Key Takeaway
In S7-300 PLCs, there are mainly three types of timers:
TON (On-Delay Timer): Delays turning ON the output after the input signal becomes true.
TOF (Off-Delay Timer): Keeps the output ON for a specified time after the input turns OFF.
TP (Pulse Timer): Generates a pulse of fixed duration when the input turns ON.
Timers can be used in LAD (Ladder Logic), FBD (Function Block Diagram), and STL (Statement List) programming styles.
Siemens also provides SFBs/SFCs (System Functions/Blocks) for extended timing functions like pulse train generation.
Timers are crucial for sequencing, delay management, and ensuring controlled operations in machine automation, process timing, and sequential logic applications.
Overview of Timer Instructions in STEP 7
In STEP 7, Siemens has kept timer functions pretty intuitive. You’ll find timers under the IEC Timer Instructions, and these are used widely across automation projects.
Timers help in managing delays, sequencing actions, creating pulse durations, and more. When working with real-world applications—like starting a motor after 3 seconds or turning off a light after a delay—timers are your go-to tool.
There are two main categories in S7-300:
S5 Timers – the old-school timers.
IEC Timers (TON, TOF, TP) – modern, easy to use, and recommended for newer projects.
Most engineers today prefer IEC timers due to their simplicity and flexibility. They’re easier to read, debug, and maintain—especially when projects grow big.
TON (On-Delay Timer)
The TON or On-Delay Timer is like saying: “Wait for a while before doing something.”
Here’s how it works:
When the input signal is TRUE, the timer starts counting.
After the preset time expires, the output turns ON.
If the input goes FALSE before the time is up, the timer resets.
This is useful in cases like:
Starting a conveyor belt 3 seconds after a sensor is triggered.
Adding a delay before turning on a motor or an alarm.
The TON is one of the most commonly used timers in automation. It brings predictability and controlled startup behavior in your systems.
Pro Tip:
Always ensure your PLC cycle time is considered while setting TON values—especially in high-speed applications.
TOF (Off-Delay Timer)
The TOF or Off-Delay Timer works in the reverse way.
Here’s how it behaves:
When the input signal turns TRUE, the output goes ON immediately.
When the input turns FALSE, the timer starts.
After the preset time expires, the output turns OFF.
This is useful when:
You want to keep a fan running 5 seconds after a motor is turned OFF.
You need to maintain output even after input is gone—for a specific period.
This timer is powerful when you want to handle post-process events—especially in cooling, alarms, or residual actions after equipment stops.
TP (Pulse Timer)
The TP or Pulse Timer is like a one-shot output.
Let’s say you want something to be ON only for a fixed time whenever triggered—no matter how long the trigger lasts. That’s what TP does:
As soon as the input becomes TRUE, the output turns ON.
It stays ON for the defined time duration.
After the time, it turns OFF automatically—even if the input is still TRUE.
This timer is often used when:
You need to trigger a short-duration pulse for a solenoid or signal.
You want to prevent long operations when inputs stick due to sensor issues.
TP ensures your outputs behave in a consistent time-controlled manner.
Use Cases and Examples for Each Timer
Let’s compare their behavior with relatable industrial scenarios:
Timer Type
Trigger
Output Behavior
Example
TON
Input TRUE
Turns ON after delay
Delay start of motor
TOF
Input FALSE
Stays ON for delay
Fan runs after motor off
TP
Input TRUE
Turns ON for set time
Punch machine, buzzer
These examples are real-life. You’ll see TON in startup sequences, TOF in shutdown handling, and TP in quick-trigger applications.
Knowing which timer to use where will make your logic cleaner, your machines safer, and your operations smoother.
Conclusion
The Siemens S7-300 PLC supports three key IEC timer instructions—TON, TOF, and TP. Each has its own role in managing time-based logic. When used wisely, they help automate processes efficiently and reliably.
Whether you’re creating a delay before action, extending operation after an event, or just generating a fixed pulse, these timers form the backbone of timed automation logic.
If you’re starting out with STEP 7 and S7-300, mastering these three timers is non-negotiable. Once you understand their behavior and strengths, you’ll find building time-controlled processes much easier and more effective.
So start experimenting, test your timers in real projects, and bring precision into your automation logic!