Teaching Digital Logic for Industrial Control and Automation Students
Industrial automation students learn digital logic concepts faster when the examples look like real machines. A straightforward solution is to use the PLC itself as a logic trainer with graphical function block programming augmented with Sequential Function Chart (SFC). The resulting lessons are better aligned to the language and techniques used in an industrial setting. Perhaps more importantly, the technique lowers the barrier to entry into state-machine design. At its core, this is a time allocation problem. Changing the course sequence allows state-machine concepts to be introduced earlier with a longer reinforcement time.
Key Takeaways
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PLCs such as the Crouzet Millenium Slim are programmed graphically using both a gate-level logic representation and Sequential Function Chart (SFC).
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The D flip-flop example demonstrates that complex circuits may be constructed from individual gates within the PLC. The introductory lessons are semantically similar to 74-series logic and gate-level capture in an FPGA.
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An example vault with a binary combination demonstrates that complex sequential circuits may be constructed with a combination of gate-level logic and SFC.
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The same teaching pattern works in PLCs other than Crouzet. For simplicity, only one toolchain solution is shown.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Teach It → Digital Logic
Difficulty: 
Engineer — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 16 Jun 2026
How to Sequence a Digital Logic Course
Gate-level logic is a non-negotiable part of the technical language.
Combinational Logic
The traditional sequence for combinational logic remains unchanged. The following topics are still covered:
- AND, NAND, OR, NOR, XOR, and NOT gates
- Sum of products
- Binary numbering systems
To this we add an industrial flavor using the language of interlocks and permissives. We also include sensors and actuators to keep the ideas grounded in industrial systems. For example, “the pump is locked out when the tank is empty” is preferable to the bland “F equals A AND B_Not.” It’s the same Boolean logic, but students can now see why it matters.
Sequential Logic
This is where we depart from the traditional emphasis on gate simplification, K maps, Quine-McCluskey, 74 series breadboards, and FPGA tools which pull curriculum toward computer architecture. These are all valuable lessons, but few are directly in the critical path for students studying industrial controls. What they need is additional time on task with state machine representations. This is provided by the Crouzet SFC representation with supporting decisions made using graphical gate-level tools.
Tech Tip: Naming individual wires is critical for understanding states. Crouzet allows both schematic-like signal routing and naming connections as described in this article. Naming select wires with the state name provides a strong learning signal as the state machine becomes visible. This solidifies the conversation as the learner can point to a particular wire in the debug window and declare, “We are in the idle state.”
Example 1: D Flip-Flop Built from Logic Gates
The D flip-flop constructed from individual gates is the bridge between combinational and sequential logic. This demonstration provides the best evidence that the Crouzet PLC may be used as a logic gate trainer.
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Figure 1 shows a 5-state shift register constructed using the D flip-flop macro. The operator can clock in data using physical pushbuttons. The PLC is shown in the debug states with the binary value of
10011maintained in the shift register. -
Figure 2 presents a Crouzet-based D flip-flop constructed using individual gates. It consists of two D latches in a master-slave configuration so that the input
Dis transferred to the outputQon the rising edge ofCLK.
Together, these screen captures demonstrate that the Crouzet gate-level logic is accessible to construct complex combinational circuitry. It also features macro encapsulation allowing students to conceptually think about circuits one level higher than the individual gates.
Figure 1: Shift register constructed using five D flip-flop macros with an image of the PLC trainer.
Figure 2: Crouzet PLC gate-level representation of a D flip-flop.
Tech Tip: There is a false dichotomy between 74-series, PLC graphical, and FPGA-based approach. All are valid, but each has a unique emphasis. The 74 series places emphasis on optimization for a minimum number of gates, circuit layout, and elimination of gremlins such as contact bounce. The FPGA demands abstraction as it naturally bends toward datapath and register-transfer in circuits that begin to resemble microcontrollers. The PLC de-emphasizes these lessons in favor of state control of machinery.
Example 2: Binary Vault Combination Using SFC
Figure 3 presents the code for a state machine simulating a combination safe. Using the three pushbuttons, the operator enters the combination as 7, 5, 1, and 4. This method of state machine coding is highly desirable as the SFC makes both the active state and state transitions visible. For example, Figure 3 shows that the token in the third state waiting for the Key_3_as_1 button press.
Observe:
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The state machine is based on SFC.
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Near the top is a resettable initial start block with which the token is instantiated. The state machine is reset when the D2 pushbutton has been pressed for five seconds.
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The state is advanced based on the
Key_N_as_Xvariables. This is based on an AND of the three inputs as well as theNewCombostrobe. -
The
NewCombostrobe is set 250 ms after the operator first pushes a button. -
The vault’s state machine does not leak information. The user’s visibility is limited to the initial start state after a reset and the final timed opening of the vault solenoid. All other states are hidden.
Tech Tip: The SFC is visualized as a token passing machine. Each block has a gate symbol with a left-hand slide activation. When the gate is opened, the token falls through to the next state. The exception is the initial start block with the square-within-square symbol. When reset, the token will be moved from the currently active state to the initial state.
Figure 3: Debug for the state machine. It is waiting for the 3rd key in the combination.
When Not to Use a PLC as the Main Logic Trainer
This technique does not replace the valuable lessons of metastability, fan out, the challenge of clean breadboarding, or the elegance of HDL. However, those topics are not generally required by the industrial technician or technologist. Instead, the focus is shifted to concepts such as interlocks, permissives, resets, and the fault handling techniques that control a machine. The PLC approach also provides a hardware anchor as students wire machine appropriate sensors, pilot devices, and actuators. This naturally provides a continuous opportunity to troubleshoot industrial systems.
Will students still learn digital logic?
Yes. The PLC-centric approach does not remove the foundational lessons in combinational circuits. The gate-level thinking remains as do patterns such as sum-of-products. What it does is recenter sequential logic lessons with a method that promotes state-based thinking. If time permits, students can use the D flip-flop to build the state machinery.
From a competency-based education perspective, advanced digital logic is simply another module that builds on the digital lessons. This could include the FPGA, microcontroller, or PCB design with high speed digital circuits.
Parting Thoughts
Industrial automation students absolutely need the language of digital logic. The primitive gates will forever be part of the technician’s and engineer’s language. These essential concepts don’t end with the gate-level representation. They continue in ladder logic and structured text which assume the Boolean operators are ingrained in the learner’s mental model.
That students learn digital logic was never the question. Instead, we are wrestling with how to identify the best learning sequence to teach the fundamentals while staying within the swim lane of industrial automation. A PLC-based technique preserves the foundation while staying within the environment students are most likely to apply those lessons.
Continue Exploring Industrial Control Systems
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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.