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Introduction to STEP 5
2. Cycle Stability
The formula shows that the extension D is a non-linear function. An important criterion is
therefore what additional interrupt load will be added to an existing interrupt load. Typical quotas
for loading through interrupts are specified in the table below.
Example:
The following example shows how loading through interrupts affects the cycle.
1st case:
A system with no load with a cycle time of, for example, 100 ms is extended
through interrupts with a quota of 25 % by a factor of D = 1.333 (the cycle time is
extended by 33 % to 133 ms).
2nd case:
A system which is already loaded with a quota of 50 % and which has, for
example, a cycle time of 200 ms is loaded with addtional interrupts by an additional
quota of 25 %. The cycle time already extended by a factor of D = 2 is extended
by the factor of D = 4 to 400 ms.
Rule:
Limit the system load through additional interrupts to max. 50 %, so that
sporadic peaks cannot result in overproportional increases in the PLC cycle time.
If the cycle time extension exceeds the factor D = 2, the S5-95F may fail to perform an automatic
warm restart following a power outage.
3. Typical Execution Time Quotas of Additional Interrupts
1. Programmer, depending on operation
2. SINEC L1
3. OB2
4. OB3 (triggered by interrupt DI)
5. OB3 (triggered by counter)
6. OB13 without direct access
7. OB13 with direct access
7-22
Interrupts
Execution Time Quota Q
7-20 %
max.
20 % with one LAN
max.
30 % with two LANs
Quota depends on:
SINEC L1 polling list
•
Frame length
•
Safety time
•
Message frequency
•
0.7 ms+T
Q=
OB2
average OB2 interval
0.7 ms+T
OB3
Q=
average OB3 interval/ms
0.15 ms+T
Q=
OB3
average triggering time/ms
T
/ms
OB13
Q=
OB13 interval time/ms
1 to 10 ms+T
Q=
OB13 interval time
S5-95F
/ms
/ms
/ms
/ms
OB13
EWA 4NEB 812 6210-02
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