Most manufacturers and utilities with an established base of vintage PLC systems have either started chipping away at modernization or nearing that phase. These projects can seem simple – after all, they won’t be touching the field devices or processing equipment, and there’s no change to operation. You think that all that you need to do is swap in new PLC hardware in accordance with the manufacturer’s guidelines and convert the program, but there’s more to the story.
Those steps only get you 80 percent of the way there, and those last few percent can be maddening without some forethought and expertise. Let’s take a quick swing through some of those maddening details in order to shed a little light on the challenges. For the purpose of this blog, we’ll focus on Allen-Bradley PLCs, but similar concepts will apply for other manufacturers.
First and foremost, read the Friendly Manual. Most particularly, I recommend the reference manual “Converting PLC-5 or SLC 500 Logic to Logix-Based Logic” from Rockwell Automation. “Logix 5000 Design Considerations” is also a captivating read. Seriously, you should at least scan these manuals in advance so you can recognize trouble before it’s too late. Avanceon has created a 25-page guide we use internally to augment these Rockwell publications.
On the hardware side of things, your first decision (for a PLC-5) will be whether to use the conversion kit. This can be a big time-saver with the physical cutover, but there’s a price to pay in terms of complexity, maintenance, and of course hardware cost. It also may tend to make you start thinking of the hardware replacement in strict 1-to-1 terms, which it may not be. There are cases where you might need 2 new I/O cards to replace an old one. And fun fact… did you know that ControlLogix racks are actually slightly wider than their PLC-5 counterparts? Measure your panel space carefully!
The Octal to Decimal problem also needs to be dealt with. The PLC-5 comes from a time before the numbers 8 and 9 were invented. Sort of like how Alaska and Hawaii were late to the United States party. What this means is that your I/O address that was once I:152/3 may now be I.3. This depends on arcane factors like which version of the Migration Utility you use, but you need to make conscious decisions about whether to fix the code or the drawings and wire labels.
Another complication that sometimes isn’t fully considered is the other PLCs and HMIs that our target PLC is communicating with. The HMI applications will definitely need to be updated, and it’s likely that other PLCs on the plant network will at least need some minor tweaks to communicate with the new PLC. Usually, these tasks are fairly simple, but only if you have those applications and the development software that accompanies them.
We haven’t talked much about the code conversion itself, but here’s one to consider. The PLC-5 is a 16-bit processor, and the ControlLogix is 32-bit. As such, all your INT registers are converted to DINT tags. Bonus! We have twice as many bits! That’s great unless you have something like indirect addressing relying on 16 bits per word. And on the topic of indirect addressing: no more double-indirects (where you indirect both the file and the element), and your old indexed addressing will be out the window too.
We can’t cover our whole 25-page conversion guide here in less than 600 words, nor can we encapsulate our wide experience doing projects like this. So, in summary, conversion projects are simple in some ways, but it’s a mistake to confuse 80 percent with 100 percent, and some detailed engineering effort is needed to get you from Point A to Point B effectively. But the trip is worth it – think of all those 8’s and 9’s you have now!
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