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Easy-to-use Modbus RTU and Modbus ASCII implementation for Python

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Easy-to-use Modbus RTU and Modbus ASCII implementation for Python.

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MinimalModbus is an easy-to-use Python module for talking to instruments (slaves) from a computer (master) using the Modbus protocol, and is intended to be running on the master. The only dependence is the pySerial module (also pure Python).

There are convenience functions to handle floats, strings and long integers (in different byte orders).

This software supports the ‘Modbus RTU’ and ‘Modbus ASCII’ serial communication versions of the protocol, and is intended for use on Linux, OS X and Windows platforms. It is open source, and has the Apache License, Version 2.0.

Tested with Python 2.7, 3.5, 3.6 and 3.7.

Support for Python2 will be dropped in 2020.

This package uses semantic versioning.


Release 1.0.2 (2019-08-11)

  • Adjusted settings for hardware tests

  • Improved developer documentation

Release 1.0.1 (2019-08-10)

  • Corrected version number

Release 1.0.0 (2019-08-10)

New features:

  • Implements reading and writing multiple bits simultaneously.

  • Support more byteorders (endianness) for floats and long integers.

Breaking changes:

  • Renamed method arguments ‘numberOfDecimals’, ‘numberOfRegisters’ to ‘number_of_decimals’, ‘number_of_registers’

  • Removed example drivers for Eurotherm 3500 and Omegacn 7500, as I no longer have access to these instruments for testing. It would great if someone would pick up support for these instruments in a separate project.

  • Requires pyserial 3.0 or later.

  • Removed module level constants for default values, as they were confusingly named.

Other fixes:

  • Allow slave addresses also in the reserved range (up to 255). Reported by GitHub user gnbl.

  • Serial port read and write buffers are cleared before each request to the instrument. Pull request from GitHub user mrrs6.

  • Check whether the serial port is open before trying to open it. Reported by Matthias Bolte.

  • Custom exceptions for Modbus errors, by Russ Garrett.

  • Silent period between messages is at least 1.75 ms to fulfill Modbus standard. Reported by GitHub user draput.

  • Use time.monotonic if available. Suggested by Matthias Bolte.

  • Implemented write timeout, to avoid hanging when writing. Instead it will raise an exception. Reported by Austin Stover.

  • Better checking of number of registers when reading and writing.

  • Rename internal methods and variables to be PEP8 compliant.

  • Improved documentation.

Release 0.7 (2015-07-30)

  • Faster CRC calculation by using a lookup table (thanks to Peter)

  • Handling of local echo (thanks to Luca Di Gregorio)

  • Improved behavior of dummy_serial (number of bytes to read)

  • Improved debug messages (thanks to Dino)

  • Using project setup by the cookie-cutter tool.

  • Reshuffled source files and documentation.

  • Moved source to GitHub from Sourceforge.

  • Moved documentation to

  • Using the tox tool for testing on multiple Python versions.

  • Using Travis CI test framework

  • Using code coverage measurement framework

  • Added support for Python 3.3 and 3.4.

  • Dropped support for Python 2.6.

Release 0.6 (2014-06-22)

  • Support for Modbus ASCII mode.

Release 0.5 (2014-03-23)

  • Precalculating number of bytes to read, in order to increase the speed.

  • Better handling of several instruments on the same serial port, especially for Windows.

  • Improved timing for better compliance with Modbus timing requirements.

Release 0.4 (2012-09-08)

  • Read and write multiple registers.

  • Read and write floating point values.

  • Read and write long integers.

  • Read and write strings.

  • Support for negative numbers.

  • Use of the Python struct module instead of own bit-tweaking internally.

  • Improved documentation.

Release 0.3.2 (2012-01-25)

  • Fine-tuned for smoother installation.

  • Improved documentation.

Release 0.3.1 (2012-01-24)

  • Improved requirements handling in

  • Adjusted not to include doc/_templates

  • Adjusted RST text formatting in README.txt

Release 0.3 (2012-01-23)

This is a major rewrite, but the API is backward compatible.

  • Extended functionality to support more Modbus function codes.

  • Option to close the serial port after each call (useful for Windows XP etc).

  • Diagnostic string output available (for support).

  • Debug mode available.

  • Improved __repr__ for Instrument instances.

  • Improved Python3 compatibility.

  • Improved validity checking for function arguments.

  • The error messages are made more informative.

  • The new example driver omegacn7500 is included.

  • Unit tests included in the distribution.

  • A dummy serial port for unit testing is provided (including recorded communication data).

  • Updated documentation.

Release 0.2 (2011-08-19)

  • Changes in how to reference the serial port.

  • Updated documentation.

Release 0.1 (2011-06-16)

  • First public release.

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