Most manufacturing operations with an established base of vintage PLC systems have either started chipping away at modernization or are nearing that phase. At first glance, these projects can seem simple—after all, you don’t have to touch the field devices or processing equipment and there’s no change to operations. That’s why many companies 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. The basic steps associated with swapping out controller hardware only get you 80% of the way there. It’s those last few percentage points that can be maddening without some forethought and expertise.
Let’s take a quick swing through some of those details to shed a little light on the challenges. We’ll focus on Allen-Bradley PLCs here, but similar concepts also apply to other controller types.
The manual is your friend
First and foremost, read the controller’s manual. Most particularly, I recommend the reference manual titled “Converting PLC-5 or SLC 500 Logic to Logix-Based Logic” from Rockwell Automation. “Logix 5000 Design Considerations” is also a good reference.
Seriously, you should at least scan these manuals in advance so you can recognize trouble before it’s too late. Avanceon has even created a 25-page guide we use internally to augment these Rockwell publications. On the hardware side of things, your first decision (when upgrading from 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 hardware cost. It also may lead you to think of the hardware replacement in strict 1-to-1 terms, which may not be the case. There are instances where you might need two new I/O cards to replace an old one.
And did you know that ControlLogix racks are slightly wider than their PLC-5 counterparts? They are, so measure your panel space carefully!
Conversions and communications
The octal to decimal issue also needs to be dealt with. The PLC-5 comes from a time before the numbers 8 and 9 were used in controller technology. What this means is that an I/O address that was I:152/3 may now be I[106].3. This depends on arcane factors such as, which version of the migration utility you use. Regardless, you still need to make conscious decisions about whether to fix the code or the drawings and wire labels.
Another complication can stem from the other PLCs and HMIs that the target PLC communicates with. The HMI applications will need to be updated along with the controller, 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 simple, but only if you have those applications and the development software that accompanies them.
When it comes to code conversion, be sure to consider the fact that the PLC-5 is a 16-bit processor and ControlLogix is 32-bit. Because of this, your INT registers are converted to DINT tags. This can be a good development because you’ll have twice as many bits to work with; that is, unless you have something like indirect addressing relying on 16 bits per word. You also won’t be able to add more double-indirects (where you indirect the file and the element). Your previous indexed addressing will also be eliminated.
We can’t cover our whole 25-page conversion guide in this column, nor can we encapsulate our wide experience working on projects like this. So, in summary, conversion projects can be simple in some ways, but it’s a mistake to confuse 80% with 100%. Detailed and strategic engineering efforts will be needed to get you from Point A to Point B effectively.
Frank Burger is an engineering manager at Avanceon, a certified member of the Control System Integrators Association (CSIA). For more information about Avanceon, visit its profile on the CSIA Industrial Automation Exchange.
