Glossary · PLC and control programming
Timer
Also known as: TON, TOF, TP, Delay timer
German: Timer
In IEC 61131-3 programming, a timer is a standard function block that measures time: TON delays switching on, TOF delays switching off and TP generates a pulse of defined length.
- PLC programming
- Standards
In one sentence
A PLC timer is a standard function block: TON delays switching on, TOF delays switching off, TP creates a pulse.
Example
A TON timer with a preset of 5 s raises a fault if a cylinder has not reached its end position 5 seconds after the valve was switched.
How it applies
- Engineering: Timers implement delays, monitoring times, pulses and filters. Each timer instance keeps its own elapsed time, so a timer used in several places needs several instances. Timer resolution and accuracy depend on the controller and on how often the instance is called.
- Fault detection: Supervision timers detect actions that take too long, such as a motor that does not reach speed. This is a simple form of Timeout monitoring.
- Safety: Standard timers in the basic control program are not safety-rated. Time delays that are part of a safety function, such as a delay before guard unlocking, are implemented in the safety system.
- Documentation: Operators and service staff often need to know delay times: how long a machine waits before a fault, how long a pump runs on. List user-relevant times in the operating manual and all configurable times in the parameter documentation.
TON vs. TOF vs. TP
TON switches the output on after the input has been TRUE for the preset time. TOF keeps the output on for the preset time after the input goes FALSE. TP produces an output pulse of the preset length on a rising edge, regardless of how long the input stays TRUE.