This article provides pragmatic recommendations for selecting a PLC. It assumes the reader has some PLC knowledge but excludes advanced topics such as networking, IO-Link, and motion control.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Select It → PLCs
Difficulty: 
Engineer — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 06 Mar 2026
Introduction
Thousands of people have asked this question. It’s the type of question a highly motivated individual asks when they are ready to cross the bridge from the technician “fix it” mindset to the “build it” mindset of the industrial control professional.
On a personal note, I have yet to meet a person with a linear career path into industrial automation. Instead, I see a wonderful complexity and near-accidental crossing into the field. All are intense, highly driven individuals with good stories to tell.
What matters when selecting a PLC?
Since this is one of your first projects there are a few rules of thumb:
Stay Within the PLC Family
- Look around the shop and select a PLC from the same family. This allows you to leverage the local knowledge base. Chances are high that DigiKey offers a PLC from the same family.
Determine the Required Number of Inputs and Outputs (I/O)
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Inventory the machine to determine:
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Number of sensors
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Number and type of actuators, such as motor starters, pneumatic solenoids, and panel lamps.
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Provide a Safety Margin
Since this is your first project, accept the fact that you will make mistakes. Therefore, it is prudent to err on the side of too much. For example, you don’t want to be short in your I/O calculations, as an error would require a new PLC or possibly an expansion module. As a safety margin, multiply your initial I/O estimates by 25%.
Safety
Let me leave you with one final, but often overlooked, consideration. The general-purpose PLC is not safety rated and should never be trusted to shut down a machine.
For example, wiring an emergency stop into a general-purpose PLC is unacceptable. Instead, the emergency stop should be wired into a failsafe device rated for the hazard level of your machine. Click here for an introduction.
Safety: Always work with your leadership to implement safety protocols that align with all local, state, federal, and international regulations. This would include, but is not limited to, machine guarding, human factors analysis of operator, provisions for Lockout/Tagout (LOTO), and inclusion of safety-related equipment.
Tech Tip: It’s common practice for shops to standardize on one or perhaps two PLCs. This is a pragmatic decision to save developer and technician overhead. For example, if you are an “OMRON shop”, you only need training and software support for the OMRON PLCs.
However, this is not as static as it would seem. I’ve heard stories of OEMs changing their PLCs. Without getting into specifics, this is not a simple technical consideration. Cost, effective support, short- as well as long-term availability, and yes, even reputation matter.
And that’s where DigiKey comes in.
Parting Thoughts
If you found this piece useful, have I earned the right to tell you where to purchase a PLC?
Come on over to DigiKey and explore the 1,865 offerings (as of mid early 2026).
Let us know if you have any questions by leaving a comment below or by starting a new thread in the DigiKey TechForum.
Best wishes,
APDahlen
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.