Siemens SIMATIC S5-90U, Системное руководство

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S5-90U/S5-95U 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
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The schematic shows the ”n
th
+ 1”
processing cycle since timer T 17
*
was
started. Although the timer ran out
shortly after the statement ”= Q32.5”,
output Q32.5 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 32.5
n: number of program scanning cycles
t
p
: program scan time
0 1
Except for ”Reset timer”, all timer operations are started only on an edge of the RLO. (The RLO al-
ternates 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 6115-02b
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