Updating The Timer Current Value; Example Of Automatically Retriggered One Shot Timer - Siemens SIMATIC S7-200 System Manual

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 (1), 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 (2), 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 (3), 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.
(1)
(2)
(3)
Figure 9-4
S7-200 Programmable Controller System Manual
A5E00066097-02
Wrong Using a 1-ms Timer
T32 T32
IN TON
/
300 PT
T32
Q0.0
Wrong Using a 10-ms Timer
T33 T33
/ IN TON
30 PT
Q0.0
T33
Correct Using a 100-ms Timer
T37 T37
/
IN TON
3 PT
Q0.0
T37

Example of Automatically Retriggered One Shot Timer

SIMATIC Instructions
Corrected
Q0.0 T32
TON
IN
/
300 PT
T32
Q0.0
Corrected
Q0.0 T33
/ TON
IN
30 PT
Q0.0
T33
Better
Q0.0 T37
/
IN TON
3 PT
Q0.0
T37
9-19