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Timer
A Timer in PLC ladder logic is a function that measures time intervals and is used to create delays or time-based control actions within a PLC program. Timers can be configured to operate in different modes, such as ON-delay, OFF-delay, or retentive timing, depending on the specific requirements of the application.
There are two main types of timers commonly used in PLC programming:
- ON-Delay Timer (TON): This timer starts counting when the input condition is true. After the preset time elapses, the timer's output becomes true. If the input condition goes false before the time elapses, the timer resets.
- OFF-Delay Timer (TOF): This timer starts counting when the input condition goes false. The timer's output remains true for the preset time after the input condition goes false, and then it becomes false.
This can be seen in the below graphs:
Using a Timer
There are two main values associated with a timer:
- Preset Time (PT): This is the time duration that the timer is set to count before changing its output state.
- Elapsed Time (ET): This is the current time that has elapsed since the timer started counting.
Timers are used in various applications, such as controlling the timing of processes, creating delays for safety interlocks, and managing sequential operations in industrial automation systems.
Behavior Table
The below table summarizes the behavior of an Output Coil:
| Input Condition | Timer State | Output State |
|---|---|---|
| False (No Signal) | Not Timing | Output Off |
| True (Signal Present) | Timing | Output On after PT |