PLC-5 Migration Path DecisionsYou’ve made the decision that your PLC-5 platforms are unsustainable, but now what? Mapping out a path forward is challenging and requires planning to be successful. As in most controls projects, two areas of concern must be addressed: software and hardware.

Software Considerations

Let’s address the software first. There are two basic approaches.

  • Rewrite the code or,
  • Use Rockwell’s conversion tool.

Here are the pros and cons of each, along with a 3rd hybrid option. Click the (+) below to expand.


  • A lower cost solution as it takes less time to execute.
  • Minimizes the risk of bad logic.


  • The tag names use a format that resembles the old PLC-5 file structure. The old address of B3/1 would become B3[1].

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  • Though tag descriptions can be used when working with HMIs or messaging with other PLCs, B3[1] doesn’t mean much.
  • Does not take advantage of new programming standards or commands.


  • The opportunity to take advantage of the latest software commands, standards, and technologies, allowing you to increase performance.
  • Because you can use readable tag names, the entire program becomes much easier to follow.
  • HMI development is more understandable and straightforward.


  • The additional time it takes to rewrite the code will be more expensive than conversion. 
  • Some risks involved in the recreation of the logic code.


  • Less expensive than a total rewrite.


  • More expensive than a path that only uses the conversion tool.

Hardware Considerations

Second, let’s look at the hardware. There are three basic options.

  • The first is to add a new ControlLogix Processor and replace the PLC-5 processor with a scanner card that turns the existing rack into remote I/O.
  • The second is to use Rockwell’s conversion kit to link the new ControlLogix Processor to the old 1771 I/O using Rockwell’s Remote IO protocol.
  • The third option is to upgrade all of the 1771 cards and chassis, replacing it all with a ControlLogix Processor and I/O. In essence a complete hardware conversion.


  • Little to no downtime needed for rewiring.
  • Minimal hardware costs. No new ControlLogix I/O cards needed.
  • No bridges are burned. Can return to the original PLC-5 processor very quickly.


  • The obsolete 1771 I/O remains in place.


  • Eliminates all the 1771 I/O cards and the PLC-5 processor.
  • Minimal downtime is required to install the hardware conversion kit.


  • Some difficulty in troubleshooting.
  • Adds additional connectors that create additional passive points of failure.


  • Eliminate all legacy components.
  • Easier to troubleshoot moving forward.
  • Less cost for hardware since you don’t have to buy the conversion kit.


  • Requires the most downtime. 
  • Every wire must be re-terminated.
  • A complete I/O checkout is required.

LSI Can Help

Rockwell Automation Gold SI - LSI LogoWhether you have a stand-alone controller, a small network of controllers or a large complex interconnected control system, LSI can help. We begin by listening to your unique needs, working to understand your processes and objectives. Then we help you to develop and execute a roadmap for upgrading your hardware and optimizing your processes. Learn more about our process below.


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