As systems integrators, we’re frequently confronted with many different brands and generations of PLC Hardware. Sometimes, this means we get to work with the latest and greatest offerings of today—but sometimes this means our employees (not even the youngest ones!) say “This PLC is LITERALLY older than me based on the install date written on the label!”

This poses a few problems for our clients and ourselves.

Unraveling Legacy Hardware Issues

Knowledge

The first issue surrounding a span of 40+ years of hardware in the field is having the knowledge to work with anything and everything we might encounter. While at their core, PLCs use ladder logic, and with some exceptions most ladder logic functions the same across all brands. But, there can be many differences with how you connect with and interact with PLCs.

If you send someone out into the field who is accustomed to working with ControlLogix PLCs from Allen Bradley with Studio 5000, they can access the latest L8 series of processors as well as something like an old L1 processor with Logix 5000 and have a pretty good idea of how to get in and make changes. Faced with a SLC PLC, they might be shocked at the lack of tag names in Logix 500, but they can likely get around pretty easily with the right programming cables. They will definitely have some questions about how to get everything set up and connected to the PLC. And they’ll need to learn some of the idiosyncrasies of the platform.

But, sit them down in front of a PLC-5 with a copy of Logix 5, and even with the right cables and there will be some hurdles to overcome. Granted, while there will be challenges, they will be able to pick it up quickly if they are familiar with PLCs.

You will find similar trajectories with Siemens, Modicon, or any of the other major PLC vendors on the market.

For this post, we will ignore the inevitable reality that the longer a PLC has been on the plant floor, the likelihood of finding a set of drawings to help you decipher everything gets closer to zero at an almost exponential rate!

Tech Support

Assuming you have your TechConnect number handy, you can call Rockwell with any questions on a Studio 5000 capable PLC and get a quick answer. You’ll run into some delays asking about SLC processors, although you’ll likely get an answer without having to be transferred. If you are calling about a PLC-5 you will likely need to get a callback, because the technician will have to retrieve the manuals from “the archives”. Even then, you will still likely get their best guess and probably even a “let me know how it goes, I’m curious to see if this will work” answer.

You might be lucky and have someone you can call who spent time in the 90s on older hardware, but even then you’ll get a lot of “well…I don’t know, it’s been a while since I have used a PLC-5”.

To put it simply, the longer a PLC is on the plant floor, the smaller the resource pool for help on the older platform becomes. If you have to go back into a PLC older than a PLC-5, then good luck—you might as well be looking at the technology equivalent of cave paintings!

Longevity

Even though PLCs are designed to be “bullet proof,” they are a piece of technology with a lifespan. Eventually they will break, get fried due to an electrical issue, lose their program when power cycled on a dead backup battery, or run into some other catastrophic problem. If you are working with newer hardware you can simply call up your distributor and get a replacement. NBut if you are using a SLC or a PLC-5 in the Allen Bradley world, you will need to scour eBay for a replacement—or hope there’s a spare in the pile of old hardware in the warehouse.

That said, you still might not find an exact replacement and will need to modify your programs to accommodate a different processor revision. This brings us back to point #1 regarding knowledge…

The other issue with PLC longevity is that they typically outlast a lot of other technology in a manufacturing facility. For example, the communication protocols available when the PLC was installed have likely been obsolete for YEARS already. As the rest of the plant matures into new technology and SCADA systems take over, it can be difficult to get old PLC hardware to communicate with the new systems. They’ll often require protocol converter retrofits which will lead us right back to tech support and knowledge hurdles to overcome.

How to Address Legacy Hardware Issues

Education

The only way to combat the knowledge gap with legacy hardware is to invest in both educating your people before they need to dive into legacy hardware (so they know what to expect), and to document everything along the way. Since you can’t rely as heavily on tech support with legacy hardware, you need to build your own knowledge base and walk-throughs for how to go online with processors, manage I/O, logic, and troubleshoot, etc.

This documentation will need to include at least rudimentary drawings when you are mapping out I/O, or updating old drawings as you find discrepancies. The likelihood of the panel now matching the drawings done when it was installed 10, 20, or 30 years ago will be pretty low.

Also, you might need to reach out to industry veterans to understand how to work with legacy hardware. If so, document everything you can when learning from them. Record video while working with the hardware, and screen recordings of the software, so you have a go-by for next time you need it.

Technology

It is likely you will need to access old computers to get the software, or at the very least set up a virtual machine with the right programming environments. It is important to back these up in a few places, as it can be a hassle to find all of the right disks, downloads, or links to what you will need along the way.

It is also important to take backups of the existing programs ASAP. The last thing you want to do is to re-write a program from scratch because of a bad backup battery and a power outage causing erasing the program from the PLC memory.

We’ll dive into this in more detail in the next section, but the other key piece of the puzzle is to make sure you have the right adapters for the hardware. This can be DH+ to USB adapters, RS-232 or RS-485 to USB adapters, or potentially some protocol converters like units from Prosoft to put legacy PLCs on to your plant network.

Document which adapters you need for which PLCs, how to connect to the PLC using those adapters, and what settings you need in your VM for everything to function properly.

Teaching an Old Dog New Tricks

If you can’t replace the legacy PLCs with newer hardware, you might be in luck for integrating the legacy PLCs with your plant network. You can likely use modules from Prosoft to get your PLCs connected to your SCADA system. These modules convert the native protocols from your PLCs to ethernet. Prosoft can usually support DH+, DF1, ControlNet, Profibus, and just about any other protocol you might encounter. You will also need to configure these modules both on the PLC side and the ethernet side. All of this information and walkthroughs should go into your documentation.

Depending on the hardware vendor, you might also have options to do a “half” upgrade. In this case, you’ll use a mounting system to put a new PLC onto your existing legacy I/O. This can be a good alternative to replacing an entire system. You’ll replace the brains with a modern processor without having to re-wire everything. This gives you a taste of the best of both worlds. In many cases, the I/O cards will be less prone to failure than the PLCs, especially in instances of losing a PLC program. While it saves wiring, this option can still be pretty costly. So, doing a rip and replace with new hardware still might be worth your while.

Upgrading to Modern Hardware

The other option when confronted with legacy hardware is to upgrade it with new hardware. Eventually, this will be a necessity. At some point, your hardware will reach its end of life and you won’t be able to find suitable replacements.

The main reason to do an upgrade at any given point in time is to better control the headaches resulting from an upgrade. If you can, plan a shutdown to take out the old hardware and replace it with new. This way, you won’t be subjected to the pain of your equipment deciding when it will fail. This is especially important in today’s supply chain climate. We are seeing some of the Allen Bradley hardware quoted at 8-12 week lead times—and then still not showing up after 45 weeks. Some brands like B&R Automation are quoting lead times of 50+ weeks for their customers. So, if you need to buy a new PLC now, it could be well over a year before you have the new hardware in hand.

Can you afford to be down for a year while you wait for new hardware? If not, you should stock up on plenty of spares of your legacy hardware before it fails.

The benefit of new hardware is you get access to all the advancements in technology that have accrued over the last few decades. The hardware will be easy to integrate with modern control systems. You’ll enjoy increased speed, ease of programming, and it will all buy you another decade or so before you need to worry about it failing. But, you’ll still want to make sure to have plenty of spares on hand until the supply chain issues are resolved!

Wrapping Up

This post isn’t intended to be entirely doom and gloom. It is a fact of life that people don’t fix things that aren’t broken. And in many cases, legacy PLC hardware isn’t broken…yet. The problems arise when a critical piece of your system’s infrastructure fails, causing major headaches—and there is no way to know for sure when that will happen. While the timing of when it fails might be entirely out of your control, you can control how to mitigate the risks of it failing and minimize the damage by properly training your people.

For best results, provide them with solid documentation, backups of your code, and spare hardware on the shelf to swap out while you roll out your modernization plans. Need help modernizing your hardware? Contact Us!