I have been using a GitHub project called pylogix by GitHub user dmroeder to do some basic data retrieval from ControlLogix and CompactLogix PLCs. While the project has been around for a while there has been some more recent activity and the code has become pretty reliable. I have used it to pull data from PLCs where that data does not change readily, for example recipe data or setpoint data. Another great use is for turning Raspberry PIs into remote IO devices by having them push data to a PLC. This can be useful for monitoring a fire pump run status in a remote pump house or for monitoring the temperature and humidity of a control room where there is no PLC and cost is an issue. I will try to post more examples of industrial uses for Raspberry PIs in the near future, but wanted to give a shout out for this project since it is quite good.