Some will argue that there’s no reason to remove part of a control system that today is running as good as it has for the last twenty years (or longer.)

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Especially when that legacy hardware has been as durable as any tank used during Desert Storm, and runs as well as an 80’s Oldsmobile Cutlass in mint condition.

But even though parting with your legacy system could be as hard a letting go of your favorite pair of blue jeans, it could be time to do just that.

When you can no longer find replacement parts, or the cost of replacement parts is higher than the cost of a new control system, it’s time to consider moving on.

Image by Brandon Cooper

After moving through the stages of grief that often follows a decision to replace old but reliable equipment, the next step is to map out a migration path.

Creating a solid plan will take time and preparation, and below I’ll share some points you should consider while planning your path forward.

What was the existing method of communication to the HMI/SCADA system, and how will it change with the new system?

The legacy system’s communication protocol might have been Token Ring, Data Highway, Modbus, or some other legacy protocol that’s not likely the default means of communications of the replacement system.

In fact, most current Control Systems have Ethernet based communications, like Ethernet/IP, PROFINET, and CC-Link IE.

But while the cabling for legacy networks like Data Highway Plus can be daisy chained up to 10,000ft without additional hardware, modern CAT5E and CAT6 Ethernet network cabling is limited to runs of roughly 328ft in any one direction.

So depending on the distance to each drop, multiple Ethernet switches or Fiber Optic cabling and transceivers may be needed to replace long runs of legacy communication cabling.

Will the legacy system that you are replacing still need to communicate with other legacy systems that will stay in operation for some amount of time?

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When using a phased migration approach, it’s not uncommon for the new control system to need to continue to communicate with other legacy systems that will stay in operation for some time.

To address this, you may need to install one or more “Legacy Network Gateways,” like EtherNet to Serial Converters, Ethernet to Legacy Network Bridges, or some other type of gateway depending on the protocols of the old and new systems.

Either way, if all the “pieces” of the control system are not moving forward at the same time, intermediate steps need to be implemented so the new system can communicate to the existing systems.

When installing my new I/O modules, should I re-wire or use conversion kits?

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For many legacy upgrades, you’ll have the choice to rewire from old to new I/O modules with a “fan-out” type of wiring lead back to terminal strips, or you can purchase conversion kits that enable you to bypass the re-landing of hundreds (or even thousands) of wires.

These conversion kits typically use more cabinet space and often leave the existing chassis in place, but they also dramatically decrease the amount of time it takes to cut-over to the new system.

And while they add more to the material costs, the overall project cost is usually offset by the labor cost that is saved.

In the end the decision to use conversion kits usually comes to a matter of personal preference, or cut-over time constraints.

Moving Forward By Leaving Your Legacy Behind

As the legacy control equipment is powered down for the last time, all the planning and considerations you have taken in prior weeks should ease the pain of knowing that the “old reliable” system will not see operation again.

So wipe that teardrop from your cheek and embrace the new opportunities, (and frustrations and nuances) that you’ll gain by installing the new system.

Hopefully, it will help you sleep at night for the next ten to twenty years.

And at some point in the future, you’ll be able to talk about “the good old days” of that legacy equipment you miss so much.

Written by Brandon Cooper
Senior Controls Engineer and Freelance Writer

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Brandon Cooper


  1. I am interested in using an Allen Bradley 1747-DEMO-7 Training Kit with a SLC 5/02 processor module in a demonstration. I would like to have the PLC act as a Modbus server/slave so that I can use some software to act as a client/master and collect data. I think that the SLC 5/03, 5/04, and 5/05 processors allowed the SLC to act as a Modbus client/master but not as a slave.
    I found 1747-KE or 1770-KF3 that would bridge DH-485 to DF1; and found some other things that would convert DF1 to Modbus server/slave.
    Did anyone make anything that converted DH-485 to Modbus server/slave so that it could be scanned by a Modbus client/master?


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