Allen-Bradley PLC Questions: A Quality Inspector’s Straight Answers

I’m a quality/compliance manager at an industrial controls company, not a salesperson. I review about 200 Allen-Bradley PLC deliveries and control panels every year. In 2024, I rejected roughly 12% of first submissions—not because the parts were broken, but because the details were wrong: wrong power supply, wrong I/O variant, wrong wiring. Small stuff that would have become expensive field issues.

These are the questions I actually get asked, and my straight answers.

1. Is the Allen-Bradley PLC MicroLogix 1400 still worth buying?

If you’re supporting existing machines, yes. If you’re designing a new line, I’d look at CompactLogix first.

The MicroLogix 1400 (1766-L32BXB is the one I see most) is still a workhorse for small panel retrofits. As of 2025, Rockwell Automation’s Product Lifecycle page still lists it as active. It has embedded Ethernet, and RSLogix 500 is stable. But “active” doesn’t mean “future roadmap.” Rockwell’s development focus is clearly on Studio 5000 and the CompactLogix platform. I’d only approve a new machine with a MicroLogix 1400 if the customer’s maintenance team already knows it intimately.

For a new line, a CompactLogix 5380 gives you better communications and a longer support horizon. It costs more upfront; it also costs less to change later. Which, honestly, is the part that matters to me.

2. Mexico PLC market share: Siemens vs Allen-Bradley—who has more?

I get this one often, and I don’t trust the single-number answer.

Global supplier rankings from ARC Advisory Group (2024) place both Siemens and Rockwell Automation/Allen-Bradley in the top tier. But Mexico-specific share depends on the vertical. In maquiladora plants that follow U.S. automotive specs, I see Allen-Bradley panels everywhere. In food and beverage lines with European machinery, I see Siemens more.

So if someone quotes you a precise “Mexico PLC market share Siemens vs Allen-Bradley” percentage, ask for the sector and methodology. (Which, honestly, is what I do.) Both brands are strong; I don’t treat it as a brand war.

3. Can an Allen-Bradley PLC control a rapid battery charger?

Yes. We’ve shipped several using a MicroLogix 1400 to sequence pre-charge, fast charge, and top-off. The PLC reads current feedback through an analog input, switches a DC-rated contactor with a digital output, and shuts down on fault.

But the PLC is only part of the system. The rapid battery charger’s interlock—the contactor, the connector sensing, the hardware shutdown—matters more than the controller. We specify those interlock requirements on the drawing and ask the customer to approve them.

One charger project taught me about language. We were using the same words but meaning different things. “Interlock” meant “PLC logic” to the power electronics engineer, and “physical contactor auxiliary contact” to me. Discovered this when the test failed because the PLC never saw the contactor close. Now I define “interlock” on every BOM.

I’d rather work with a specialist who knows their limits. If someone says “we can do the whole charging system, no problem,” I get skeptical. The good vendors say, “We know the charging circuit, you know the PLC—let’s define the interface.” That gets my respect.

4. Why would you reject a control panel because of a control panel light?

Because a pilot light is not decoration. It’s an operator’s first clue about machine state.

Pilot lights have voltage ratings just like contactors. A 120 V light on 24 V will be dim; a 24 V light on 120 V will fail—sometimes immediately. So the control panel light has to match the circuit. That’s a spec check, not an aesthetic review.

In 2023, we rejected a batch of 12 panels because the green run light stayed on after the E-stop was pressed. The LED wasn’t broken; it was back-fed through a commoned neutral. It took about 20 minutes to find, but it delayed delivery by a week. In a real emergency, that light would have told an operator everything was fine while the machine was stopped for an active fault.

A control panel light that lies to you is a safety issue. Not acceptable.

5. How do I test a power supply with a multimeter?

For a 24 V DC PLC supply, set the multimeter to DC volts. Put the red lead on the +24 V terminal, the black lead on the common terminal, and read.

IEC 61131-2 gives a nominal 24 V DC supply tolerance of -15%/+20%, which works out to 20.4–28.8 V. If you see below 20.4 V, the supply is weak, failing, or undersized. If you see above 28.8 V, you risk damaging input cards.

Test under load, not just no-load. I still kick myself for once trusting a no-load reading. The supply showed 24.2 V on the bench, then dropped to 18.9 V when I connected the modules and field sensors. If I’d loaded the supply before shipping, we’d have avoided a service call.

6. What’s the most common issue you find before power-up?

Wiring that doesn’t match the drawing. Not bad PLCs, not failed components—wiring.

I said “terminate per drawing.” They heard “we know how to land these wires.” Result: a 120 V signal on a 24 V input card. The module survived, but the controller faulted on first scan. The most frustrating part? We went over the field wiring plan three times. You’d think written specs prevent that. They don’t.

Now I verify field termination before every power-up. It costs an extra hour, but it’s cheaper than rework after shipment. Actually, let me correct myself: it costs an extra hour when I plan it; it costs two days when I don’t.

7. What should I check before buying a used Allen-Bradley PLC?

Ask for the full part number, not the family name. “MicroLogix 1400” isn’t enough. A 1766-L32BXB has 24 V DC inputs; a 1766-L32AWA has 120 V AC inputs. I once bought “a MicroLogix 1400” and received the wrong I/O type. The vendor understood the family but not the variant. Hindsight: I should have sent a one-line spec table instead of assuming.

Check the firmware revision against the Rockwell Product Compatibility and Download Center. Test every I/O point physically—force outputs, short inputs, verify the status LEDs. A used PLC can look like a great deal and still have a dead output channel. Also ask for the original invoice or a photo of the shipping label. If the label doesn’t match the part number, that’s a red flag.

Not ideal, but workable—after you test it. Otherwise, not workable.