The Practical Use of SFC46 (STP) in Siemens S7 PLC Systems

Abstract

SFC46 (STP) is a critical system function call in Siemens Step 7 PLC programming, specifically designed for stopping and terminating the execution of the current processing cycle in a controlled manner. This function is vital in certain industrial control scenarios where system shutdowns, resets, or process interlocks require immediate and secure halting of operations. This paper discusses the practical reasons for using SFC46, how it operates, and presents a case study in the context of a complex industrial environment.

Introduction

In automated control systems, ensuring safe and controlled process termination is crucial for maintaining both system integrity and operational safety. Siemens Step 7 PLC offers the SFC46 (STP) system function call, which allows the PLC to stop processing current tasks based on specific conditions. This functionality is often critical in error handling, emergency stop sequences, or processes that must halt before a certain threshold is reached.

The goal of this post is to highlight the operational need for SFC46, explore its technical background, and provide a detailed case study to demonstrate its practical application in a real-world scenario.

The Role of SFC46 (STP) in Siemens PLCs

SFC46 is responsible for stopping the cyclic execution of a program. It achieves this by either entering a state in which no further execution takes place (i.e., a stop mode) or preparing the system for manual restart or diagnosis.

Key Features of SFC46:

• Controlled Stop: Halts the current cycle or program without abrupt interruption, allowing the PLC to execute termination routines and avoid leaving outputs in an undefined state.
• Event-Driven: Triggered by specific conditions, such as critical process faults, hardware failures, or emergency stops.
• Process Safety: Ensures outputs and running processes are halted in a safe manner to prevent damage or hazardous conditions.

Why Use SFC46 (STP)?

There are several practical reasons why an SFC46 system function may be used in a Siemens S7 environment:

1. Emergency Stop: In industrial environments, safety mechanisms require systems to immediately halt upon detecting hazards, equipment failure, or manual emergency intervention. SFC46 enables a fast and reliable stop while avoiding catastrophic damage.
2. Process Interlocks: When certain conditions in the automation process dictate that specific tasks or machinery must stop, SFC46 allows the system to enter a safe mode while waiting for reset or diagnostics. For example, it can prevent pumps from running dry or motors from overheating.
3. Error Recovery and Diagnostics: If an error occurs that requires human intervention, SFC46 can stop the program, allowing engineers or operators to diagnose the issue without any risk of further damage to the equipment.
4. Restart Management: By using SFC46, the PLC can enter a state where further execution halts until an operator manually restarts the system. This is important in scenarios where automated restarts could be dangerous or lead to undesired process behavior.

Technical Overview of SFC46

SFC46 is typically called from within a Step 7 PLC program under fault conditions or upon operator command. The function itself is not invoked by default but must be programmed as part of error-handling routines.

When triggered:

• The current execution cycle is halted.
• Outputs are safely transitioned into a known state.
• Depending on the program logic, SFC46 can lead to a total stop or a pause, awaiting further instructions.

Case Study: Application of SFC46 in an Industrial Pump System

Scenario:

In this case study, we examine an industrial pumping station that manages the transfer of water between reservoirs. The system includes multiple pumps controlled by Siemens S7 PLCs, and the entire process is monitored by a Supervisory Control and Data Acquisition (SCADA) system.

Problem Statement:

During normal operation, an issue was observed where one of the pumps continued running even when the water level in the reservoir was critically low. This caused the pump to operate dry, leading to overheating and eventual mechanical failure. In response, a control algorithm was implemented to shut off the pumps under certain conditions, but issues with false triggers or failure to halt properly persisted.

Solution Using SFC46:

The control engineers decided to use SFC46 to safely stop the program when the water level dropped below a certain threshold, or when a critical hardware fault occurred. By integrating SFC46 into the error-handling routines of the Step 7 program, the following advantages were realized:

1. Emergency Water Level Detection: When the SCADA system detected that water levels were too low, it sent a signal to the PLC, triggering the SFC46 function. This safely stopped the pump before any damage could occur.
2. Pump Failure Interlocks: If the sensors detected a fault in the motor or power supply, SFC46 was invoked to halt the entire process, avoiding further damage and preventing the pump from continuing in an unsafe state.
3. Safety and Restart Management: After the SFC46 function stopped the process, the system awaited manual intervention before restarting. This allowed maintenance teams to inspect and repair the equipment without fear of unintentional restarts.

Results:

• Reduced Downtime: By using SFC46 to safely stop the pumps during faults, the company reduced unplanned downtime.
• Prevented Damage: The function prevented pumps from running dry, reducing the number of mechanical failures.
• Increased Operator Confidence: Operators reported fewer false positives in the emergency stop system and greater confidence in the system’s ability to handle faults automatically.

Conclusion

SFC46 (STP) is an essential tool in Siemens S7 PLC programming for halting processes safely and securely. In industrial environments where equipment safety, error handling, and process integrity are paramount, this function ensures controlled stop sequences, thereby protecting both machinery and personnel. As demonstrated in the case study, integrating SFC46 into process control systems can significantly improve operational safety and reliability.

Mostafa Sayed