Siemens Simatic CFU, Commissioning Manual

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Figure 5-5 Example: Counting process controlled via user program (load value) and effects on the
comparison event
5.4 Measure frequency
In "Frequency measurement" mode, the function works in parallel to the digital input. The
functions of the digital input remain available.
Operating principle in "Frequency measurement" mode
The CFU determines the frequency applied in Hz. The frequency value is displayed in the unit
Hz in REAL format.
You can read the frequency value in the feedback interface.
The interval from the beginning of measuring to the end of measuring is called the measuring
window. Within the measuring window, the measured values are updated cyclically.
The measurement window is calculated automatically by the CFU and adapted according to the
frequency. Different update rates result from this.
The frequency is determined with the following formula:
Frequency [Hz] = (Number of rising edges at digital input) / (measuring window [s] )
Note
Detection of the input signal
If frequency values are outside the measuring range, the status is undefined.
● Frequencies above 1 kHz cannot be correctly resolved (minimum pulse length 500 µs).
● Frequencies under 0.1 Hz cannot be correctly resolved (pulse length greater than 10 s).
A statement on the actual values is not possible.
Operating modes and functions
5.4 Measure frequency
SIMATIC CFU
Commissioning Manual, 08/2019, A5E39252870-AD
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