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A Python Ethernet/IP library for communicating with Allen-Bradley PLCs.

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pycomm3 started as a Python 3 fork of pycomm, which is a Python 2 library for communicating with Allen-Bradley PLCs using Ethernet/IP. The initial Python 3 port was done in this fork and was used as the base for pycomm3. Since then, the library has been almost entirely rewritten and the API is no longer compatible with pycomm. Without the hard work done by the original pycomm developers, pycomm3 would not exist. This library seeks to expand upon their great work.


pycomm3 includes 3 drivers:

  • CIPDriver
    This driver is the base driver for the library, it handles common CIP services used by the other drivers. Things like opening/closing a connection, register/unregister sessions, forward open/close services, device discovery, and generic messaging.
  • LogixDriver
    This driver supports services specific to ControlLogix, CompactLogix, and Micro800 PLCs. Services like reading/writing tags, uploading the tag list, and getting/setting the PLC time.
  • SLCDriver
    This driver supports basic reading/writing data files in a SLC500 or MicroLogix PLCs. It is a port of the SlcDriver from pycomm with minimal changes to make the API similar to the other drivers. Currently this driver is considered legacy and it’s development will be on a limited basis.


PLCs can be used to control heavy or dangerous equipment, this library is provided “as is” and makes no guarantees on its reliability in a production environment. This library makes no promises in the completeness or correctness of the protocol implementations and should not be solely relied upon for critical systems. The development for this library is aimed at providing quick and convenient access for reading/writing data inside Allen-Bradley PLCs.


The package can be installed from PyPI using pip: pip install pycomm3 or python -m pip install pycomm3.

Optionally, you may configure logging using the Python standard logging library. A convenience method is provided to help configure basic logging, see the Logging Section in the docs for more information.

Python and OS Support

pycomm3 is a Python 3-only library. The minimum supported version of Python is 3.6.1 and has been tested up to 3.9. There should be no OS-specific requirements and should be able to run on any OS that Python is supported on. Development and testing is done primarily on Windows 10. If you encounter an OS-related problem, please open an issue in the GitHub repository and it will be investigated.


Python 3.6.0 is not supported due to NamedTuple not supporting default values and methods until 3.6.1


This README covers a basic overview of the library, full documentation can be found on Read the Docs or by visiting


If you’d like to contribute or are having an issue, please read the Contributing guidelines.

Highlighted Features

  • generic_message for extra functionality not directly implemented
    • working similar to the MSG instruction in Logix, arguments similar to the MESSAGE properties
    • See the examples section for things like getting/setting drive parameters, IP configuration, or uploading an EDS file
    • used internally to implement some of the other methods (get/set_plc_time, forward open/close, etc)
  • simplified data types
    • allows use of standard Python types by abstracting CIP implementation details away from the user
    • strings use normal Python str objects, does not require handling of the LEN and DATA attributes separately
    • custom string types are also identified automatically and not limited to just the builtin one
    • BOOL arrays use normal Python bool objects, does not require complicated bit shifting of the DWORD value
    • powerful type system to allow types to represent any CIP object and handle encoding/decoding the object


  • simple API, only 1 read method and 1 write method for tags.
    • does not require using different methods for different data types
    • requires the tag name only, no other information required from the user
    • automatically manages request/response size to pack as many requests into a single packet
    • automatically handles fragmented requests for large tags that can’t fit in a single packet
    • both support full structure reading/writing (UDTs, AOIs, etc)
      • for read the Tag.value will be a dict of {attribute: value}
      • for write the value should be a sequence of values or dict of {attribute: value} , nesting as needed
        • does not do partial writes, the value must match the complete structure
        • not recommended for builtin type (TIMER, CONTROL, COUNTER, etc)
      • both require no attributes to have an External Access of None
  • uploads the tag list and data type definitions from the PLC
    • no requirement for user to determine tags available (like from an L5X export)
    • definitions are required for read/write methods
  • automatically enables/disables different features based on the target PLC
    • Extended Forward Open (EN2T or newer and v20+)
    • Symbol Instance Addressing (Logix v21+)
    • detection of Micro800 and disables unsupported features (CIP Path, Ex. Forward Open, Instance Addressing, etc)

LogixDriver Overview

Creating a driver is simple, only a path argument is required. The path can be the IP address, IP and slot, or a full CIP route, refer to the documentation for more details. The example below shows how to create a simple driver and print some of the information collected about the device.

from pycomm3 import LogixDriver

with LogixDriver('') as plc:
    # OUTPUT:
    # Program Name: PLCA, Device: 1756-L83E/B, Revision: 28.13

    # OUTPUT:
    # {'vendor': 'Rockwell Automation/Allen-Bradley', 'product_type': 'Programmable Logic Controller',
    #  'product_code': 166, 'version_major': 28, 'version_minor': 13, 'revision': '28.13', 'serial': 'FFFFFFFF',
    #  'device_type': '1756-L83E/B', 'keyswitch': 'REMOTE RUN', 'name': 'PLCA'}

Reading/Writing Tags

Reading or writing tags is as simple as calling the read and write methods. Both methods accept any number of tags, and will automatically pack multiple tags into a Multiple Service Packet Service (0x0A) while making sure to stay below the connection size. If there is a tag value that cannot fit within the request/reply packet, it will automatically handle that tag independently using the Read Tag Fragmented (0x52) or Write Tag Fragmented (0x53) requests.

Both methods will return Tag objects to reflect the success or failure of the operation.

class Tag(NamedTuple):
    tag: str  # the name of the tag, does not include ``{<# elements>}`` from request
    value: Any  # value read or written, may be ``None`` if an error occurred
    type: Optional[str] = None  # data type of tag, including ``[<# elements>]`` from request
    error: Optional[str] = None  # ``None`` if successful, else the CIP error or exception thrown

Tag objects are considered successful (truthy) if the value is not None and the error is None.


with LogixDriver('') as plc:'tag1', 'tag2', 'tag3')  # read multiple tags'array{10}') # read 10 elements starting at 0 from an array'array[5]{20}) # read 20 elements starting at elements 5 from an array'string_tag')  # read a string tag and get a string'a_udt_tag') # the response .value will be a dict like: {'attr1`: 1, 'attr2': 'a string', ...}

    # writes require a sequence of tuples of [(tag name, value), ... ]
    plc.write('tag1', 0)  # single writes do not need to be passed as a tuple
    plc.write(('tag1', 0), ('tag2', 1), ('tag3', 2))  # write multiple tags
    plc.write(('array{5}', [1, 2, 3, 4, 5]))  # write 5 elements to an array starting at the 0 element
    plc.write('array[10]{5}', [1, 2, 3, 4, 5])  # write 5 elements to an array starting at element 10
    plc.write('string_tag', 'Hello World!')  # write to a string tag with a string
    plc.write('string_array[2]{5}', 'Write an array of strings'.split())  # write an array of 5 strings starting at element 2
    plc.write('a_udt_tag', {'attr1': 1, 'attr2': 'a string', ...})  # can also use a dict to write a struct

    # Check the results
    results ='tag1', 'tag2', 'tag3')
    if all(results):
        print('They all worked!')
        for result in results:
            if not result:
                print(f'Reading tag {result.tag} failed with error: {result.error}')


Tag names for both read and write are case-sensitive and are required to be the same as they are named in the controller. This may change in the future.

Unit Testing

pytest is used for unit testing. The tests directory contains an L5X export of the testing program that contains all tags necessary for testing. The only requirement for testing (besides a running PLC with the testing program) is the environment variable PLCPATH for the PLC defined.

User Tests

These tests are for users to run. There are a few tests that are specific to a demo plc, those are excluded. To run them you have the following options:

with tox:

  • modify the PLCPATH variable in tox.ini
  • then run this command: tox -e user

or with pytest:

pytest --ignore tests/online/

(or the equivalent in your shell)


Test coverage is not complete, pull requests are welcome to help improve coverage.


pycomm3 is distributed under the MIT License

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