PLC (Programmable Logic Controller) programming is the backbone of modern industrial automation. Whether you are running a conveyor system, managing a water treatment plant, or controlling a complex manufacturing line, the quality of your PLC code determines the reliability, safety, and efficiency of your entire operation.
Yet even experienced engineers fall into common traps that can lead to unexpected downtime, safety hazards, and costly rework. At ILEK Technologies, having delivered automation solutions across industries, we have seen these mistakes repeatedly - and more importantly, we know exactly how to prevent them.
In this blog, we break down the 5 most common PLC programming mistakes and share practical tips to help you avoid them.
Mistake #1: Poor or No Program Documentation
The Problem
One of the most widespread mistakes in PLC programming is writing code without proper comments, tags, or documentation. Programmers often rush to complete the project and skip labelling rungs, naming I/O tags meaningfully, or adding any inline comments. The result? A program that works on day one but becomes a nightmare to troubleshoot six months later especially if someone else has to maintain it.
Real-World Impact
Imagine a motor fault alarm going off at 2 AM. The on-call technician opens the PLC program and sees hundreds of rungs with tag names like "BIT1", "CONTACT_A", or "TIMER_7". Hours of production downtime result from what should have been a 10-minute fix.
How to Avoid It:
- Use descriptive tag names - e.g., "ConveyorMotor_Run" instead of "M1_Coil"
- Add a comment to every rung explaining what it does in plain language
- Maintain an external document with I/O list, tag descriptions, and wiring details
- Follow a consistent naming convention throughout the entire project
- Document any changes with date, engineer name, and reason for change
Mistake #2: Ignoring Scan Time and Program Efficiency
The Problem
PLC programs execute in a continuous scan cycle - read inputs, execute logic, write outputs, repeat. Every rung in your program adds to the scan time. Programmers who do not consider program efficiency end up with bloated code that causes high scan times, delayed responses, and in worst cases, missed process events or safety trips.
Common Culprits:
- Excessive use of complex math functions in every scan
- Polling external devices unnecessarily on every scan
- Using too many nested subroutines without clear logic
- Running communication blocks without limiting their trigger frequency
How to Avoid It:
- Monitor scan time during commissioning and set acceptable thresholds
- Use periodic task routines (e.g., every 100ms) for non-critical calculations
- Break large programs into smaller, modular subroutines called only when needed
- Limit communication instruction execution using one-shot triggers
- Test scan time under maximum load conditions before go-live
Mistake #3: Inadequate Fault Handling and Safety Logic
The Problem
Many PLC programs are written assuming everything will work perfectly. Sensors will always respond, motors will always start, and communication links will never drop. This is a dangerous assumption. When faults occur and they will a program with no fault-handling logic can freeze, behave unpredictably, or worse, cause equipment damage or operator injury.
Real-World Impact
A sensor wire breaks and the PLC does not detect it - the logic continues as though the sensor is "OFF". A conveyor runs into an obstruction because no feedback fault was detected. A simple $50 sensor problem turns into a $5,000 mechanical repair.
How to Avoid It:
- Implement watchdog timers to detect communication loss or logic hangs
- Add feedback verification confirm motors have started within a timeout period
- Program a safe state: define exactly what happens when a fault is detected
- Use PLC diagnostic instructions to catch CPU faults, memory errors, and I/O failures
- Separate safety-critical logic from standard control logic where possible
- Test fault scenarios during FAT (Factory Acceptance Testing) - simulate broken wires, lost comms, and sensor failures
Mistake #4: Not Using Modular / Structured Programming
The Problem
Many beginner and some experienced programmers write all their logic in a single massive program file. This "spaghetti code" approach makes the program hard to read, hard to debug, and nearly impossible to reuse. When the same logic is copy-pasted across multiple sections instead of being called from a reusable function block, any change must be made in 10 different places - and forgetting even one is a guaranteed bug.
How to Avoid It:
- Divide your program into logical sections: Startup, Normal Run, Fault Handling, HMI Communication
- Use Function Blocks (FBs) or Add-On Instructions (AOIs) for repeated equipment types like motors, valves, and pumps
- Parameterise your function blocks so they can be reused for any instance of the same equipment
- Review your program structure before you start coding - plan first, code second
Mistake #5: Skipping Proper Testing and Simulation
The Problem
Time pressure is real in industrial projects. Deadlines lead programmers to skip thorough testing and go straight to live commissioning on the plant floor. This approach is risky - bugs discovered during live commissioning can cause equipment damage, production loss, and in some cases, safety incidents. The cost of fixing a bug after installation is 10x higher than catching it in simulation.
Real-World Impact
A sequence logic error causes two actuators to operate simultaneously when they should interlock. During live commissioning, the result is a mechanical clash causing thousands of rupees in damage - a scenario that 30 minutes of offline simulation would have caught.
How to Avoid It:
- Use PLC simulation tools (e.g., Siemens S7-PLCSIM, Rockwell RSLogix Emulate) to test offline
- Write a test plan that covers every mode: normal run, emergency stop, manual override, fault recovery
- Perform a Factory Acceptance Test (FAT) with the customer before site installation
- Simulate all fault conditions - power loss, sensor failure, communication drop
- Do a staged commissioning: test one section at a time before integrating the full system
- Involve maintenance personnel in testing they know the real-world failure modes best
Final Thoughts
PLC programming is both an art and an engineering discipline. The difference between a reliable automation system and one that constantly causes headaches often comes down to the quality of the programming practices behind it. By avoiding these five common mistakes - poor documentation, ignoring scan time, inadequate fault handling, unstructured code, and skipping testing - you can dramatically improve the reliability, safety, and maintainability of your automation systems.
At ILEK Technologies, we follow best-in-class programming standards for every project we deliver - from small machine automation to full plant-wide SCADA integration. Our goal is not just to make a system work on day one, but to make sure it works flawlessly for years to come.
Have questions about your PLC project? Need a second opinion on your existing automation code? Contact ILEK Technologies today - we are happy to help.