Identifying the “best” PLC language has consumed gallons of ink. Many argue for IEC 61131-3 Ladder Logic (LD) as the universal solution while others prefer Structured Text (ST).
Fair enough.
However, this assumes the technician has access to the program. In many cases they do not.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Teach It → PLC Language for the Classroom
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 12 Mar 2026
PLCs are Locked by Default
This is self-evident to all of us who have started a new PLC program. The first step is to set a lockout password for the PLC or to wade through the system and disable the password, leaving the PLC wide open for anyone to upload, study, modify, or copy the program.
Discussion Points
Yes, there is a good argument that the service technician should have access to the PLC. This may even be desirable for simple machine control. However,
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There comes a point where the program complexity is beyond what the technician and technical leads can quickly identify. Here the term “quickly” implies that the line is down, and the equipment must be repaired quickly.
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There are safety and liability considerations. As an example, a well-intentioned technician or installer could inadvertently modify the program to disable a safety-critical control, resulting in damage to the machine or injury to personnel. For instance, we have all heard the story of the engineer who learned the meaning of mechanical stop when an out-of-control machine missed the electrical interlocks and then violently sheared the mechanical interlocks.
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Modification of the program may void the equipment warranty.
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The original programmer may preserve their intellectual property by locking down the PLC.
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The original programmer can ensure long-term revenue via support contracts by providing specialized services that are difficult to reproduce in house.
Collectively, these threads get tied up with right-to-repair. Just because you own the machine does not imply that you are able to repair or modify the machine.
Bottom line: If the end user doesn’t own the program, the programmer should use best in-house practices. Do you agree this is the best solution for all parties over the life of the machine?
Diagnostics Mitigate the Technician’s need to View the Program
We can expect the original programmer to mitigate the lack of program access with good diagnostic tests. Examples include an HMI with technician-friendly diagnostic tools (Figure 1). It would also include the ability to manually step or control the process if it is safe to do so.
Figure 1: Screen capture of the live debug for structured text showing that the IF statement evaluates as true.
Technician Education Consideration
As a rule, the curriculum is saturated with topics. I’ve been there as part of curriculum design. Instructors struggle to cover the basics from elementary electronics, theory, mathematics, test equipment operation on up to advanced topics. As a result, industrial automation topics don’t receive that much attention as we would like.
Is PLC learning considered training or education?
At best, we see a few PLC classes with the opportunity to showcase industrial control in a capstone class. Also, I’ve talked to more than a few educators who disregard industrial automation claiming it is “training” as opposed to “education.” The bottom line is that students leave many colleges and technical schools with little beyond ladder logic.
There are a few bright spots in the trade schools. I visited one that was truly impressive across multiple dimensions with excellent instructors, equipment, and support systems. They dedicate the time to explore the topics surrounding the PLC, including safety, motor drives, and networking.
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Related Foundational Articles
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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.