Siemens SIMATIC S7-1500 System Manual page 185

"Multiple periods" measurement method
Requirement:
• CPU firmware version V3.0 or higher
• Hardware function version FS 11 or higher
In this measurement method, the period duration is determined based on all periods in a
measuring cycle, whereby the measuring cycle length corresponds to one application cycle
(e.g. servo clock).
The number of 41.67 ns increments between the last rising edge of the last measuring cycle
and the last rising edge of the current measuring cycle is counted.
The current count (32-bit value) is updated on the feedback interface with each application
cycle (e.g. servo clock).
• The event counter and the period duration counter are circular counters.
• A counter overflow is not displayed.
• To determine the count, the difference between the current count and the previous count
must be calculated by the application, taking into account any overflow.
N
P(new)
N
P(old)
N
E(new)
N
E(old)
If current count < old count is true for the counters, then an overflow has occurred. In this
case, the value 2
must be added to the new count for the period duration counter. A maximum of one
overflow is possible.
The "Multiple periods" measurement method yields a more accurate measurement.
• For short periods (i.e. a few 41.67 ns increments per period), in particular, the sampling
jitter (1 increment) has less effect because the period duration is determined over many
periods.
• Measured value fluctuations are averaged. For example, if rotational speeds are acquired
using a slotted disk and fork light barrier, the tolerances of individual slots have less of an
effect because the measurement is not based on a single period (of an individual slot).
SIMATIC Drive Controller
System Manual, 11/2023, A5E46600094-AD
: Period duration counter, current (new) count
: Period duration counter, previous (old) count
: Event counter, current (new) count
: Event counter, previous (old) count
must be added to the new count for the event counter, and the value 2
16
9.9 Configuration of digital inputs/outputs (X142)
Configuring
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