Siemens SM2 monitoring system User Manual
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U3585-J-Z125-8-76
Accuracy of the SM2 data
Notes on SM2 operation
Monitoring method inaccuracies
Different inaccuracies can occur depending on the monitoring method used:
1. Event-driven monitoring method
This method supplies very precise data at the cost of increased system workload.
Problems can occur only when the duration of events is monitored.
Problems can occur only when the duration of events is monitored.
As shown above, the duration of an event (and, if applicable, an activity count) is
assigned to the second monitoring cycle even though part of it should be assigned to
the first cycle. The relative magnitude of the monitoring error decreases in inverse
proportion to the length of the monitoring cycle.
assigned to the second monitoring cycle even though part of it should be assigned to
the first cycle. The relative magnitude of the monitoring error decreases in inverse
proportion to the length of the monitoring cycle.
2. Sample-driven monitoring method
The accuracy of this monitoring method is subject to the laws of statistics.
A requirement for the validity of the monitored data is that the samples are independent
of the monitored events. SM2 uses the system timers to control sampling by having
itself activated at regular intervals.
Hardware interrupts are used for this purpose. However, such an interrupt is not
permitted whenever the CPU is in a non-interruptible state. This results in a sampling
delay and thus in a certain dependency on system events.
A requirement for the validity of the monitored data is that the samples are independent
of the monitored events. SM2 uses the system timers to control sampling by having
itself activated at regular intervals.
Hardware interrupts are used for this purpose. However, such an interrupt is not
permitted whenever the CPU is in a non-interruptible state. This results in a sampling
delay and thus in a certain dependency on system events.
Certain unavoidable system activities cause further delays between interrupt accep-
tance and sampling by SM2. If statistical independence of the samples is assumed, the
accuracy of the monitored data depends on the number of samples.
tance and sampling by SM2. If statistical independence of the samples is assumed, the
accuracy of the monitored data depends on the number of samples.
An assessment of the accuracy can be obtained by using confidence intervals (e.g.
deviation of not more than 1% in 99% of all cases).
It should be noted that a high sampling rate increases the system workload. Therefore,
a long monitoring cycle is preferable to excessively frequent sampling.
deviation of not more than 1% in 99% of all cases).
It should be noted that a high sampling rate increases the system workload. Therefore,
a long monitoring cycle is preferable to excessively frequent sampling.
Monitoring cycle
Monitoring cycle
. . . .
Duration