Siemens SIMATIC S7 System Manual page 385

Example: automatically retriggered One-shot timers
The corrected examples use the normally closed contact Q0.0 instead of the timer bit as the
timer enabling input. This ensures that output Q0.0 is turned ON for one scan cycle, each
time a timer reaches the preset value.
1 ms timer
Q0.0 is turned ON for one scan whenever the timer's current value is updated after the
normally closed contact T32 is executed and before the normally open contact T32 is
executed.
10 ms timer
Q0.0 is never turned ON, because the timer bit T33 is turned ON from the top of the scan to
the point where the timer box is executed. Once the timer box has been executed, the
timer's current value and its T–bit are set to zero. When the normally open contact T33 is
executed, T33 is OFF and Q0.0 is turned OFF.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Program instructions
7.17 Timer
385