Siemens PLC Timer Coil Addressing with Practical Examples
Published on Jun09, 2025 | Category: step7Siemens PLCs support two types of timer instructions. The first is a functional type that includes multiple inputs and outputs for flexible control. The second is the coil-type timer, which is simpler and functions like a standard output coil. With coil-type timers, you only need to assign a timer address and set a preset time value. These timers are straightforward to use and act as output instructions in the ladder logic.
Coil timers are commonly used like regular coils and can also serve as input conditions using normally open (NO) or normally closed (NC) contacts with the same timer address. Timers are crucial in PLC programming for tasks that require time delays, fixed-duration outputs, or time-based sequencing.
In Siemens PLC programming, timers help control operations such as delayed motor starts, blinking indicators, or safety mechanisms. Each timer is uniquely identified by an address starting with the letter T followed by a number, such as T0 to T255. These addresses allow you to easily reference and reuse timers within your program.
Types of Timers coil in Siemens PLC
Siemens PLCs provide several types of timers coil, including:
- SP (---( SP ) – Pulse Timer Coil): Represents a coil in ladder logic that triggers a short pulse.
- SE (---( SE ) – Extended Pulse Timer Coil): Ladder coil for generating extended pulse durations.
- SD (---( SD ) – On-Delay Timer Coil): Ladder logic coil used to implement an On-Delay Timer.
- SS (---( SS ) – Retentive On-Delay Timer Coil): Ladder coil for retentive on-delay timing logic.
- SA (---( SA ) – Off-Delay Timer Coil): Coil that maintains the output after input condition turns false for the specified time.
What is Siemens PLC Timer Coil Address and Preset Value?
Siemens PLCs reserve a dedicated memory area for timers within the CPU. Each timer occupies a single 16-bit word in memory. The system supports up to 256 timers, with addresses ranging from T0 to T255. These timer addresses are used in ladder logic programming to define timing operations.
Each timer address starts with the letter "T" followed by a number (e.g., T0, T1, T2, ..., T255). The availability and exact range of timer addresses can vary depending on the specific Siemens PLC model in use. For accurate information, always consult the official Siemens documentation.
The timer preset value is specified using the S5Time format: S5T#aH_bM_cS_dMS, which is both user-friendly and flexible. In this format:
- H = Hours
- M = Minutes
- S = Seconds
- MS = Milliseconds
Siemens PLC Pulse Type Timer Coil SP
The SP pulse timer coil generates a short-duration output when its input condition turns ON. It works as an output coil in ladder logic and is used to trigger momentary operations like pulse outputs or one-shot triggers. The timer address (e.g., T0) can later be used as a normally open (NO) or normally closed (NC) contact to control other output devices.
Siemens PLC Extended Pulse Timer Coil SE
The SE (Extended Pulse Timer Coil) keeps its output ON for the entire preset time, even if the input turns OFF earlier. If the input is triggered again while the timer is still active, it resets and extends the output pulse. It’s ideal for applications requiring consistent ON time regardless of brief input pulses.
Siemens PLC On-Delay Timer Coil SD
The SD (On-Delay Timer Coil) delays the activation of the output. Once the input condition becomes true, the timer begins counting. After the preset time, the output turns ON. It is commonly used for delayed start operations like conveyor systems or motor runs.
Siemens PLC Retentive On-Delay Timer Coil SS
The SS (Retentive On-Delay Timer Coil) behaves like an on-delay timer but saves the elapsed time if the input turns OFF before completion. When the input is re-triggered, the timer resumes from where it stopped. This is useful in cases where timing needs to continue across input cycles.
In Siemens PLC programming, a retentive timer (such as the SS coil) accumulates elapsed time even if the input signal turns OFF temporarily. To reset the accumulated time and start fresh, you need to use a reset coil with the same timer address. The reset coil is placed in the ladder logic where you want to clear the timer’s value. When this reset coil energizes, it clears the retentive timer’s memory, effectively resetting the timer count to zero. This allows the timer to restart timing from the beginning when triggered again.
Siemens PLC Off-Delay Timer Coil SF
The SF (Off-Delay Timer Coil) immediately activates the output when the input turns ON and maintains the output ON even after the input turns OFF. The output stays ON for the defined preset time. This is useful for post-process delays, such as keeping a fan running after shutdown.
