Fault Diagnosis: Common Causes of Siemens PLC Program Freezing and Quick Troubleshooting Methods

As an automation engineer, I cannot count how many times I have been called to the site to handle the issue of “the equipment suddenly stopped working.” In most cases, this is related to the PLC program freezing. Today, I will share with you the common causes of Siemens PLC program freezing and the quick troubleshooting techniques I have accumulated.

What Does PLC Freezing Look Like?

“PLC freezing” simply means that the PLC has stopped its normal scanning cycle. Just like our phones occasionally freeze and become unresponsive, PLCs can also “strike”. This usually manifests as: the device stops responding, output points do not activate, the human-machine interface cannot communicate, the STOP light is constantly on or flashing, etc.

PLC freezing is different from ordinary program logic errors, as it is usually a system-level issue that can paralyze the entire control system.

Common Causes of Freezing

1. Hardware Failures

Hardware-related issues are often the most direct and easily overlooked:

• Power Supply Issues: Unstable voltage or poor connections can lead to CPU resets or abnormal operation
• Communication Failures in Expansion Modules: Especially in distributed I/O systems
• I/O Card Failures: Damaged input/output cards can sometimes cause bus communication interruptions
• Memory Card Failures: Loose or damaged memory cards in the S7 series can also prevent the system from starting normally

I once encountered an interesting case: a device would inexplicably stop working around 3 PM every day. After two days of investigation, I discovered that at that time, sunlight was directly hitting the control cabinet, causing the internal temperature to rise, and an aging power module was outputting unstable voltage at high temperatures!

2. Software Logic Issues

• Infinite Loops: The program contains loop structures that cannot be exited
• Excessive Recursion Depth: Nested calls of FC/FB lead to stack overflow
• Division by Zero Errors: Performing division when the variable value is 0
• Illegal Memory Access: Accessing non-existent DB blocks or out-of-bounds addresses
• Watchdog Timer Timeout: Program execution time is too long, triggering the system protection mechanism

3. Communication-Induced Freezing

• Improper Handling of Communication Interrupt Timeouts: Such as interruptions in communication with HMI or host computers
• PROFIBUS/PROFINET Network Failures: Heavy bus load or severe interference
• Bus Contention from Multiple Masters: Configuration errors leading to bus conflicts

Quick Troubleshooting “Three-Step Method”

After years of practice, I have summarized a set of quick localization methods:

Step 1: Observe Diagnostic Indicator Lights

The indicator lights on Siemens PLCs are the most intuitive basis for judgment:– SF (System Fault) Light On: Hardware or I/O fault– BF (Bus Fault) Light On: Communication network issue– STOP Light Flashing: Program execution error– All Lights Off: Power supply issue

Step 2: Use Programming Software for Diagnosis

• Connect to TIA Portal or STEP 7, and check the diagnostic buffer
• Analyze error codes and downtime points
• Check system status and variable values in the forced table

Note: If unable to connect to the PLC, it may be due to a communication interface failure or the PLC being in a deep freeze state, requiring a power cycle to restart.

Step 3: Program Backup and Isolation Testing

• First, back up the current program
• Comment out suspicious code blocks one by one
• Test hardware functionality using a simplified program
• Gradually restore functionality and monitor system status

Real Case: Freezing Caused by Counter Overflow

Last year, in a packaging line project, the customer reported that the equipment would inevitably freeze after running for a few days. Through the diagnostic buffer, I found that there was an access error to a counting DB block before each downtime. It turned out that the production counter used a 16-bit INT variable, which overflowed after continuous production exceeding 32767!The choice of data type may seem trivial, but it can hide significant risks.

Prevention is Better than Cure

To avoid PLC freezing issues, it is recommended to take the following measures:

1. Properly Comment the Program, clearly indicating the functions of each module and the purposes of variables
2. Use Structured Programming to Avoid Complex Jumps and Nesting
3. Reasonably Set Watchdog Times and Loop Monitoring
4. Add Validity Checks for Key Variables, such as checking if the divisor is zero before division
5. Add Fail-Safe Mechanisms to Ensure Controlled Shutdown of the System During Abnormal Conditions
6. Regularly Back Up Programs and Parameters

Practical Exercise Recommendations

Select a simple Siemens PLC (such as S7-1200), intentionally write a program containing infinite loops or division by zero errors, observe the system response and diagnostic information. By simulating various fault conditions, familiarize yourself with the characteristics of different types of errors. Use the diagnostic buffer tool to analyze error codes and establish a correspondence between error phenomena and causes, which will greatly enhance fault diagnosis capabilities.

Remember:Systematic troubleshooting is more efficient than blind attempts, first check the hardware, then the software, and approach from the outside in is the professional way.