Messy old control cabinet

Upgrading a Legacy Controls System

Resource Type: Blog |

The process of upgrading a control system is nuanced and requires careful analysis of the equipment to which it’s connected. While mechanical equipment upgrades can result in more tangible benefits (increased throughput, efficiency, etc.), the improvements resulting from a controls upgrade are more nuanced. In general, a well executed controls upgrade creates a safer, more reliable system with less downtime, better access to spare components, and the potential for savings on utilities. However, because the controller is the central hub of the system, the project needs to be approached in a thoughtful manner, with strong knowledge of the interfacing equipment, the protocols used for communication between them, and the programming methodologies used by both the old and new PLCs.

This article, part one of a two-part series, discusses the reasons to consider a controls upgrade. Part two of this series discusses the preparation required for its successful completion.

By Director of Indiana Operations / Sr. Electrical Engineer John Shipley and  Director of Texas Operations Nick Hitchcock


The Role of the Control System

A control system is an industrial computer – the central intelligence component within an industrial manufacturing process responsible for controlling the operation. It sends and receives information from the equipment, sensors, switches, human machine interfaces (HMIs), and potentially other programmable logic controllers (PLCs) within the system. 

Depending on the particular controller, some means of connectivity include:

  • Analog inputs
    • 4-20 mA current control signal
    • 0-10 V, 0-5 V voltage control signal
  • Discrete digital inputs to monitor on/off states of signals 
  • Communication inputs
    • Industrial Ethernet (i.e. PROFINET, IP, CC-Link IE, TCP/IP)
    • Serial-based fieldbuses (i.e. PROFIBUS, Modbus RTU, RS-232, RS-485)
    • IO-Link
    • AS-Interface (ASi) bus

Brief Overview of Controls System Operation

At a basic level, a controller executes sequential code based on current input conditions from connected equipment. These input signals are first converted to an appropriate digital quantity that can be input into the microprocessor. It then executes sequential-based code, evaluating current input signals conditions, and creates output signals accordingly. These digital quantities are then converted into their appropriate target output format, and sent to connected devices. In recent years, industrial control languages have become more streamlined, guided by IEC 61131-3 specification. Most control system vendors provide support for most, if not all of the IEC-required languages (Ladder Diagram LD, Function Block Diagram FBD, Instruction List IL (called STL for Siemens systems), and Structured Text (ST)). 

Why Consider a Control System Upgrade?

There are a variety of reasons that may be pertinent to upgrading a control system within a manufacturing facility. Depending on the specific circumstances, one, or several, of these factors may be valid concerns driving the decision.

1. Difficulty Obtaining Replacement Parts 

As a control system ages, it can become progressively more challenging to obtain the necessary spare parts needed to keep it in operation. If the older system was designed with custom parts, custom IC components, or relies on older communication protocols, over time any of these may become obsolete, making support for the system very challenging.

Common parts that can be difficult to source for a legacy system include:

  • Microprocessors
  • Input and/or output cards
  • Batteries 
  • Servos
  • Variable frequency drives (VFDs)

2. Compatibility Issues

A control system upgrade may be needed to support newer software platforms and/or equipment that isn’t backwards compatible. 

Sometimes, a supporting reason for a controls upgrade is to be able to run newer software such as Siemens TIA Portal, and access its benefits.  

Upgrading to Siemens TIA Portal From the Simatic Manager Step 7 Classic 

Released in the 2010s, TIA Portal is Siemen’s most up-to-date software platform offering significant improvements in functionality, wide ranging device compatibility, and general integration within the automation environment. A topic in and of itself, TIA Portal streamlines and therefore simplifies the engineering workflow required to control a manufacturing line, offering a singular environment that can communicate with a variety of components. 

However, older control systems like Simatic Manager Step 7 Classic(I don’t love this link, but can’t readily find something better) do not necessarily have the hardware to support TIA Portal. Their processors may be insufficient, or likewise, there may be insufficient memory within the computer. TIA Portal requires a means of high-speed communication of which some older controllers may not be capable. In addition, older HMIs may not meet the requirements needed to run TIA Portal. 

In a recent upgrade project, a client of Patti Engineering wanted to move their software system to TIA Portal, however, the hardware within the control system was too old to support it. To accomplish the task, Patti Engineering integrators first upgraded all I/O cards, and then proceeded with a PLC and control system upgrade to meet the goal of running TIA Portal.

Pandemic Supply Chain Problems Lead to Differences in the Same Part 

Due to supply chain problems that occurred alongside the COVID-19 pandemic, some controls manufacturers were forced to switch sources for their IC (integrated circuit) components. In particular, Siemens has a very popular HMI Comfort panel, the TP1500. As a result of supply chain problems, Siemens opted to redesign the entire panel (inside and out). Therefore, a TP1500 purchased today as a spare part may not be directly compatible with older versions of the same model. 

3. Downtime Issues 

As equipment gets older, the risk of failures increases. The environment in which the system is operating plays a substantial role in the system lifespan, with higher humidity and higher temperature environments presenting a larger strain on the operating components. 

Some failure examples include:

  • Variable Frequency Drive (VFD) Burning – Overheating Damage

A VFD is composed of power semiconductors in both its rectifier and inverter stages that perform the regular switching operations defining its function. Over time, these components deteriorate, become more resistive and less efficient, increasing energy dissipation as heat. In addition, harsher electrical environments can also contribute to component wear and damage over time. 

  • Relay Output Card Failures – Contact Wear Damage

A relay is a switching device at its core. Over time, the mechanical contacts of the device may become damaged leading to poor, unreliable connections. 

Intermittent component failures are an additional problem with older control systems, resulting in downtime. 

In a recent Patti Engineering project, a client was experiencing intermittent faults within the control system. Each fault resulted in four to eight hours of line downtime. Patti’s integrators were able to determine that one particular communication module within the SIMATIC S7-400 PLC rack was intermittently failing, causing the fault. Replacing the particular card resolved the problem.

4. Cycle Time Gains

A newer controls system will likely have a much faster overall processing speed. As a result, the time it takes to receive and interpret inputs, and then drive the resulting outputs will be much less than that of a legacy device. A more in depth discussion on the topic of improving cycle time will be covered in an upcoming blog on the multi-faceted topic of Overall Equipment Effectiveness (OEE).

5. Brand Standardization 

Streamlining to one particular brand of control system components, for example, Siemens, simplifies support and maintenance, staff training, resulting in less overall monetary investment. 

6. Cybersecurity and Safety

Today, two particular technology areas commonly driving the need to upgrade a controls system include improved cybersecurity and safety optimization.


Depending on the system’s age, there may be limited means of meeting cybersecurity protection requirements. While a very old system may have no network connectivity at all (and therefore no cybersecurity risk), some more recent systems may be connected to the OT network with limited means of addressing cybersecurity concerns. This limitation can leave more than just the particular device vulnerable to a cybersecurity attack; it effectively leaves an open door for a threat to find its way to several, or possibly all, networked devices. 

Meeting Safety Standards

It’s important to note that safety standards within manufacturing have been continuously evolving in recent years. A system deemed “safe” as recently as 2005 would likely be deemed unacceptable by today’s standards. In the United States, it is the owner of the manufacturing facility who is responsible, and ultimately liable, for site safety.

As systems are upgraded, a safety risk assessment must be completed. While upgrading to meet today’s safety standards is not a strict legal requirement, failure to meet the standard leaves a manufacturer open to liability and fines should an incident occur. Ensuring equipment meets safety standards proves that a manufacturer has done their due diligence in creating a safe work environment, serving as some legal protection should someone be injured. 

A control system upgrade may be required to meet today’s more stringent safety standards. Older control systems have very limited safety control capabilities, while newer systems have more integrated means of including robust safety features in a production line. This is an important requirement when completing a required safety risk assessment and selecting devices that are safe, will fail safe, or are configurable such that a single point of failure will not cause a human hazard or injury. This topic is discussed further in our recent eBook, Common Considerations Integrating Robotics in Manufacturing.

A recent Patti Engineering upgrade project’s business justification involved both of these concerns. The desire was to upgrade an assembly line to have greater cybersecurity robustness as well as stronger system safety. To meet these requirements, the control system had to be upgraded as it had limited safety features and a known cyber-vulnerability issue within its firmware that due to age, the manufacturer would not be supporting/addressing. 

7. Hydraulic to Servo: The Potential for Utility Cost Savings and Environmental Safety

Newer PLCs incorporate better means of connecting servo systems than their older counterparts, opening up new ways to upgrade current equipment. They support the high-speed communication protocols needed for more precise control and performance. Additionally, many newer PLCs have built-in motion control and functionality, streamlining the integration of a servo system.

Cost Savings on Utilities

In the past, servo control systems were more costly to purchase and operate. Today, servo systems are far more competitive in price and are generally more energy efficient than both hydraulic and pneumatic systems. Depending on the application, an upgrade from a hydraulic or pneumatic system to a servo system may reduce overall electricity expenses. In addition, the possibility of costly air leaks (pneumatics), and installation of flow meters to monitor for them, is removed. 

Environmental Safety 

A move away from hydraulics eliminates the possibility of leaking oil concerns and the potential for explosion in environments that include flammable substances.

8. Software Functionality Improvements

Upgrading to a newer control system allows you to operate your facility using the most up-to-date, integrated software systems, improving code efficiency, downtime recovery, and data connectivity (Industry 4.0). The topic of software is discussed in further detail above, as well as in part two of this blog series.

When to Consider Full Replacement 

There are times when it’s a better decision to fully replace outdated equipment rather than try to update the controller. This section addresses that option. 

1. Overall Cost of Upgrade

Determining the best investment between a controls upgrade and a full equipment replacement is somewhat nuanced. Upgrading controls is likely a lesser and short-term monetary investment compared to a full equipment replacement. On the other hand, the latter option comes with a longer lifespan for all system parts. A rule of thumb used at a prominent automotive company for guidance in selecting between these two options may be helpful to those making a similar decision: If the overall cost to upgrade is 50% or more of a new unit, consider full replacement.

2. Current Condition of Equipment

In line with the first point above, the condition of the connected equipment plays a large role in determining the best course of action between a system replacement and controls upgrade. If the mechanical portion of the system is in generally good condition, a controls upgrade may be the best approach that balances project cost with benefit. On the other hand, if the physical portion of the system is in poor condition, it’s likely that investing in a full replacement provides the best return on investment. 

3. Consider Lead Time

When determining the best course of action, the lead time for machine replacement may be a relevant factor. It’s not uncommon for equipment to have a six-month lead time, a timeframe that may be longer than that of an upgrade.

Maintaining the control system ensures that the entire manufacturing line is managed by a reliable, up-to-date system, providing robust safety and cybersecurity. In the next blog of this series, we discuss the preparatory steps needed for a control system upgrade. 

Related categories: Blog Control Systems Integration

Sam Hoff's Bio


Samuel M. Hoff, Chief Executive Officer, started the company from his home in 1991. Since then he’s expanded his business to more than 35 college-degreed engineers. Patti Engineering has engineering offices in Auburn Hills, MI, Austin, TX, and Indianapolis, IN.