Allen-Bradley PLC Advantages and Disadvantages: A Cost Controller's 6-Point Buying Checklist

I'm a procurement manager at a mid-sized packaging plant. For the past six years, I've owned the automation budget—roughly $180,000 in cumulative spending, and I've documented every invoice in our cost tracking system. My job isn't to design control systems; it's to make sure we're paying for what we actually need. That means I ask awkward questions like "why is this software module more expensive than the CPU?" and "what happens if that spare part takes six weeks?" This checklist is the one I wish someone had handed me before my first Allen-Bradley PLC purchase.

If you're evaluating Allen-Bradley PLCs—especially the ControlLogix family—for a new machine, a retrofit, or a plant-wide standard, this post is for you. It walks through the business side, not the wiring diagrams. There are six steps, and each one has caught a hidden cost at some point.

Step 1: Define What the PLC Actually Has to Control

Before you compare processors, draw a one-line diagram of the process around the PLC. What field devices will it command? VFD motor controllers? Solenoid valves? HVAC contactors? Belt drives? Every one of those needs an output card, a network module, or a relay interface. The controller is just one piece, and it's usually not the piece that surprises you.

I've seen engineers pick a high-end ControlLogix processor because it's the “safe” choice, only to discover that the existing I/O chassis is a different revision. That mistake cost $1,200 in adapter hardware in our facility. If the team had spent one hour listing the field devices first, they would have realized a CompactLogix could handle the job for half the price.

Rule: match the CPU to the I/O count and communication protocols you actually need, not to the brochure.

Step 2: Compare the Real Advantages and Disadvantages

Let's be honest about Allen-Bradley PLC advantages. The hardware is generally durable. The software is widely known. The support network is about as deep as it gets in North America—which is a big deal when you're looking at uptime. If your plant already runs Studio 5000, staying with the same family, whether ControlLogix or CompactLogix, saves you training and avoids integration surprises.

But there are real disadvantages too. There's a price premium for the brand. The software licensing structure is confusing from a procurement standpoint—I've never fully understood why a software license can cost more than the controller we bought. And some specialty communication cards have lead times that stretch far beyond the base PLC delivery.

So don't just list pros and cons on paper. Ask yourself: do my people already know this platform? If yes, that's a huge hidden advantage. If no, add training and first-year tech support into the total cost.

Step 3: Demand a Transparent, Line-Item Quote

This is the step where I've earned my salary. When asking for quotes, I now say: “Please list what is NOT included along with what is included.” It sounds trivial, but it changes everything. A vendor who lists all fees upfront—even if the total looks higher—usually costs less in the end. Hidden-fee patterns often hide in software licensing, terminal blocks, shipping, cables, documentation, or “engineering support.”

I remember comparing two Allen-Bradley bids for a redundant PLC setup. The lower-priced quote looked like a clear winner until I dug into the details. Programming was billed separately. Spare parts were not included. The warranty seemed thinner. The total turned out to be nearly the same as the higher bid, but with fewer protections. That “cheap” quote would have generated a $1,200 redo when quality failed later.

Per FTC guidelines (ftc.gov), advertising claims must be truthful and not misleading. I believe the same standard should apply to vendor quotations. You're not being difficult by asking for the actual total before signing. Transparent pricing builds trust; vague quotes don't.

Step 4: Check the Supporting Cast: VFDs, Contactors, Power Supplies

A PLC is the brain, but it doesn't do much without muscles. We learned this when we had to replace a contactor on a rooftop HVAC unit. My first instinct was to search for “hvac contactor near me” to get a quick local replacement. But I almost ordered the wrong contactor because the coil voltage didn't match the PLC output card. The part was cheap. The mismatch would have been expensive.

Likewise, when a VFD motor controller fails, don't just match horsepower. Check the communication protocol. Many VFDs use analog signals or discrete run contacts, but newer ones use Ethernet/IP or serial. If the PLC and VFD don’t speak the same language, you’re adding wiring, setup time, and possibly another module.

Add a line item for every interface type. A quick data-sheet check before ordering can save you from a week of downtime.

Step 5: Use a Multimeter for a 30-Second Battery Test

One of the most embarrassing moments in my career was a frantic service call for a “dead” PLC processor on our packaging line. The integrator arrived, opened the controller, pulled out the battery, and tested it with a multimeter. The voltage was 1.8 volts instead of the expected 3.0. The processor wasn't dead. The backup battery was.

This is exactly the same method you'd use to test a car battery with a multimeter: set the meter to DC volts, touch the probes to the terminals, and read the voltage. For a lithium cell in a ControlLogix CPU, anything below 2.8V is questionable. Replacing the battery costs about ten dollars and takes two minutes. Not testing it can cost a $400 service call and hours of downtime.

I want to say the battery part number for the 1756-L8x is something like 1756-BA2, but don't quote me on that—check your manual. The point is to put it on the preventive maintenance schedule. PLCs don't stop working when the battery dies. They stop working the next time the power flickers.

Step 6: Calculate 10-Year TCO, Not Just Today's Price

Let's talk total cost of ownership. When I audited our 2023 automation spending, software updates and training made up about 9% of the budget. Nobody quoted that number at the beginning, but it's part of owning Allen-Bradley PLCs.

Think about the next decade. You'll need a spare I/O card, maybe a power supply, definitely a battery. You'll need someone who can program the unit, and that skill has a price. Software maintenance agreements may be required to keep compatibility with your laptops. These aren't hidden fees; they're predictable costs. But they're easy to miss if you focus only on the controller price.

Urgency is the enemy of honest procurement. If a production line is down, the temptation is to accept the first vendor's rush quote. Even in an emergency, a quick battery check and a spec check can keep you from buying a more expensive processor than you actually need.

Common Mistakes I Still See

Here are three patterns that keep showing up in our industry. First, engineers fixate on the CPU but ignore the battery, the firmware version, and the spare parts situation. Second, buyers assume a higher price always means better quality. Not true. It might just mean more margin. Third, nobody asks “what's NOT included” until the surprise invoice arrives. I made that mistake early in my career. It took two budget overruns to teach me.

Allen-Bradley PLCs—including ControlLogix—are proven tools. They're not cheap, but they're dependable. My job is to make sure the price we pay actually gives us the reliability we expect. This checklist helped me stop guessing and start asking the right questions. I hope it does the same for you.