This brief introduces critical PLC aspects that are often overlooked. Instead of focusing on the hard datasheet specifications, we examine the soft (human) specifications such as consistent usage across the factory, software, workforce training, future modifications, and obsolescence.
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: 01 May 2026
Who is involved in the selection of a PLC throughout its lifecycle?
There are many people who will interact with the PLC and associated hardware/software including:
- System integrator and machine designers will select a PLC
- Engineers, programmers or technicians will code the PLC
- Technicians and engineers will commission the PLC-based equipment
- Technicians will service the PLC-based equipment
- Engineers, programmers, or technicians will modify the equipment and software
- Engineers, programmers, or technicians may add a remote interface or industry 4.0 data sharing capabilities
- Engineers, programmers, or technicians may replace the PLC in the distant future
Involvement varies with skill level
The number of people involved with this process varies by the complexity of the system. Sometimes a single skilled person will perform all tasks within a factory. The most complex system will have a contracted team working with in-house personnel to design, implement, and maintain PLC based systems. The selection process is further complicated when we consider turnover and promotions. It is unlikely that the team will remain unchanged over the decade’s long lifecycle of the PLC.
What are the soft (human) requirements for PLC selection?
PLC and associated components are used in industrial control of automation processes. System downtime is an ever-present concern as the cost can range from hundreds to thousands of dollars per minute in idle workforce, wasted product, loss of goodwill, and overtime to boost product numbers. In other words, the initial cost of the PLC is insignificant when compared to the associated lifecycle cost.
Selecting the best PLC is a holistic process that involves the hard PLC requirements along with many in-house considerations. Here are a few of those soft requirements:
Consistency across the factory
Some factories will select a single PLC or family of PLC for use on the factory floor. This is beneficial as technicians can focus on a single PLC and associated software package. Logistics is also simplified as the PLC family will likely share many accessories such as a plug-in or expansion modules.
Software Consistency from Designer to Technician
Software is one of the most contentious arguments surrounding the PLC and PLC selection. On one side we find purists who insist that all programs be written in ladder logic. On the other we find people who will write in the latest python, C++, Node-RED, MATLAB etc., and then use docker to push the changes to the production system. Both are correct, provided they account for the skill and knowledge level of their team with the understanding of the long lifecycle and the high-cost system downtime. DigiKey has solutions to fit both camps.
Future Expansion
Future expansion is an important consideration for PLC selection. It’s not unusual to expand the PLC to include advanced control modes, additional sensors/actuators, or to add networked remote-control and data monitoring capabilities. This soft requirement implies that a PLC should be over specified in terms of computational power and memory. It should also have ample resources for expanded hardware and networking capabilities.
High Cost of Modifications
What happens when the equipment requires modification ten years after the original programmer has retired?
Modifications of PLC-based equipment carries a high cost. The programmer must understand the PLC software and the process/machine that is being controlled. This includes all of the corner cases such as those involved with startup, shutdown, and error handling. As a result, manufacturers are hesitant to change PLCs for operational equipment. While they may think nothing about changing a workstation every few years, the PLC will likely remain unchanged for the decades-long life of the machine.
Education Pipeline and Support for Long-Term Viability
The final soft specification is a recognition of support offered by manufacturers, trade schools, engineering schools, textbook publishers, and the larger community. This includes contributions to social media in the form of how-to videos. PLCs with a larger support system are likely to be more popular.
Wrapping Up
Bottom line: PLCs tend to be selected on popularity as a hedge against future obsolescence.
- This includes an expectation that the PLCs and associated parts will be available into the next decade.
- There is also an expectation that a thriving PLC repair industry will be available to service the equipment.
Real World Example of PLC Longevity
Your PLC is obsolete. How many years can you continue with the existing PLC?
The Rockwell Automation (not currently carried by DigiKey) introduced the SLC 500 over 30 years ago and then discontinued it about 10 years ago. Second hand or repaired modules such as the venerable 1746-OV16 (16 semiconductor output) are likely to be available for several decades to support the many SLC500 PLCs still in service.
Continue Exploring Industrial Control Systems
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- Guide to Troubleshooting Industrial Control and Automation Equipment
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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.