Internal Architecture

This page describes the internal structure of the python-wagoplc library.

Mode of operation

What happens if you run python main.py at the command line? The following graphic illustrates the internal workflow.

The top-level __init__.py file contains the main() function, which is called when the application script executes. It is responsible for reading the configuration, creating a Scheduler instance and starting the task execution cycle.

Configuration handling

The read_config module contains a function of the same name, which is called by the main() function. It reads the configuration file controller.yaml and interprets an optional Tasks instance given from the script. With this in mind, it creates a unified variable mapping, a Controller object and a list of Task objects. In the process, any invalid configuration is identified and results in an exception. This involves validating the schema of the tasks section using the schema library.

Task management

Task management happens inside the tasks module, which contains the Tasks, Task and Scheduler classes. At the moment, the scheduler supports running cyclic tasks, with support for event-based and periodic tasks planned. It runs until an external interrupt occurs, after which the PLC outputs are reset and any open file descriptors closed.

Standard library

python-wagoplc also contains a standard library of function blocks as defined by IEC 61131-3, which can be imported from the fb module. The function block object is created behind the scenes in read_config, which imports the corresponding class either from the standard library or a user-defined module.

Controller-specific functionality

At the heart of the python-wagoplc are the packages that define controller-specific functionality. Each controller series must have its own package, each controller must have its own module and class. The first and base controller of a series must both define the Controller interface and be a superclass for each following version, if applicable. Inside a controller package, controllers may also be grouped into generations. See the CC100_v1 class and its subclasses as an example.