Getting Started

Creating a Driver

Drivers are simple to create and use, the quickest way is to use them within a context manager (with statement). Most of the examples in the documentation will shown them used in that way. If you are using them as part of a larger program or creating long-lived connections, you may not want to use the context manager in this case. When used outside a context manager, you will need to call the open() method first and the close() method on shutdown. Failing to close the connection could cause issues communicating with the device. Each driver opens a single connection to the device, you may use multiple instances to create multiple connections. It is also the user’s responsibility to maintain the connection, the drivers do not implement any periodic handshaking. The default timeout is fairly long, but a long lived connection will need to issue a request usually at least once a minute or the PLC may close the connection.

Each driver requires a path argument, this is a CIP path to the destination device. The paths used in pycomm3 are similar to how they appear in Logix.

There are three possible forms:

IP Address Only (

Use for devices without a backplane (drives, switches, Micro800 PLCs, etc) or for PLCs in slot 0 of a backplane.

IP Address/Slot (

Use for PLCs in a backplane that are not in slot 0.

CIP Routing Path (

This is a full CIP route to a device, it should appear similar to how paths are shown in Logix. For port selection, use backplane or bp for the backplane and enet for the ethernet port. Both slash (/) and backslash (\) are supported.


Both the IP Address and IP Address/Slot options are shortcuts, they will be replaced with the CIP path automatically.

>>> from pycomm3 import CIPDriver
>>> with CIPDriver('') as drive:
>>>     print(drive)
Device: AC Drive, Revision: 1.2

The default behavior is to use the Extended Forward Open service when opening a connection. This allows the use of ~4KB of data for each request, the standard is only ~500 bytes. Although this requires the communications module to be an EN2T or newer and the PLC firmware to be version 20 or newer. Upon opening a connection, the CIPDriver will attempt an Extended Forward Open, if that fails it will then try using the standard Forward Open.

Creating a LogixDriver

The LogixDriver has two additional arguments:

init_tags (default True)

When true, the driver will upload all tags in the PLC and the definitions for any UDTs and AOIs. These definitions are required for the read() and write() methods to work.

init_program_tags (default True)

When uploading the tag list, if True all program scoped tags are uploaded. Set False to upload controller-scoped tags only. This arg is only checked if init_tags is True.

There is some data that is collected about the target controller when a connection is first established. It will call both the get_plc_info() and get_plc_name() methods. get_plc_info() returns a dict of the info collected and stores that information, making it accessible from the info property. get_plc_name() will return the name of the program running in the PLC and store it in info['name']. See info for details on the specific fields.

After the controller info has been retrieved, the driver will begin uploading the tag list unless init_tags option has not been set False. Depending on the number of tags, the PLC model, and other factors, the tag list could take some time to upload. A very large tag list on an old processor with high CPU utilization could take 10-15 seconds, while a small tag list or a new processor might take <1 second. If you are setting up multiple drivers on the same PLC, startup time can be saved by uploading the tag list in the first driver and disabling init_tags in the others. Then you can pass the uploaded tag list from the first driver to the other drivers, shown below.

from pycomm3 import LogixDriver
first_plc = LogixDriver('')  # uploads the tag list
second_plc = LogixDriver('', init_tags=False)
second_plc._tags = first_plc.tags  # doesn't upload any tags

Creating a SLCDriver

Currently, there is no additional configuration for a SLCDriver over a CIPDriver.

Response Tag Object

Many methods return a Tag object, like generic_message() or the read and write methods of the LogixDriver or SLCDriver. The truthiness of a Tag object represents the status of a request. A successful request will have a value that is not None and the error attribute is None. Anything otherwise will be a failed request. The error attribute will contain either the CIP error message or exception raised during the request.

class pycomm3.Tag(tag, value, type, error)[source]

True if both value is not None and error is None, False otherwise

property tag

tag name of tag read/written or request name (generic message)

property value

value read/written, may be None on error

property type

data type of tag

property error

error message if unsuccessful, else None

Data Types

Data types are a major component of pycomm3, they are classes used to represent any tag or CIP object. They are able to encode and decode to and from Python values and bytes. Atomic and structure values along with arrays of either are supported. Each elementary (primitive) data type is provided as well as some common derived (structure of elementary types) types. See the Data Types for all available CIP types and Custom Types for any pycomm3 provided custom types. The type classes provide two class methods: encode and decode. These are class methods, meaning they do not require an instance of they type to be created. In fact, the only time an instance of a type is used is when added members (with a name) to a structure. The encode method takes a Python object and encodes it to bytes. The decode method takes bytes and returns the corresponding Python object.

Elementary Types

Also known as primitives, these types are the building blocks for all CIP data types. These are basic types that store a single value, like integers, floats, strings, etc. All of these types can be imported directly from pycomm3, for a full list of the types refer to Data Types.

>>> from pycomm3 import DINT, SHORT_STRING
>>> DINT.encode(112233)
>>> DINT.decode(b'\x12\x34\x56\x78')
>>> SHORT_STRING.encode('Hello there!')
b'\x0cHello there!'
>>> SHORT_STRING.decode(b'\x0eGeneral Kenobi')
'General Kenobi'

Structure Types

Structures are complex types composed of any number of different elementary or struct member types. The Struct() factory is used to create new struct types. To create a new struct, a list of members is required. Members must be DataType, either classes (unnamed) or instance (named). Creating named members is really the only time a user would create an instance of a type. When decoding a struct, the value is returned as dictionary of {member_name: value}. Any unnamed members will be excluded from the return value, also since the return value is a dict, member names should be unique.

>>> from pycomm3 import Struct, DINT, STRING, REAL
>>> MyStruct = Struct(DINT('code'), STRING('name'), REAL('value'))
>>> struct_values = {
...     'code': 80,
...     'name': 'my name',
...     'value': 123.45
... }
>>> MyStruct.encode(struct_values)
b'P\x00\x00\x00\x07\x00my namef\xe6\xf6B'
>>> YourStruct = Struct(DINT, DINT('code'), DINT('type'))
>>> YourStruct.decode(your_bytes)  # assume your_bytes is an encoded YourStruct
{'code': 34, 'type': 73}  # notice the first member is unnamed and not included

Both dictionaries and sequences are supported for encoding structs. In the first example, we could have done: struct_values = [80, 'my name', 123.45] and gotten the same result. When encoding a struct with multiple unnamed members, using a list of values is the easiest solution. To use a dict you must include a None key and value to be used for the unnamed members. But, if there are multiple unnamed members of incompatible types, you will have to use a list/sequence instead.


Arrays are a homogenous sequence of a DataType (either elementary or structs). Any type can be used to create an array of that type using the [] operator or the Array() factory. There are two important components for an array, the element type and the length. The element type is the DataType and the length specifies the number of elements. The length has 3 options:


Where the length is specified as an int, the array length is fixed to that number of elements.

>>> SINT[5].encode([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])  # notice it only encodes/decodes 5 elements
>>> SINT[5].decode(b'\x01\x02\x03\x04\x05\x06\x07\x08\t\n')
[1, 2, 3, 4, 5]

Where the length is specified as a DataType. When decoding an array, the length will be decoded first using the type specified and then decoded that many elements. Encoding will encode however many values are supplied, but does not add the encoded length.

>>> SINT[SINT].encode([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])  # length type is not used when encoding
>>> SINT[SINT].decode(b'\x05\x01\x02\x03\x04\x05\x00\x00\x00')
[1, 2, 3, 4, 5]

Where the length is None. When decoding, the array will consume the entire byte buffer and decode as many elements as possible. Encoding will encode however many values are supplied.

>>> SINT[None].encode([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])  # length type is not used when encoding
>>> SINT[None].decode(b'\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0A')
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]


This library uses the standard Python logging module. You may configure the logging module as needed. The DEBUG level will log every sent/received packed and other diagnostic data. Set the level to higher than DEBUG if you only wish to see errors, exceptions, etc. A helper method called configure_default_logger is provided to setup basic logging. There are two optional parameters, level and filename. level (default logging.INFO) is the logging level. filename (default None) if set, will also log to the specified file.

from pycomm3.logger import configure_default_logger


Produces output similar to:

2021-02-26 14:37:41,389 [DEBUG] Opening connection to
2021-02-26 14:37:41,393 [DEBUG] pycomm3.cip_driver.CIPDriver.send(): Sent: RegisterSessionRequestPacket(message=[b'\x01\x00', b'\x00\x00'])
2021-02-26 14:37:41,397 [DEBUG] pycomm3.cip_driver.CIPDriver.send(): Received: RegisterSessionResponsePacket(session=184719106, error=None)
2021-02-26 14:37:41,398 [INFO] pycomm3.cip_driver.CIPDriver._register_session(): Session=184719106 has been registered.
2021-02-26 14:37:41,398 [INFO] pycomm3.logix_driver.LogixDriver._initialize_driver(): Initializing driver...

pycomm3 also uses a custom logging level for verbose logging, this level also prints the contents of each packet send and received. If submitting a bug report, this level of logging is the most helpful.

from pycomm3.logger import configure_default_logger, LOG_VERBOSE
configure_default_logger(level=LOG_VERBOSE, filename='c:/tmp/pycomm3.log')

Verbose output:

2021-02-26 14:42:36,752 [DEBUG] Opening connection to
2021-02-26 14:42:36,765 [VERBOSE] pycomm3.cip_driver.CIPDriver._send(): >>> SEND >>>
(0000) 65 00 04 00 00 00 00 00 00 00 00 00 5f 70 79 63     e•••••••••••_pyc
(0010) 6f 6d 6d 5f 00 00 00 00 01 00 00 00                 omm_••••••••
2021-02-26 14:42:36,766 [DEBUG] pycomm3.cip_driver.CIPDriver.send(): Sent: RegisterSessionRequestPacket(message=[b'\x01\x00', b'\x00\x00'])
2021-02-26 14:42:36,768 [VERBOSE] pycomm3.cip_driver.CIPDriver._receive(): <<< RECEIVE <<<
(0000) 65 00 04 00 02 98 02 0b 00 00 00 00 5f 70 79 63     e•••••••••••_pyc
(0010) 6f 6d 6d 5f 00 00 00 00 01 00 00 00                 omm_••••••••
2021-02-26 14:42:36,768 [DEBUG] pycomm3.cip_driver.CIPDriver.send(): Received: RegisterSessionResponsePacket(session=184719362, error=None)
2021-02-26 14:42:36,769 [INFO] pycomm3.cip_driver.CIPDriver._register_session(): Session=184719362 has been registered.
2021-02-26 14:42:36,769 [INFO] pycomm3.logix_driver.LogixDriver._initialize_driver(): Initializing driver...
2021-02-26 14:42:36,769 [VERBOSE] pycomm3.cip_driver.CIPDriver._send(): >>> SEND >>>
(0000) 63 00 00 00 02 98 02 0b 00 00 00 00 5f 70 79 63     c•••••••••••_pyc
(0010) 6f 6d 6d 5f 00 00 00 00                             omm_••••
2021-02-26 14:42:36,769 [DEBUG] pycomm3.cip_driver.CIPDriver.send(): Sent: ListIdentityRequestPacket(message=[])
2021-02-26 14:42:36,771 [VERBOSE] pycomm3.cip_driver.CIPDriver._receive(): <<< RECEIVE <<<
(0000) 63 00 45 00 02 98 02 0b 00 00 00 00 5f 70 79 63     c•E•••••••••_pyc
(0010) 6f 6d 6d 5f 00 00 00 00 01 00 0c 00 3f 00 01 00     omm_••••••••?•••
(0020) 00 02 af 12 c0 a8 01 ec 00 00 00 00 00 00 00 00     ••••••••••••••••
(0030) 01 00 0c 00 bf 00 14 13 30 00 90 be 1e c0 1d 31     ••••••••0••••••1
(0040) 37 36 39 2d 4c 32 33 45 2d 51 42 46 43 31 20 45     769-L23E-QBFC1 E
(0050) 74 68 65 72 6e 65 74 20 50 6f 72 74 03              thernet Port•