Know Your Options: 6 Key Considerations when Replacing a PLC-5

Programmable Logic Controllers (PLCs) control multiple systems, so replacing one (or many) can seem daunting. There are a few options and considerations when replacing a PLC-5 to take into account before jumping in.

The Front-End Engineering Design (FEED) is an audit that identifies the gaps in OT control systems, this service pinpoints needs and considers the capability and capacity of your facility.  This includes:

• prioritizing the critical components that will need to be replaced,
• confirming the site infrastructure capacity (power and electrical, environmental control, network connectivity (IT and OT), location and space requirements),
• identifying what improvements can be realized during the reprogramming,
• identifying how HMIs will be impacted,
• identifying whether alarms will be impacted by peripheral work, and mitigating any impacts,
• confirming how and what data will be historized,
• and ensuring networks are up-to-date and compatible.

1. IO Migration

There are multiple ways to replace the controller: completely rewiring the new one or using the conversion tool to connect existing swing arms to the new controller.

Swing arm method

Pros

• Time saving: Rather than spending time on rewiring, the swing arm method allows you to simply connect the new IO modules directly to the existing PLC-5 IO chassis with the use of an IO adapter plate, conversion modules, and pre-wired cables. A technician can complete the wiring of a single PLC in less than 10 minutes.
• Reduced error: Fitting the same swing arms to the new system reduces the chance of errors during rewiring.
• Unlikely damage: There is a reduced chance of breakage since the wires will not be worked on individually.

Cons

• Accessibility: The replacement PLC chassis mounts directly over the existing PLC-5 chassis, which limits accessibility. When you need to access the PLC-5 chassis, you must power down the system.
• Costly: This requires purchasing the IO adapter plates, conversion modules, and specialized pre-wired cables.

Rewiring

Pros

• Reliable: Wiring IO directly into the new IO modules re-terminates the connections, making them more reliable. This ensures greater safety for plant processes.
• Facilitated troubleshooting and accessibility: Single IO connections can be accessed for troubleshooting. With adequate planning, it’s possible to conduct a live swing over or hot swap.

Cons

• Time-consuming: The time it takes to rewire is extensive and requires system expertise.
• Room for error: The more connections to wire, the more room for errors.
• Potential damage: Depending on when the system was last updated, there may be brittle wires and handling them could cause breakage. Replacing wires can be costly and time consuming if they need to be re-run.

2. Software Migration

Along with changing out the hardware, migrating to a new controller also requires reprogramming. This can be done the traditional way, writing the code, or using a conversion tool, which automates this process.

Using a Conversion Tool

This is only an option if converting within Allen-Bradley products.

Like installing the conversion tool hardware, the software conversion tool converts code simply by automating the logic migration, rather than rewriting the program.  However, this method will not make use of all the software capabilities, such as User Defined Datatypes (UDT) or Add-on Instructions (AOI).

Standard Reprogramming

This allows users to take advantage of new function block programming for greater optimization and software capabilities, such as User Defined Datatypes (UDT) or Add-on Instructions (AOI)). Also reprogramming allows for updating to a tag-based naming system within the program which will reduce troubleshooting and will support future project expansions.

Manually reprogramming the logic will also allow the user to implement any site programming standards that the client has, ensuring consistency with other PLC programs across the organization.  This program standardization not only helps with troubleshooting but also allows new staff to adapt to the system easily.

3. Human Machine Interfaces (HMIs)

Applicable only if you connect your PLC-5 to an HMI.

HMIs must be one of the considerations when replacing a PLC-5, because you will need to update your HMI at a minimum. If it is integrated with a SCADA, a DCS, or other PLCs, you must migrate it. For HMIs there is a choice between a modern solution, like a touchscreen, sticking with a legacy system with pushbuttons, or choosing a hybrid solution which can utilize both.

Legacy

Pros

• No changes: Operation staff faces less of a learning curve if they continue using your current system.
• No extra cost.

Cons

• Outdated.
• Less flexible: When IO is added or moved around, altering pushbutton displays and adding new labels makes this a much greater task.
• Finnicky programming: Reworking legacy graphics may take more effort or not be as intuitive as expected. If the program fails to convert the code properly, it can be detrimental to your facility.
• Graphic scalability: Legacy graphics may not be compatible with new monitors (display may be stretched or distorted, the resolution may not be compatible, etc.)

Modernized

Pros:

• Standardization:You can align layouts with company standards to facilitate use for operators.
• Remote accessibility.
• Flexibility and compatibility: Touchscreen HMIs allow for more flexibility and adaptability when setting up new equipment and IO. If other external control systems (like DCS, SCADA, or other PLCs) need to be added, or if operators could benefit from new trending tools, these can be easily integrated.

Cons:

• Costly: A new HMI is a costly piece of equipment, especially when considering purchasing alongside a new controller.

4. Hardware

Power infrastructure and communication modules: Not all controllers have the same power requirements so it’s important to confirm that the site power infrastructure is adequate for the new hardware and cabinet design. Additionally, verify that the program uses communication modules compatible with those currently in use on-site.

Location and fit: Each cabinet has its own individual constraints (size, location, accessibility to power), and you must consider these when designing and working on them. In a brownfield facility, adding new hardware or combining or adding PLCs (which means doubling the IO) may be more complicated than it seems or may not even be possible. Additionally, the cabinet location should be optimal for field wiring. Cabinet design requires confirming the interior dimensions of the cabinet and location of the cabinet for optimal integration.

5. Alarming

Alarms are highly important in a facility to alert operations staff of events or situations that need attention, and must be one of the considerations when replacing a PLC-5.  If alarms are not properly re-configured when conducting a PLC migration, the impacts for site safety and operational efficiency can be severe. Alarms do not convert well with the conversion tool, so it is best to work with an integrator who can ensure your system will function correctly.

6. Network Design

Network Architecture: If the site updates its communication type (e.g., from RS232/serial connections to Ethernet/IP), this may require firewalls and network switches for added protection.

Cybersecurity protection: Upgrading control systems and software and designing networks to meet new cybersecurity standards will reduce external threats and increase security.

Remote access: A new system introduces the ability to operators to control site assets remotely, improve their visibility on site and of alarm occurrences, reducing potential windshield time. Reducing the amount of time spent in hazardous locations, increases safety for operation staff.

Conclusion

There are many factors for considerations when replacing a PLC-5, and there is no one-size-fits-all; you need a solution that is specific to your facility and company needs. At InnoTech, we’ve completed projects in varying sizes, all using our scalable and standardized project execution processes. Find out more about our migration process, download our guide to modernizing obsolescent PLC-5s in industrial facilities (below), or speak to one of our experts today!

Talk to one of our experts

Sources:
InnoTech SMEs
Larson, K. (2023, June 20). Lessons chevron learned in modernizing its plcs. Control Global.
Magill, K. (2022, October 12). Schneider Electric moves to modernize equipment and machinery at 2 US plants. Supply Chain Dive.
Nicolas. (2023, March 16). Industrial Equipment Modernization. Solutions TREXO.
PLC-5 controllers: Allen-Bradley. Rockwell Automation. (n.d.).
ABB Top aluminum producer and recycler novelis to increase uptime following ABB Modernization. News. (2023, August 16).