Siemens SIMATIC S5-100U, System Manual

Page: 177 / 552

S5-100U STEP 5 Operations
Starting a timer
In the programmable controller, timers run asynchronously to program scanning. The time that has
been set can run out during a program scanning cycle. It is evaluated by the next time scan. In the
worst case, an entire program scanning cycle can go by before this evaluation. Consequently,
timers should not activate themselves.
Example:
1s - n · t
p
Schematic Representation Explanation
The schematic shows the “n
th
+ 1”
processing cycle since timer T 17* was
started. Although the timer ran out
shortly after the statement “= Q 1.0”,
output Q 1.0 remains set. The change is
not considered until the next program
scanning cycle.
*
KT 100.0 is equal to 1 s.
Program
Signal from
timer 17
L KT 100.0
SP T 17
A T 17
= Q 1.0
n: number of program scanning cycles
t
p
: program scan time
0 1
The following rules apply to timers:
• Except for “Reset timer”, all timer operations are started only when there is an edge change.
The RLO alternates between “0” and “1”.
• After being started, the loaded time is decremented in units corresponding to the time base until
it reaches zero.
• If there is an edge change while the timer is running, the timer is reset to its initial value and
restarted.
• The signal state of a timer can be scanned with Boolean logic operations.
EWA 4NEB 812 6120-02b
8-19