Technical Article

Designing and Planning Control System Retrofits

June 05, 2020 by Dennis Lynch

This article discusses the designing and planning of control system retrofits.

Once the decision has been made to replace or retrofit your plant or facilities control system, it’s time to start a crucial part of the process, which is design and planning. It is very important that this stage is completed diligently and methodically as a poorly designed or planned control system will be one that does not last or perform well. Before design and planning can start, it is also important that the right control system has been selected. You can learn more about that process in a previous article.


Design and Planning

Once you have your budget and made your new control system choice, it's time to come up with a plan and start designing your system. Designing a replacement control system can be more difficult than designing a control system from scratch. In designing a replacement to an existing control system, you may have to make decisions you normally would not have to make if it was a brand new installation. Your existing plant may need to be operational during the retrofit or the replacement may need to occur swiftly during an outage. You may also have to have alternative designs or be flexible with your design in case something doesn’t quite turn out as you expect.

The first key objective to planning your replacement is documenting the existing installation. This may involve gathering existing drawings, documents, databases, control system logic, HMI screen configurations, etc. Many times, existing documentation can be suspect or altogether incorrect. 

It is a good idea to go over the existing documentation and verify key parts of it with the existing installation. If existing documentation is not available, something must be done to document the existing status of the system. This can be accomplished with notes, sketches, photographs, video, plant personnel interviews, or other means. Even if the existing documentation seems accurate, photographs or video is an easy way to ensure that you have captured as much of the existing installation as possible. Photographs and videos can also help if you are working on the design away from the control system site, as it gives you an easy reference to installation without physically being there.


These pictures won’t win any photo contests, but they can be a valuable reference during a control system retrofit. 


When documenting with photographs or video, make sure your pictures or video are clear and complete. Don’t forget to capture every area of the existing control system. Organizing your pictures right after you take them is a good way to make sure they make sense days, weeks, or even months down the road as you work on the design of the new system. Videos can help augment pictures as you can sweep an entire area or control cabinet slowly with the video and go back later and pause the video to see certain areas you may have missed with photographs.

Getting the electronic configuration of the existing system is just as important as the physical documentation. If the existing configuration can be maintained electronically on the original hardware while you are implementing the configuration on the new hardware, this can be a handy way to duplicate and test the new configuration. You can refer back to the original configuration as needed to complete the new configuration as many times as needed. 

In addition to an electronic copy, a hard copy of the complete configuration and logic should also be created as a reference. This documentation may be used months, even years after the new control system has been replaced to verify a suspicious setting, range, or piece of logic that has been questioned once the system has been up and running.



There are numerous ways to tackle your retrofit or replacement. Your plan may involve a phased approach if you need to keep things up and running or have very little downtime available.  You may also opt for the “rip and replace” method where you completely remove the old system and replace it with the new system. Each of these methods can have additional options within themselves as well. You may be re-using the existing field terminations or utilize a manufacturer's conversion system for field terminations. 

Some phased approaches can even use the existing I/O hardware along with the field terminations. This can be adventitious for a number of reasons but does not eliminate obsolete hardware if that is one of your concerns.


I/O wiring conversion system. Image used courtesy of Allen-Bradley.


When converting existing logic, there are several methods available to accomplish this part of the process. Many control systems have auto-conversion tools available to speed up the conversion process. Be careful with some of these tools as they get the job done quickly, but may not get it done in the manner you would if you were programming manually. Many are not complete either. You have to “read the fine print” if you will, on the capabilities of some of these tools. 

Sometimes the auto-conversion tools prevent you from taking advantage of newer or more advanced functions in the new control system. Formatting might be another area where auto-conversion tools fall short. The auto-conversion tool is a program that has been programmed to convert the logic in a set way. This may not be the most advantageous way and the format may not fit your taste or plant guidelines.

Manual programming, using the current program as a reference, may be slower than the auto-conversion process, but may afford you the best results when comparing all the alternatives.  With manual programming, you can take advantage of new or more advanced features of the new control system, while still having peace of mind that you are using trusted logic that hopefully was fully functional in the past. You have more control over the formatting of the program and can review the program for errors, inconsistencies, or inefficient logic. Existing logic may have evolved over the years and been under the care of multiple individuals who may have added logic and not followed consistent programming practices.  Manual programming offers the perfect opportunities to clean things like this up.

If the existing logic is not available or is a mess and has been prone to errors and caused the users nothing but headaches in the past, then neither of the above methods seem to be a good fit for the new control system logic. Sometimes, starting from scratch can be the best way to get the best results from your new control system. Careful planning and review of existing documentation will help get a total re-programming of a system underway just a little faster than if you were designing a brand new installation.


Image used courtesy of Allen-Bradley.


To summarize, below are some key recommendation to a successful retrofit or replacement are:

  • Documentation - As stated above in the design and planning stage, document the existing installation thoroughly. Provide very clear and detailed drawings and documents for the new installation as well.
  • Pre-Assembly - Assemble as much of the new system as you can upfront while the old system is still operational.
  • Simulation - If possible, simulate or test the new system offline, before taking the new system down. If you can operate both the old and new systems side by side, you can eliminate many errors, by pre-checking your implementation of the new system while referencing the old system.
  • Be Flexible - Have alternate solutions readily available for those that you can predict. For other issues that come up that you didn’t see coming, you will need to be prepared to provide alternate solutions on the fly.
  • Educate Yourself - Visit other facilities, read articles, or talk to others that have completed retrofits, especially of the same type and get their feedback.
  • Research Solutions - Research your proposed conversion and integration solutions thoroughly, as the advertised results from the manufacturer are not always as they seem. There are usually trade-offs when using a manufacturer's conversion system as opposed to completely replacing hardware or terminations.
  • Manage Closely - Do not rely solely on drawing and documents to get your design across to the installers. Make frequent visits and go over the upcoming scope of work with the installers and make sure all is clear.