

Version 3.0.0dev5 is the current prerelease of 3.0.0 (Supports Python >=3.8).

Remark: “Supports” means that we only test with those versions, lower versions (e.g. 3.7) might work depending on the functionality used.


Note 3.0.0 is a major release with a number of incompatible changes.


Pymodbus is a full Modbus protocol implementation using a synchronous or asynchronous (using asyncio) core.

Supported modbus communication modes: tcp, rtu-over-tcp, udp, serial, tls

Pymodbus can be used without any third party dependencies (aside from pyserial) and is a very lightweight project.

Pymodbus also provides a lot os ready to use examples as well as a server/client simulator which can be controlled via REST Api and can be easily integrated into test suites.

Requires Python >= 3.8

The tests are run against Python 3.8, 3.9, 3.10 on Windows, Linux and MacOS.


Client Features

  • Full read/write protocol on discrete and register

  • Most of the extended protocol (diagnostic/file/pipe/setting/information)

  • TCP, RTU-OVER-TCP, UDP, TLS, Serial ASCII, Serial RTU, and Serial Binary

  • asynchronous(powered by asyncio) and synchronous versions

  • Payload builder/decoder utilities

  • Pymodbus REPL for quick tests

  • Customizable framer to allow for custom implementations

Server Features

  • Can function as a fully implemented modbus server

  • TCP, RTU-OVER-TCP, UDP, TLS, Serial ASCII, Serial RTU, and Serial Binary

  • asynchronous and synchronous versions

  • Full server control context (device information, counters, etc)

  • A number of backend contexts (database, redis, sqlite, a slave device) as datastore

Use Cases

Although most system administrators will find little need for a Modbus server on any modern hardware, they may find the need to query devices on their network for status (PDU, PDR, UPS, etc). Since the library is written in python, it allows for easy scripting and/or integration into their existing solutions.

Continuing, most monitoring software needs to be stress tested against hundreds or even thousands of devices (why this was originally written), but getting access to that many is unwieldy at best.

The pymodbus server will allow a user to test as many devices as their base operating system will allow (allow in this case means how many Virtual IP addresses are allowed).

For more information please browse the project documentation: or

Example Code

For those of you that just want to get started fast, here you go:

from pymodbus.client import ModbusTcpClient

client = ModbusTcpClient('')
client.write_coil(1, True)
result = client.read_coils(1,1)

For more advanced examples, check out the Examples included in the repository. If you have created any utilities that meet a specific need, feel free to submit them so others can benefit.

Examples Directory structure

├── common.      -> Common examples for clients and server (sync/async), Payload encoders and decoders.
├── contrib.     -> Examples contributed by contributors. Serial Forwarder, Database contexts etc.

If you are looking for UI, checkout Modbus Simulator or Modbus Cli

Also, if you have a question, please create a post in discussions q&a topic, so that others can benefit from the results.

If you think, that something in the code is broken/not running well, please open an issue, read the Template-text first and then post your issue with your setup information.


Note For async clients, it is recommended to use `asyncio` as the async facilitator.

Pymodbus REPL (Read Evaluate Print Loop)

Pymodbus REPL Client

Pymodbus REPL comes with many handy features such as payload decoder to directly retrieve the values in desired format and supports all the diagnostic function codes directly .

For more info on REPL Client refer Pymodbus REPL Client

Pymodbus REPL Server

Pymodbus also comes with a REPL server to quickly run an asynchronous server with additional capabilities out of the box like simulating errors, delay, mangled messages etc.

For more info on REPL Server refer Pymodbus REPL Server


You can install using pip or easy install by issuing the following commands in a terminal window (make sure you have correct permissions or a virtualenv currently running):

pip install -U pymodbus

This will install a base version of pymodbus.

To install pymodbus with options run:

pip install -U pymodbus[<option>,…]

Available options are:

  • repl, installs pymodbus REPL.

  • serial, installs serial drivers.

  • datastore, installs databases (SQLAlchemy and Redit) for datastore.

  • documentation, installs tools to generate documentation.

  • development, installs development tools needed to enable test/check of pymodbus changes.

Or to install a specific release:

pip install -U pymodbus==X.Y.Z

Otherwise you can pull the trunk source and install from there:

git clone git://
cd pymodbus
pip install -r requirements.txt

To get latest release (for now v2.5.3 with Python 2.7 support):

git checkout master

To get bleeding edge:

git checkout dev

To get a specific version:

git checkout tags/vX.Y.Z -b vX.Y.Z


pip install -r requirements.txt pip install -e .

This installs pymodbus in your virtual environment with pointers directly to the pymodbus directory, so any change you make is immediately available as if installed.

Either method will install all the required dependencies (at their appropriate versions) for your current python distribution.

Repository structure

The repository contains a number of important branches and tags.
  • dev is where all development happens, this branch is not always stable.

  • master is where are releases are kept.

  • All releases are tagged with vX.Y.Z (e.g. v2.5.3)

  • All prereleases are tagged with vX.Y.ZrcQ (e.g. v3.0.0.0rc1)

If a maintenance release of an old version is needed (e.g. v2.5.4), the release tag is used to create a branch with the same name, and maintenance development is merged here.

Current Work In Progress

The maintenance team is very small with limited capacity and few modbus devices.

However, if you would like your device tested, we accept devices via mail or by IP address.

That said, the current work mainly involves polishing the library and solving issues:

  • Get version 3.0.0 released

  • Fixing bugs/feature requests

  • Architecture documentation

  • Functional testing against any reference we can find

  • The remaining edges of the protocol (that we think no one uses)

Development Instructions

The current code base is compatible python >= 3.8. Here are some of the common commands to perform a range of activities


pip install -r requirements.txt install all requirements pip install -e . source directory is “release”, useful for testing

tox -e py38 (or py39, py310, pypy38) Run pytest on source code

tox -e pylint Run pylint on source code tox -e codespell Run codespell on source code tox -e bandit Run bandit on source code tox -e flake8 Run flake8 on source code tox -e black Run black on source code

Generate documentation


cd doc make clean make html


Just fork the repo and raise your PR against dev branch.

Here are some of the items waiting to be done:

License Information

Pymodbus is built on top of code developed from/by:
  • Copyright (c) 2001-2005 S.W.A.C. GmbH, Germany.

  • Copyright (c) 2001-2005 S.W.A.C. Bohemia s.r.o., Czech Republic.

  • Hynek Petrak,

Released under the BSD License