Documentation Status


Note This is a Major release and might affect your existing Async client implementation. Refer examples on how to use the latest async clients.


Pymodbus is a full Modbus protocol implementation using twisted for its asynchronous communications core. It can also be used without any third party dependencies (aside from pyserial) if a more lightweight project is needed. Furthermore, it should work fine under any python version > 2.3 with a python 3.0 branch currently being maintained as well.


Client Features

  • Full read/write protocol on discrete and register
  • Most of the extended protocol (diagnostic/file/pipe/setting/information)
  • TCP, UDP, Serial ASCII, Serial RTU, and Serial Binary
  • asynchronous(powered by twisted) and synchronous versions
  • Payload builder/decoder utilities

Server Features

  • Can function as a fully implemented modbus server
  • TCP, UDP, Serial ASCII, Serial RTU, and Serial Binary
  • asynchronous(powered by twisted) and synchronous versions
  • Full server control context (device information, counters, etc)
  • A number of backing contexts (database, redis, a slave device)

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.sync 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 respository. If you have created any utilities that meet a specific need, feel free to submit them so others can benefit.

Also, if you have questions, please ask them on the mailing list so that others can benefit from the results and so that I can trace them. I get a lot of email and sometimes these requests get lost in the noise: or at gitter:


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):

easy_install -U pymodbus
pip install  -U pymodbus

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

git clone git://
cd pymodbus
python install

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

If you would like to install pymodbus without the twisted dependency, simply edit the file before running easy_install and comment out all mentions of twisted. It should be noted that without twisted, one will only be able to run the synchronized version as the asynchronous versions uses twisted for its event loop.

Current Work In Progress

Since I don’t have access to any live modbus devices anymore it is a bit hard to test on live hardware. However, if you would like your device tested, I accept devices via mail or by IP address.

That said, the current work mainly involves polishing the library as I get time doing such tasks as:

  • Make PEP-8 compatible and flake8 ready
  • Fixing bugs/feature requests
  • Architecture documentation
  • Functional testing against any reference I can find
  • The remaining edges of the protocol (that I think no one uses)

Development Instructions

The current code base is compatible with both py2 and py3. Use make to perform a range of activities

$ make
   Makefile for pymodbus


 make install    install the package in a virtual environment
 make reset      recreate the virtual environment
 make check      check coding style (PEP-8, PEP-257)
 make test       run the test suite, report coverage
 make tox        run the tests on all Python versions
 make clean      cleanup all temporary files


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

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,
  • Twisted Matrix

Released under the BSD License