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ie-databus

This module makes reading from the IE Databus as simple as possible, allowing access to all of the data streamed through MQTT in real-time. The latest update also allows for writing to the IE Databus, which means data can be written from an edge app to the PLC. This module can even be used on flaskboard-lite.

This README is a word-for-word copy of my original User Guide which can be found here.

The ie-databus module itself can be found here.

Actual documentation can be found here.

The example code used throughout this guide can be found here.

Basic Usage

Only three lines are necessary to set up and get connected to the edge device’s IE Databus.

from ie_databus import IEDatabus
databus = IEDatabus('edge', 'edge')
databus.start()
  1. Pretty straightforward - simply imports the IEDatabus class from the ie_databus package.
  2. This line initializes the IEDatabus class and prepares the MQTT client for connection. The two parameters are the username and password for the IE Databus, respectively.
  3. Calling the start() method connects to the edge device’s IE Databus and processes the tag headers. This method blocks until tag data is available.

Note: The rest of this guide assumes that the above code snippet is already included.

Tags Overview

Data received from the IE Databus is organized in a dictionary of Tag objects. Tag objects expose all of the key-value pairs from the raw MQTT JSON data as object attributes.

Accessing this dictionary is simple:

print(databus.tags)
{'Q_VFD1_Temperature': Tag(name='Q_VFD1_Temperature', id='101', data_type='Real', qc=3, ts='2022-08-02T18:02:50.430Z', [...]

The output is a Python dict, specifically of type Dict[str, Tag]. The keys represent the name of a PLC tag and the value is a Tag object with data about its respective tag.

Tag objects have 7 relevant attributes:

Name Type Description
name str The name of the PLC tag
id str The ID of the PLC tag
data_type str The original data type of the val attribute
qc int
qx int
ts str The timestamp of when this data was received
val float The current value of the PLC tag

See Reading PLC Tags for more info on how to use Tag objects.

Reading PLC Tags

Reading a specific PLC tag is simple; all you need is the name of the PLC tag you’re trying to read.

print(databus.tags['Q_VFD3_Temperature'].val)
68.12994384765625

The example above prints out the current value of Conveyor 3’s temperature.

You can easily read multiple tags too.

current_tags = databus.tags
tag_names = ['M_R01_S', 'M_R01_L', 'M_R01_U', 'M_R01_R', 'M_R01_B', 'M_R01_T']
for tag in tag_names:
    print(current_tags[tag].val)
0.0018717440543696284
0.0007486974936909974
89.99854278564453
0.0012352190678939223
-90.00113677978516
0.001852828892879188

The example above will print out the values for each tag listed in tag_names. Notice how the tags attribute of the databus is only accessed once – otherwise, it is possible that a new set of data could come in and newer values would be accessed. Having tags with different timestamps could potentially create undesirable effects.

Values aren’t the only thing that can be read from PLC tags. Any of the attributes of the Tag class can be accessed using the databus.tags dictionary.

print(databus.tags['I_R03_GripperLoad'])
print(databus.tags['I_Conveyor1Status_NIST_A'].ts)
Tag(name='I_R03_GripperLoad', id='112', data_type='Int', qc=3, ts='2022-08-02T18:15:38.178Z', val=1691)
2022-08-02T18:18:28.180Z

Writing to PLC Tags

Writing data to a PLC tag is also very simple. Similar to reading data, all you need is the name of the PLC tag and the data you want to write.

databus.write_to_tag('I_TwoWayCommunicator', True)

The example above writes True to the I_TwoWayCommunicator tag. Data written can be of any type that is serializable.

Note: the PLC tag itself must be writable; otherwise, this function will do nothing.

IEDatabus has a default write topic, but if you need to write to a tag under a different MQTT topic, you can.

databus.write_topic = 'new/mqtt/topic'
databus.write_to_tag('I_TwoWayCommunicator', True)

Advanced Usage

To disable listening for incoming MQTT data:

databus.stop()

While the databus is stopped, you can still read from databus.tags, however the data will be the same as it was when stop() was called. Basically, the data will not be up-to-date.

To re-enable listening for incoming MQTT data:

databus.start()

Examples

To get an overview of all of the available tags and their values:

from ie_databus import IEDatabus
databus = IEDatabus('edge', 'edge')
databus.start()

for key, tag in databus.tags.items():
   print(f'{key}: {tag.val}')
Q_VFD1_Temperature: 32.064971923828125
Q_VFD2_Temperature: 68.12994384765625
Q_VFD3_Temperature: 68.12994384765625
[...]

Combining reading and writing together to notify the PLC if Conveyor 4’s temperature goes above 100 degrees:

from ie_databus import IEDatabus
databus = IEDatabus('edge', 'edge')
databus.start()

import time
while True:
    if databus.tags['Q_VFD4_Temperature'].val > 100:
        databus.write_to_tag('I_TwoWayCommunicator', True)
        break
    time.sleep(1)