Between evolving requirements, additional feature requests and urgent maintenance tasks, the Programmable Logic Controllers (PLC) at the European X-Ray Free Electron Laser Facility (EuXFEL) have become subjected to an array of demands. As the maintainability effort towards the existing systems peak, the requirement for a sustainable solution become an ever pressing concern. Ultimately, in order to provide a PLC code base which can easily be supported and adapted to, a reworking was required from the ground up in the form of a new suite of libraries and tools. Through this, it was possible to bring standardised software principals into PLC design and development, conjunctively offering an interface into the existing code base for ongoing support of legacy code. The set of libraries are developed by incorporating software engineering principles and design patterns in test driven development within a layered architecture. In defining clear interfaces across all the architectural layers - from hardware, to the software representation of hardware, and clusters of software devices, the complexity of PLC development decreases down into modular blocks of unit tested code. Regular tasks such as the addition of features, modifications or process control can easily be performed due to the adaptability, flexibility and modularity of the core PLC code base.