How to Troubleshoot Obscure Errors in Custom PLC Code

How to Troubleshoot Obscure Errors in Custom PLC Code


Programmable logic controllers (PLCs) are digital computers used for automation of industrial processes. They run custom code to control machines and processes. As with any complex software system, obscure errors can occur that are difficult to troubleshoot. This article provides an in-depth guide on techniques and strategies to diagnose and fix obscure errors in custom PLC code.

Understanding the PLC System

To effectively troubleshoot PLC code errors, you need to understand how the PLC system is designed and operates.

PLC Hardware Components

The main hardware components of a PLC include:

PLC Operating System

The operating system manages tasks like:

Ladder Logic Programming

Ladder logic is the main programming language used with PLCs. It uses graphic symbols to emulate electromechanical control circuits. The main elements are:

Understanding general ladder logic programming techniques is key for troubleshooting errors.

Check PLC System Health

Before diving into application code, check the overall health of the PLC system.

Fix any underlying system issues before troubleshooting application code.

Understand Ladder Logic Execution

It's critical to understand how ladder logic executes to effectively troubleshoot errors.

Know how code executes across branches and scan cycles. Visualize program flow.

Check Ladder Logic Structure

Look for structural issues in the ladder logic that could cause obscure problems:

Cleaner and simplified code logic will reduce obscure bugs.

Verify I/O Addresses

One of the most common sources of issues is incorrect I/O addressing in code.

Meticulously confirming I/O addresses will reveal many obscure errors.

Review Data Storage Areas

Data storage areas like registers, counters, and timers are used across code sections and can be sources of problems.

Data storage is complex so carefully inspect these areas of code.

Check Error Logs

The PLC logs various errors and warnings that can provide clues on obscure bugs.

Detailed error logs often point directly to root causes.

Use Debug Tools

Debug software tools are invaluable for solving obscure errors through visibility.

Use all available debug features to gain visibility.

Leverage Ladder Logic Best Practices

Many obscure bugs can be avoided by using ladder logic best practices:

Clean and well-structured ladder logic avoids obscure logic issues.

Seek Additional Expertise

For extremely obscure errors, leverage additional expertise:

Obscure bugs often require tapping specialized knowledge.


Troubleshooting obscure PLC errors requires methodically checking system health, understanding program execution, verifying logic structure and I/O addressing, inspecting data areas, checking logs, utilizing debug tools, following best practices, and seeking help from experts. Meticulously following these troubleshooting techniques will systematically isolate and correct even the most obscure PLC errors.