Dos and Don’ts of Designing a Custom PLC

PLC Direct
Dos and Don’ts of Designing a Custom PLC

Table of Contents

    When planning a PLC system, striking the right balance between customization and simplicity is crucial. Every industrial facility is different, and configuring automation hardware to match specific process requirements gives maintenance engineers and procurement teams a measurable operational advantage. This guide covers the key dos and don'ts of custom PLC design and programming, along with practical steps for keeping a custom PLC system maintainable over time. Following these principles helps avoid the most common and costly mistakes in PLC design.

    Why Does Customization Matter in PLC Design?

    Custom PLC design allows engineers to optimize performance, build in scalability, and eliminate unnecessary hardware costs, three factors that directly affect the long-term value of an automation system. Here's why:

    • Performance: Customizable PLCs enable you to optimize your system's functionality according to your specific requirements. With a PLC tailored to your needs, you can achieve higher performance and gain a competitive edge in your industry.

    • Scalability: By designing a custom PLC that easily integrates with new technologies, you can better accommodate future expansion or upgrades, thereby increasing the lifetime value of your PLC system.

    • Cost efficiency: With a customized PLC, you no longer have to spend money on unnecessary features, thereby achieving long-term cost savings.

    Customization is crucial for leveraging the power of PLCs in your projects. But before you dive into the customization process, you should consider some essential factors.

    What Are the Dos of Custom PLC Design?

    A PLC is a complex automation solution. Therefore, there's much more to integrating it than simply purchasing a highly rated product. This is especially true if you're going to take the hands-on approach to customizing its performance. To ensure the success of your custom PLC, keep the following considerations in mind.

    Do Determine the Objectives Before Starting PLC Design

    Clearly define the functional requirements of your PLC system and create a detailed list of features you want. This will give you a clear roadmap throughout the development process and help you avoid costly, time-consuming late-stage adjustments.

    Do Choose Hardware That Matches Your System Requirements

    Carefully select PLC hardware that meets your requirements for processing capability, memory capacity, and communication options. Investigate the reliability and reputation of your chosen hardware to ensure long-lasting operation. For facilities sourcing replacement or lifecycle hardware, surplus sealed and refurbished PLC units from established platforms such as Siemens SIMATIC, Schneider Electric Modicon, and Omron Sysmac provide a cost-effective way to meet these specifications without a full platform replacement.

    Do Standardize Programming Language Across Your PLC System

    Use standard programming languages such as Ladder Logic or Structured Text to ensure interoperability and make it easier for other engineers to understand and work with your custom PLC. This ensures that everyone on your team can use the system with minimal confusion, reducing your margin of error.

    Do Prioritize Safety in Every Stage of Custom PLC Programming

    The safety of your team while using the PLC equipment is also vital. Implement safeguards and protective measures to prevent risks associated with incorrect PLC operation. Also, ensure that your custom PLC conforms to industry safety standards and guidelines.

    What Are the Don'ts of Custom PLC Programming?

    Now that we've covered some dos for designing the ideal custom PLC, it's time to discuss some don'ts. While customizing a PLC can yield numerous benefits, you must avoid pitfalls that can negatively impact its performance and maintainability. Below are some key don'ts to keep in mind when programming your customizable PLC.

    Don't Deviate From Established PLC Design Standards

    Though customization offers flexibility, you shouldn't deviate too far from established industry standards for programming languages and communication protocols. Adhering to these standards will ensure compatibility with other systems and make it easier for your team to maintain and troubleshoot the PLC.

    Don't Overcomplicate Your Custom PLC Programming Logic

    Strive for simplicity and clarity in your PLC programming. Avoid using overly complex logic or structures that are difficult to understand and maintain. Instead, focus on creating modular, self-documenting code that is easy to read, debug, and update.

    Don't Skip Testing Before Deploying Your PLC System

    Thoroughly test your custom PLC code before deployment to identify any errors, inefficiencies, or other potential issues. Use a combination of simulation tools, bench testing, and field testing to ensure your PLC performs reliably and meets your project requirements.

    Don't Neglect Documentation in Your PLC Design Process

    Failing to create and maintain comprehensive documentation for your custom PLC can make troubleshooting and updating the system more difficult. Ensure all design specifications, programming guidelines, and maintenance procedures receive detailed notes in your files and that this paperwork is easily accessible to your team.

    How Do You Keep a Custom PLC System Maintainable Over Time?

    Even once you understand how to build the right PLC for your current operations, the work isn't over. As your facility continues to evolve, you'll need to make key changes to your systems to keep up with new demands and regulations. To make updating your custom PLC easier and more efficient, consider the following strategies.

    Implement Version Control for Your PLC Code

    Use a version control system to track changes to your PLC code and maintain a revision history. This will help you quickly identify and revert to previous code versions if necessary, ensuring smoother updates and reducing the risk of errors.

    Develop a Scheduled Maintenance Plan for Your PLC System

    Establish a regular schedule for reviewing and updating your custom PLC system. This will help you proactively identify potential issues, optimize performance, and ensure your PLC remains compatible with any new technologies or requirements.

    Train Your Engineering Team on Custom PLC Programming Standards

    Provide training and resources to ensure your engineering team has the necessary skills and knowledge to maintain and update your custom PLC. Encourage ongoing professional development and staying informed about industry trends and best practices.

    Create and Maintain Thorough PLC Design Documentation

    Maintain comprehensive documentation of your custom PLC, including design specifications, programming guidelines, and troubleshooting procedures. This will facilitate smoother updates and make it easier for your team to address any issues that arise.

    To Conclude

    Designing a custom PLC offers measurable benefits in performance, scalability, and cost efficiency when planning a PLC system from the ground up or upgrading an existing installation. Following the dos and don'ts outlined here helps avoid the most common and costly mistakes in PLC design. PLC Direct supplies surplus sealed, refurbished, and used PLC hardware across leading brands, including Schneider Electric, Mitsubishi Electric, and more, giving facilities access to the hardware they need to build, maintain, or extend a custom PLC system without being limited to a single sourcing channel. All products come with a 1-year PLC Direct warranty. Contact PLC Direct to check availability on specific hardware for your installation.

    PLC Direct

    With over 10 years in industrial automation hardware, the PLC Direct Team covers control systems, drives, HMIs, sensors, safety systems, and process instrumentation across a wide range of manufacturer lines. We support customers with parts lifecycle, hardware compatibility, procurement decisions, and maintenance challenges that arise in industrial automation environments.

    Frequently Asked Questions

    Before beginning custom PLC design, clearly define the system's functional requirements: the number and type of I/O points, required communication protocols, processing and memory demands, and any relevant safety or environmental standards. A detailed requirements list at the outset prevents costly changes during development and ensures the selected hardware matches the actual application.
    Hardware selection is the most critical factor when planning a PLC system. The CPU must have sufficient processing speed and memory for the application, I/O modules must match the field device types in use, and communication modules must support the protocols required by the broader automation network. Getting these specifications right before procurement avoids compatibility problems during commissioning.
    The most common mistakes in custom PLC programming include overcomplicating logic structures, deviating from standard programming languages such as Ladder Logic or Structured Text, skipping simulation and bench testing before deployment, and failing to thoroughly document the program. Each of these issues increases the time and cost of future maintenance and troubleshooting.
    Version control in PLC programming involves tracking every revision to the control logic, storing prior versions, and maintaining a record of what changed and why. This practice is particularly important in facilities where multiple engineers work on the same system, as it prevents conflicting changes and provides a reliable rollback path if a new program version causes unexpected behavior.
    Compatible hardware for custom PLC installations, including surplus sealed and refurbished controllers, I/O modules, and communication hardware, is available from independent suppliers outside standard OEM distribution channels. This is particularly relevant for facilities that maintain older installed systems where OEM lead times make standard sourcing impractical, or where the original models have been discontinued.