TIA Portal Tutorial | Step-by-Step Simulation for Beginners | OFF-Delay Timer (TOF) in STL

Описание: Below is a complete, Siemens STL (Statement List) OFF-Delay Timer.

Siemens PLC STL Programming – OFF-Delay Timer (TOF) Logic Explained

Short Description

This example demonstrates the implementation of an OFF-delay timer using STL (Statement List) in Siemens PLCs programmed with TIA Portal. The logic delays the switching OFF of an output after the input signal becomes false.

Detailed Description

The OFF-delay timer (TOF) is a commonly used timing function in industrial automation where an output must remain ON for a predefined time after the input condition turns OFF.

In this STL-based implementation, the timer is triggered by a control signal. When the input goes from TRUE to FALSE, the timer starts counting, and the output remains active until the preset time elapses. This behavior is essential for applications such as fan run-on, conveyor overrun, and delayed stop sequences.

The logic is written in pure STL, making it ideal for engineers working with legacy Siemens PLCs or those who want low-level control and deeper understanding of PLC execution.

Working Principle

Input signal turns ON → Output turns ON immediately

Input signal turns OFF → Timer starts

Output remains ON during the timer duration

After time expires → Output turns OFF

Example STL Logic (OFF-Delay Timer)

A "Start_Signal" = "Timer_Enable" A "Timer_Enable" TOF "OffDelay_Timer", T#5s A "OffDelay_Timer" = "Output_Motor"

Explanation:

Start_Signal controls the timer input

TOF holds the output ON for 5 seconds after input turns OFF

Output_Motor deactivates only after the delay expires

Key STL Instructions Used

A – AND / load logic

= – Assign output

TOF – OFF-delay timer function

T#5s – Time preset

PLC Compatibility

Siemens S7-300 / S7-400

Siemens S7-1200 / S7-1500 (STL support required)

TIA Portal environment

Applications

Fan run-on after machine stop

Conveyor belt delayed stop

Lubrication pump overrun

Safety and shutdown sequencing

Process stabilization delays

Advantages of STL OFF-Delay Logic

Precise control over timing behavior

Efficient execution

Clear signal flow understanding

Useful for troubleshooting legacy systems

Keywords / Tags

Siemens STL off delay timer
TOF timer STL Siemens
PLC off delay timer logic
TIA Portal STL programming
Industrial automation timers
Siemens PLC timing functions