Specifications

SIMATIC Instructions
9-19
S7-200 Programmable Controller System Manual
C79000-G7076-C233-01
Updating the Timer Current Value
The effect of the various ways in which current time values are updated depends
upon how the timers are used. For example, consider the timer operation shown in
Figure 9-4.
 In the case where the 1-ms timer is used, 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.
 In the case where the 10-ms timer is used, 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 is set to zero. When the normally open contact T33 is
executed, T33 is off and Q0.0 is turned off.
 In the case where the 100-ms timer is used, Q0.0 is always turned on for one
scan whenever the timer’s current value reaches the preset value.
By using the normally closed contact Q0.0 instead of the timer bit as the enabling
input to the timer box, the output Q0.0 is guaranteed to be turned on for one scan
each time the timer reaches the preset value.
IN
PT
300
T32
T32
TON
IN
PT30
T33
T33
TON
IN
PT
3
T37
T37
TON
T32
Q0.0
T33
T37
/
/
/
IN
PT
300
T32
Q0.0
TON
IN
PT30
T33
Q0.0
TON
IN
PT
3
T37
Q0.0
TON
T32
T33
T37
/
/
/
CorrectedWrong Using a 1-ms Timer
Wrong
Correct
Corrected
Better
Using a 10-ms Timer
Using a 100-ms Timer
Q0.0
Q0.0
Q0.0
Q0.0
Q0.0
Figure 9-4 Example of Automatically Retriggered One Shot Timer