Siemens PLC Programming Debugging Techniques: Shorten Your Project Delivery Cycle by 30%

Hello everyone! I am Shui Gong, an engineer with over ten years of experience in the automation industry. Throughout my journey, from being a novice in the workplace to now being able to independently handle the entire process from project design to delivery, I have deeply realized that program debugging is the most important, time-consuming, yet often underestimated part of PLC projects. Today, I want to share some practical debugging techniques that can truly shorten your project delivery cycle by more than 30%, easily earning the recognition of your clients.

This article is especially suitable for those who are new to Siemens PLC programming, as well as engineers who already have some experience but want to further improve their efficiency. Whether it is the S7-1200/1500 or S7-300/400 series, these techniques can help you avoid detours and improve project quality.

1. Hardware Configuration and Environment Preparation: The Foundation of Debugging Efficiency

As we all know, the tallest building rises from the ground, the efficiency of project debugging largely depends on the hardware configuration and development environment in the early stages. Here, I summarize a few easily overlooked but very important preparatory tasks:

1. Select the Appropriate PLC Model and Modules

Select based on project requirements: For example, the S7-1200 is suitable for small applications, while the S7-1500 performs better in medium to large projects. Do not choose an underperforming CPU just to save money, as it will waste more time during debugging.
Reasonable module selection: For instance, does the resolution of the analog input/output module meet the accuracy requirements? Does the communication module support the protocols required by the client? Ignoring these issues may require rework during debugging.

2. Ensure Proper Hardware Wiring

Clear labeling: Every terminal and wire must be labeled. If you cannot find the corresponding signal during debugging, efficiency will be greatly reduced.
Good grounding: Especially for analog modules and communication modules, grounding issues can lead to signal interference or even equipment damage.

3. Build a Reliable Debugging Environment

The computer configuration should keep up; it is recommended to use a high-performance laptop that supports STEP 7 or TIA Portal software.
Ensure a stable network connection, using industrial-grade switches to avoid disconnections.

2. Core Debugging Approach: From Simple to Complex, Step-by-Step Verification

During debugging, many friends like to run the entire program directly at the beginning, only to find problems later and not know where to start. This is a typical debugging misconception. My experience is: break complex problems down into simple steps and verify functions step by step. Here is my debugging process to share:

1. Modular Programming + Module Testing

The benefits of modular programming: Divide the program into modules such as input processing, logic control, and output processing, making it easier to verify functions step by step.
Module testing: For example, only test whether the input module is receiving signals correctly, and then gradually test the logic and output.

2. Test I/O Signals

Input signal testing: Use the PLC’s input status monitoring function to verify whether each signal is in place.
Output signal testing: You can check whether external devices respond by manually forcing outputs.

3. Step-by-Step Verification of Core Logic

Use breakpoint debugging functions: In TIA Portal, breakpoint debugging allows you to pause program execution and check variable changes step by step.
Observe important data blocks: Make good use of DB (data blocks) to view the specific values of variables during program execution, especially during complex calculations or state transitions.

3. Code Implementation and Technical Details: Improve Program Debugging Readability and Flexibility

Writing clear and understandable programs makes debugging much easier. Here are some programming tips I have accumulated to share with everyone:

1. Variable Naming Standards

Use meaningful names, such as Motor_Start and Sensor_Status, avoiding direct addresses like M0.0.
Differentiate variable types by prefix, for example, I_ for input signals, Q_ for output signals, and DB_ for data blocks.

2. Detailed Comments are Essential

Add comments for each segment of code and complex logic, explaining functionality and key points. You will thank yourself when you look at the code three months later.

3. Flexibly Use State Machines

For complex process control, using a state machine is a very clear design approach. For example: 0=Standby, 1=Starting, 2=Running, 3=Fault, easily manage processes through state transitions.

4. Use Simulation Functions

Siemens’ PLCSIM tool can simulate program execution without hardware. Using simulation functions can help identify most logical errors in advance.

4. Function Expansion and Practical Application Cases

After completing the basic functionality debugging, clients usually propose some expansion requirements, such as adding alarm functions or implementing remote monitoring. The realization of these functions needs to consider debugging flexibility and scalability.

Case Study: Intelligent Filling Machine Control System

I once participated in a project for an intelligent filling machine, where the client required:

Precise filling control;
Fault alarms and historical records;
Remote monitoring functionality.

Key points during debugging:

Filling control: Read flow meter signals through the analog module and use PID function blocks for precise control. I set up a dedicated debugging DB block to observe PID parameters and output values in real-time, quickly adjusting control effects.
Fault alarms: Use HMI to implement alarm levels and time records, focusing on checking whether each alarm condition triggers correctly during debugging.
Remote monitoring: Achieve data exchange between PLC and SCADA systems through Profinet communication, paying special attention to network configuration and communication delays during debugging.

5. Debugging Methods and Suggestions: Practical Tips to Avoid Detours

1. Record Debugging Logs

Be sure to record changes and reasons for each program modification, making it easier to find problems later.

2. Prepare Common Tools in Advance

Including multimeters, oscilloscopes, industrial Ethernet testers, etc., which may be needed at any time during debugging.

3. Maintain Communication with Clients

If you encounter unclear functional requirements during debugging, communicate with the client in a timely manner to avoid unnecessary work.

6. Common Problems and Solutions

1. PLC Cannot Download Program Normally

Check if the communication settings are correct, such as IP address and port.
Confirm whether the PLC is in STOP mode.

2. Analog Signal is Unstable

Check if the grounding is good or if there is strong electrical interference.

3. No Response from Device After Program Runs

Check if the output signal is correctly activated.
Check if the hardware wiring is loose.

7. Summary and Insights

Debugging is a process that tests both patience and technical skills. Through reasonable hardware configuration, clear program design, and step-by-step debugging methods, efficiency can be greatly improved, reducing unnecessary rework. In this process, recording and learning are particularly important; every debugging experience will take you one step further.

Finally, I welcome everyone to share their debugging insights or challenges in the comments section, so we can discuss improvement methods together. I hope this article helps you, and let’s continue to advance on the path of automation!

Engineer Zhang May 2025

Tip: Always stay calm during debugging; remember that the more anxious you are, the more mistakes you make. Following the steps is the key to efficient debugging!