Timer Functions in Ladder Logic

The timing block functions are used with various contact arrangements and in multiples to accomplish various timing tasks.

Typical industrial timing tasks include timing of the intervals for welding, painting, and heat treating.

Timers can also predetermine the interval between two operations.

With a PLC you can utilize as many timer blocks as you need, within the PLC memory limitations.

Basic Timing Functions

Basic PLC TIME DELAY ON function and some other derived timing functions.

Typical of the derived functions are


  2. interval pulse timing,

  3. Multiple pulse timing of more than one process operation.

Timer Variables

One major advantage of the PLC timer is that its time may be a programmable variable time as well as a fixed time.

The variable time interval may be in accordance with a changing register value.

Another advantage of the PLC timer is that its timer accuracy, repeatability, and reliability are extremely high because it is based on solid-state technology.

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In ladder logic programming, timers are essential components that allow you to introduce time-based control and sequencing into your control logic.

Timer Functions in Ladder Logic Programming

Here are the commonly used timer functions in ladder logic programming:

  1. On-Delay Timer (TON): The On-Delay Timer, or TON, is used to introduce a delay in activating an output after the input condition becomes true. The timer starts counting when the input becomes true, and the output is activated only when the preset time has elapsed. The TON timer is often used for time-based operations such as time delays, conveyor control, or sequencing.

  2. Off-Delay Timer (TOF): The Off-Delay Timer, or TOF, is used to introduce a delay in deactivating an output after the input condition becomes false. The timer starts counting when the input becomes false, and the output remains activated until the preset time has elapsed. The TOF timer is commonly used for tasks like maintaining the state of an output for a specific duration after the input is removed.

  3. Retentive Timer (RTO): The Retentive Timer, or RTO, retains the accumulated time value even during power loss or PLC restart. It allows you to measure elapsed time across multiple scans of the ladder program. The accumulated time value can be used for various applications, such as cumulative operating time tracking or monitoring maintenance intervals.

  4. High-Speed Timer (TP): The High-Speed Timer, or TP (Time Pulse), is a timer that operates at a higher resolution or faster timing intervals compared to standard timers. It provides precise timing control for high-speed processes or applications that require fast response times.

  5. Time Accumulation Timer (TACC): The Time Accumulation Timer, or TACC, allows you to accumulate time values over multiple activations of an input. It is used to keep track of the total accumulated time a specific event or condition has occurred. This can be useful for tasks like calculating equipment run time or monitoring the duration of specific operating conditions.

  6. Pulse Timer (TPulse): The Pulse Timer, or TPulse, generates a pulse of a specific duration when the input condition becomes true. It allows you to generate time-based output signals or trigger specific actions for a defined duration.

These PLC timer functions can be configured with parameters such as the preset time, enable/disable conditions, and reset conditions as per the specific requirements of your control logic. They provide the necessary time-based control and sequencing capabilities in ladder logic programming, enabling precise timing and coordination of industrial processes.