There's no single "best" Allen-Bradley PLC. It depends on your situation.
When I took over purchasing for our company in 2020, I thought choosing a PLC would be straightforward. Pick the one with the most I/O points, right? Wrong. I quickly learned that the right choice depends entirely on what you're trying to do.
Here's the thing: the Allen-Bradley lineup is huge—Micro800, CompactLogix, ControlLogix, and more. Each one solves a different problem. If you pick wrong, you either waste money on features you'll never use or end up with a system that can't scale.
Let me break it into three common scenarios I've seen (and lived through).
Scenario A: The Small, Standalone Project — Budget is Tight
This is where I started. A small conveyor system for our packaging line. Maybe 16 inputs, 12 outputs. Nothing fancy. The temptation? Buy the cheapest PLC you can find. But that's a trap.
I almost ordered a MicroLogix 1100 from a surplus vendor. Price looked great—$380 compared to $650 for a new Micro850. But then I checked the software. The MicroLogix uses RSLogix 500, which is old. Real old. The Micro850 uses Connected Components Workbench (CCW), which is free and modern.
When I compared the two side by side—the outdated RSLogix 500 vs. the free CCW—I finally understood why the software ecosystem matters so much more than the hardware price.
My advice for a small, standalone project:
- Go with the Micro800 series (like the Micro820 or Micro850). They're affordable and the software is free.
- Don't cheap out on the power supply. I did once. It failed after 14 months.
- If you need motion control, the Micro850 is your best bet. The 820 doesn't support it well.
One thing I learned the hard way: I told the vendor "I need a PLC for a small conveyor." They heard "give me the cheapest thing you have." Result: I got a Micro810 with barely enough I/O. Had to order expansion modules later, which cost more in the end.
Scenario B: The Medium-Sized Retrofit — You Have Existing Gear
Our biggest project to date was upgrading a production line with about 100 I/O points, some analog sensors, and existing PanelView HMIs. This is where the CompactLogix series shines.
The mistake people make? Trying to use a Micro850 for this. It can handle it technically, but you'll fight the limited networking and data logging capabilities forever.
I said to the controls engineer, "Let's just use a Micro850. It's cheaper." He said, "Sure, if you want to spend 40 hours writing data logging from scratch." That conversation made me realize that hardware cost is only part of the equation.
For a medium retrofit:
- CompactLogix 5380 or 5480 is the sweet spot. You get ControlLogix-level features without the ControlLogix price tag.
- Make sure your existing HMI is compatible. Ours wasn't—had to upgrade that too.
- If you're using remote I/O over Ethernet/IP (which you probably should), the 5380 handles it seamlessly.
Had to decide in a hurry once—client was down, production was losing $1,200/hour. I went with the 5380 based on past reliability. In hindsight, I should have double-checked the firmware version. The first unit had an older rev that didn't support our PanelView. That cost us another day.
Scenario C: The Big New Plant — Scale & Redundancy Matter
This is the big leagues. We're talking 500+ I/O points, multiple production lines, safety systems, and the need for redundancy. This is ControlLogix territory.
I've seen this go wrong in spectacular fashion. A colleague of mine—let's call him Dave—spec'd a single CompactLogix for a plant that needed full redundancy. When the unit failed eight months in, the entire plant went dark for 6 hours. His boss was not happy.
For a new plant build:
- ControlLogix 5580 or 5570 with redundant chassis. It's expensive, but the downtime cost of not having redundancy is higher.
- Don't forget the software: Studio 5000 (which is the modern name for RSLogix 5000). It's not cheap—about $8,000 for a professional license—but it's the standard.
- Plan your network architecture upfront. One plant I sourced for had to add 4 extra switches because they didn't account for the I/O density.
To be fair, you can get away with a CompactLogix if you use dual-CPU configurations. But frankly, when you're building a $5M plant line, spend the extra $10K on ControlLogix. It's insurance.
How to Figure Out Which Scenario You're In
Still not sure? Here's a quick test I use:
- Count your I/O points (including 20% spare). Under 50? Scenario A. 50-300? Scenario B. Over 300? Scenario C.
- Check your existing plant. Do you already have ControlLogix on other lines? Stick with the same family for spare parts and training consistency.
- Look at your timeline. If you need to be up and running in 4 weeks, you're probably better off with Micro800 or CompactLogix—ControlLogix lead times are longer. That's been our experience, at least.
- Consider your team's skill set. Newer techs know Connected Components Workbench better than Studio 5000. Don't buy a platform nobody on your floor knows how to troubleshoot.
And hey—if you're still unsure, order from someone who lets you return unopened units. I've been burned by restocking fees before. Once ordered a MicroLogix 1400 for a job that ended up going to CompactLogix. The vendor wouldn't take it back. Sat on my shelf for two years.
