Discover how phased PLC migrations can minimize downtime, reduce costs and improve system performance with this comprehensive guide to planning and execution.

Learning Objectives
- Discover the key considerations for programmable logic controller (PLC) migrations.
- Find examples of breaking up a PLC migration into phases.
- Learn what to expect from a partner.
PLC migration insights
- Migrating a programmable logic controller (PLC) can be a large project that can be tackled in phases.
- Building out a plan to migrate an outdated PLC is critical.
- Even though migrating an outdated PLC is a costly and time-consuming process, the savings –– in both time and money –– could be worth the initial investment.
Consider the process of buying and renovating an older house. Before any renovations can begin, appraisals and inspections are necessary, especially for an aging home. Once the property is purchased, decisions must be made about which renovations to prioritize and the order in which to complete them. Renovating the entire house at once may not be feasible, so the work is often done in phases. Should leaky plumbing be addressed first? Or perhaps the house needs a fresh coat of paint. Determining where to begin is crucial. In many ways, upgrading obsolete PLCs in a facility mirrors this process.
As machinery ages, obsolescence and the likelihood of failure increases while options for repairs and direct replacements decreases, leading to downtime, wasted resources and frequent troubleshooting — ultimately causing significant operational headaches. Legacy PLCs can also introduce cybersecurity vulnerabilities.
Additionally, a 20-year-old PLC relying on parts sourced from online auctions can create operational risks. It no longer offers the reliability or performance that facilities need. A PLC migration can help resolve these issues while also providing cost savings through improved uptime, higher-quality throughput and reduced energy consumption. However, upgrading machines and systems all at once is often too costly and time-consuming for most facilities to manage without causing significant disruptions to production.
A phased PLC migration allows for two key benefits:
- It spreads the cost over time.
- It reduces the amount of downtime needed for each stage.
Key considerations for PLC migrations
The following factors will help in planning the right phases, determining the upgrade path, and budgeting for a phased PLC migration.

Evaluating the current state
Upgrading a PLC requires considering its role within the broader control system, starting by assessing the current system’s network, hardware and communication connections for compatibility and integration with the entire control system. This also includes looking at what the existing documentation reveals about the equipment, which network the PLC is on and what additional components will be required to ensure smooth operation after the update.
Some components to assess for potential impact during the migration include:
Human Machine Interfaces (HMI)
Older HMIs typically lack sufficient memory, processing power, and proper communication protocols to keep up with modern PLCs. Therefore, if the HMIs involved in the migration are also outdated, they typically need to be upgraded along with the legacy PLC. However, newer HMIs are often made backwards compatible, supporting older communication protocols.
Network
The industrial communication network — such as Ethernet/IP, Profinet or Profibus — that the PLC uses to communicate with its I/O cards and other devices plays a crucial role in determining the upgrade path, especially if the network, like the PLC, is also outdated. If the network relies on outdated serial protocols, such as RS-232 or RS-485, the entire network will need to be updated to support modern communication standards like Ethernet/IP or Profinet.
Obsolete motion controls
Modern PLCs may not support analog communication directly or may lack the proper interfaces needed to connect with older analog motion control systems. If the system relies on analog motion control, the upgraded PLC may not communicate with it effectively, making it necessary to upgrade the motion control systems as well.
Software
Additional system components — such as the software code — may also need to be upgraded to ensure compatibility with the new PLC. Properly updating the software is crucial to avoid integration issues and fully utilize the capabilities of the modern PLC.
Building a successful PLC migration plan
Breaking up a successful PLC migration into phases can help minimize disruption to throughput and provide greater flexibility in managing the upgrade process. In some cases, running simulations before implementing the migration can be highly effective. Performing a full simulation — replicating real-world processes, such as a virtual conveyor system — can help ensure the new software functions correctly before shutting down production for installation. Although this approach involves programming both the simulation and the actual system, it greatly reduces debugging time and lowers the risk of software issues during the migration.
There are several ways to divide the upgrade into phases; here are a few examples of independent phases to jumpstart the project:
- Upgrade smaller areas over time, which allows modernization of smaller sections of the plant, one at a time. After the first area is upgraded, the process can be fine-tuned, implementing lessons learned to improve subsequent upgrades. This approach reduces risk to the entire plant and provides valuable operations feedback on the system for future improvements.
- Upgrade HMIs first, as they are often compatible with older PLCs, allowing for smoother communication during the transition.
- Lay new network cables in parallel with the existing network to support newer protocols in advance, minimizing downtime. These cables can be connected to the new PLC once installed. Alternatively, the new network can be installed in parallel with the old one, gradually transferring the load to the modern system to reduce disruption.
- Install the new PLC in tandem with the old one, allowing both to receive system inputs. The older PLC can continue handling outputs while the newer is tested and debugged. As the migration progresses, controls are gradually transferred to the new PLC. Another option is using the new PLC only to control migrated equipment until all can be converted.
With numerous interconnected components and potential upgrade paths, creating a detailed, well-informed plan based on a thorough facility assessment is critical to ensure a smooth and successful migration.
Allocating the right resources
Adequate and thorough planning is often a challenge for PLC migrations, whether phased or all at once. A solid plan must consider every component connected to the upgraded PLC, the project timeline and the roles and responsibilities of everyone involved. Engineers, project managers, operations and maintenance teams should collaborate in the planning process to ensure no critical documentation or perspectives are overlooked.
It’s also important to remember that adding more personnel doesn’t necessarily speed up the project. Allocating the right resources is essential, but some tasks must occur sequentially.
When choosing a migration partner, it’s important to select one that focuses on finding the best upgrade path for the facility, rather than just pushing a preferred software vendor. While some facilities may prefer to stick with a specific platform, big brand hardware vendors often come with a higher price tag. In many cases, equivalent functionality can be achieved at a reduced cost without solely relying on one specific vendor.
Downtime is always required for a PLC upgrade, but a well-developed plan can significantly minimize its impact. Additionally, budget constraints or underestimating costs can complicate the process, making careful financial planning crucial. While cost management is important, overly restrictive budgets or cutting corners often result in lower-quality solutions that may not deliver the expected results.
Budgeting for a phased PLC migration
A well-developed plan that addresses key components and current performance will lead to a more accurate budget and timeline, while helping calculate ROI. The cost of the project can be justified in several ways, such as through quality improvements, increased throughput and reduced energy usage.
PLC migrations can reduce operational expenses by improving system accuracy, reliability and performance. Upgrading to modern PLCs enhances process control, leading to fewer errors, defects, and less rework. This improvement aligns with increased throughput, as reducing errors and defects allows more efficient production and a higher volume of quality output from the facility.
Additionally, older equipment often relies on pneumatics for automation, which is costly in terms of energy consumption. Replacing pneumatics with electric actuators or servo systems can result in significant cost savings. Energy consumption savings are a common way to help justify the budget for a PLC migration, with projects often paying for themselves in as little as three months due to reduced energy costs.
Selecting the right migration resource
Choosing the right partner for a PLC migration project is crucial to ensuring its success. A good migration partner will conduct a thorough on-site assessment and listen closely to the needs and concerns of the facility, ensuring the best upgrade options are recommended.
The right partner should also have extensive experience and a proven track record in the customer’s specific industry or facility type. This expertise ensures a deep understanding of the unique challenges and requirements of the environment. Beyond this, the partner should provide a 30,000-foot view of the entire migration project, helping to clarify how all the interconnected aspects fit together rather than focusing solely on isolated components. This perspective is particularly valuable for customers lacking in-house expertise to fully manage the complexities of a PLC migration.
Most importantly, the right partner will take the time to understand specific needs, constraints and timelines, working collaboratively to develop tailored solutions instead of relying on generic approaches. Transparency is key — an ideal partner will clearly explain their approach and the reasons behind it, leaving no ambiguity. Strong communication and a commitment to building a solid working relationship are essential for a successful project that aligns with long-term goals.
Pulling it all together
Simultaneously upgrading machines and systems can be costly, time-consuming, and a disruption to production. An assessment of your current system’s network, hardware and communication connections, along with the proper division of phases and allocation of the right resources, are all components needed to execute a successful phased PLC migration so you can minimize downtime, reduce costs and improve your system’s performance.
C. Todd Majors, P.E. is a controls and automation discrete manufacturing industry leader with Hargrove Controls & Automation.