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Easy-to-use Modbus RTU implementation for Python

Project description


MinimalModbus is an easy-to-use Python module for talking to instruments (slaves) from a computer (master) using the Modbus protocol. Example code includes drivers for Eurotherm and Omega process controllers. The only dependence is the pySerial module.

This software supports the ‘Modbus RTU’ serial communication version 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 Python2.6, Python2.7 and Python3.2.

Home page

Home page with full API documentation (this page if viewed on

Python package index (PyPI) with download (this page if viewed on Note that no API is available). The download section is at the end of the page.

The SourceForge project page with mailing list and subversion repository ( ).

General on Modbus protocol

Modbus is a serial communications protocol published by Modicon in 1979, according to It is often used to communicate with industrial electronic devices.

There are several types of Modbus protocols:

Modbus RTU

A serial protocol that uses binary representation of the data. Supported by this software.

Modbus ASCII

A serial protocol that uses ASCII representation of the data. Not supported by this software.

Modbus TCP/IP and variants

A protocol for communication over TCP/IP networks. Not supported by this software.

For full documentation on the Modbus protocol, see

Two important documents are:

Typical hardware

The application for which I wrote this software is to read and write data from Eurotherm process controllers. These come with different types of communication protocols, but the controllers I prefer use the Modbus RTU protocol. MinimalModbus is intended for general communication using the Modbus RTU protocol (using a serial link), so there should be lots of applications.

As an example on the usage of MinimialModbus, the driver I use for an Eurotherm 3504 process controller is included. It uses the MinimalModbus Python module for its communication. Also a driver for Omega CN7500 is included. For hardware details on these process controllers, see Eurotherm 3500 and Omega CN7500.

There can be several instruments (slaves) on a single bus, and the slaves have addresses in the range 1 to 247. In the Modbus RTU protocol, only the master can initiate communication. The physical layer is most often the serial bus RS485, which is described at

To connect your computer to the RS485 bus, a serial port is required. There are direct USB-to-RS485 converters, but I use a USB-to-RS232 converter together with an industrial RS232-to-RS485 converter. This has the advantage that the latter is galvanically isolated using opto-couplers, and has transient supression. This software has been tested using a Westermo MDW-45 RS232-to-RS485 converter.

Typical usage

The instrument is typically connected via a serial port, and a USB-to-serial adaptor should be used on most modern computers. How to configure such a serial port is described on the pySerial page:

For example, consider an instrument (slave) with address number 1 to which we are to communicate via a serial port with the name /dev/ttyUSB1. The instrument stores the measured temperature in register 289. For this instrument a temperature of 77.2 C is stored as 772, why we use 1 decimal. To read this data from the instrument:

#!/usr/bin/env python
import minimalmodbus

instrument = minimalmodbus.Instrument('/dev/ttyUSB1', 1) # port name, slave address (in decimal)

## Read temperature (PV = ProcessValue) ##
temperature = instrument.read_register(289, 1) # Registernumber, number of decimals
print temperature

## Change temperature setpoint (SP) ##
instrument.write_register(24, NEW_TEMPERATURE, 1) # Registernumber, value, number of decimals for storage

The full API for MinimalModbus is available on, and the documentation in PDF format is found on


It is better to put the details in a driver for the specific instrument. An example driver for Eurotherm3500 is included in this library, and it is recommended to have a look at its source code. To get the process value (PV from loop1):

#!/usr/bin/env python
import eurotherm3500

heatercontroller = eurotherm3500.Eurotherm3500('/dev/ttyUSB1', 1)  # port name, slave address

## Read temperature (PV) ##
temperature = heatercontroller.get_pv_loop1()
print temperature

## Change temperature setpoint (SP) ##

Correspondingly, to use the driver for Omega CN7500:

#!/usr/bin/env python
import omegacn7500

instrument = omegacn7500.OmegaCN7500('/dev/ttyUSB1', 1) # port name, slave address

print instrument.get_pv() # print temperature

More on the usage of MinimalModbus is found on

Default values

Most of the serial port parameters have the default values defined in the Modbus standard:

instrument.serial.port          # this is the serial port name
instrument.serial.baudrate = 19200   # Baud
instrument.serial.parity   = serial.PARITY_NONE
instrument.serial.bytesize = 8
instrument.serial.stopbits = 1
instrument.serial.timeout  = 0.05   # seconds

instrument.address     # this is the slave address number

These can be overridden:

instrument.serial.timeout = 0.2

For details on the allowed parity values, see


Python versions 2.6 and higher are supported (including 3.x). Tested with Python2.6, Python2.7 and Python3.2.

This module relies on pySerial to do the heavy lifting, and it is the only dependency. You can find it at the Python package index:

Download and installation

From command line (if you have the pip installer, available at

pip install -U minimalmodbus

or possibly:

pip install -U pyserial
pip install -U minimalmodbus

You can also manually download the compressed source files from (see the end of that page). In that case you first need to manually install pySerial from

There are compressed source files for Unix/Linux (.tar.gz) and Windows (.zip). To install a manually downloaded file, uncompress it and run (from within the directory):

python install

or possibly:

sudo python install

If using Python 3, then install with:

sudo python3 install

There is also a Windows installer (.exe) available. Just start it and follow the instructions.

For Python3 there might be problems with easy_install and pip. In that case, first manually install pySerial and then manually install MinimalModbus.

To make sure it is installed properly, print the _getDiagnosticString() message. See the support section below for instructions.

Implemented functions

These are the functions to use for reading and writing registers and bits of your instrument. Study the documentation of your instrument to find which Modbus function code to use.





read_bit() Function code 2 [or 1]

write_bit() Function code 5 [or 15]


read_register() Function code 3 [or 4]

write_register() Function code 16 [or 6]

Modbus implementation details

Note that the computer (master) actually is a client, and the instruments (slaves) are servers.

In Modbus RTU, the request message is sent from the master in this format:

Slave address [1 Byte], Function code [1 Byte], Payload data [0 to 252 Bytes], CRC [2 Bytes].
  • For the function code, the allowed range is 1 to 127 (in decimal).

  • The CRC is a cyclic redundancy check code, for error checking of the message.

  • The response from the client is similar, but with another payload data.

Function code (in decimal)

Payload data to slave (Request)

Payload data from slave (Response)

1 Read bits (coils)

Start address [2 Bytes], Number of coils [2 Bytes]

Byte count [1 Byte], Value [k Bytes]

2 Read discrete inputs

Start address [2 Bytes], Number of inputs [2 Bytes]

Byte count [1 Byte], Value [k Bytes]

3 Read holding registers

Start address [2 Bytes], Number of registers [2 Bytes]

Byte count [1 Byte], Value [n*2 Bytes]

4 Read input registers

Start address [2 Bytes], Number of registers [2 Bytes]

Byte count [1 Byte], Value [n*2 Bytes]

5 Write single bit (coil)

Output address [2 Bytes], Value [2 Bytes]

Output address [2 Bytes], Value [2 Bytes]

6 Write single register

Register address [2 Bytes], Value [2 Bytes]

Register address [2 Bytes], Value [2 Bytes]

15 Write multiple bits (coils)

Start address [2 Bytes], Number of outputs [2 Bytes], Byte count [1 Byte], Value [k Bytes]

Start address [2 Bytes], Number of outputs [2 Bytes]

16 Write multiple registers

Start address [2 Bytes], Number of registers [2 Bytes], Byte count [1 Byte], Value [n*2 Bytes]

Start address [2 Bytes], Number of registers [2 Bytes]

For function code 5, the only valid values are 0000 (hex) or FF00 (hex), representing OFF and ON respectively.

It is seen in the table above that the request and response messages are similar for function code 1 to 4. The same can be said about function code 5 and 6, and also about 15 and 16.

For finding how the k Bytes for the value relates to the number of registers etc (n), see the Modbus documents referred to above.

Issues when running under Windows

When running under Windows, the underlying pySerial may complain that the serial port is already open. This seems to occur especially when communicating with more than one instrument. It is possible to make MinimalModbus close the serial port after each call. Use it like:

#!/usr/bin/env python
import minimalmodbus
minimalmodbus.CLOSE_PORT_AFTER_EACH_CALL = True

instrument = minimalmodbus.Instrument('/dev/ttyUSB1', 1) # port name, slave address (in decimal)
print instrument.read_register(289, 1)


Apache License, Version 2.0.


Send a mail to

Describe the problem in detail, and include any error messsages. Please also include the output after running:

>>> import minimalmodbus
>>> print minimalmodbus._getDiagnosticString()

Note that it can be very helpful to switch on the debug mode, where the communication details are printed. See the ‘Develop’ section below.


Jonas Berg,


Significant contributions by Aaron LaLonde.


If you find this software useful, then please leave a review on the SourceForge project page (Log-in is required).

Please also subscribe to the (low volume) mailing list (see so you can help other users getting started.


To switch on the debug mode, where the communication details are printed:

#!/usr/bin/env python
import minimalmodbus

instrument = minimalmodbus.Instrument('/dev/ttyUSB1', 1) # port name, slave address (in decimal)
instrument.debug = True
print instrument.read_register(289, 1)  # Remember to use print() for Python3

With this you can easily see what is sent to and from your instrument, and immediately see what is wrong.

The data is stored internally in this driver as byte strings (representing byte values). For example a byte with value 18 (dec) = 12 (hex) = 00010010 (bin) is stored in a string of length one. This can be done using the function chr(18) or typing the string \x12.

Note that the letter A has the hexadecimal ASCII code 41, why the string \x41 prints ‘A’.

These strings can look pretty strange when printed, as values 0 to 31 (dec) are ASCII control signs (not corresponding to any letter). For example ‘vertical tab’ and ‘line feed’ are among those. To make the output easier to understand, use:

print repr(bytestringname)

Then you can find the value and use an ASCII table to see if it is correct.

The details printed in debug mode (messages and responses) are very useful for using the included dummy_serial port for unit testing purposes. For examples, see the file test/

More implementation details are found on

Unit testing

Unit tests are provided in the test subfolder. To run them:


Also a dummy/mock/stub for the serial port, dummy_serial, is provided for test purposes. See

The test coverage analysis is found at To see which parts of the code that have been tested, click the corresponding file name.


Text revision

This README file was changed (committed) at $Date: 2012-01-24 14:04:54 +0100 (Tue, 24 Jan 2012) $, which was $Revision: 127 $.

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