Emerson PACSystems RSTi-EP User Manual page 226

User Manual
GFK-2958J
Figure 127: Principal: Function Frequency Counting
Calculation of Results
The input process data Rising edges register the amount of rising edges for each channel
within the referring measurement cycle period. Counting starts with the last rising edge of
the previous measurement cycle and ends with the last but one edge of the current
measurement cycle. The time between first and last counted edge is defined as measured
period. This is a 27-bit value with a resolution of 125 ns and a precision of 1μs (valid value
range between 0x00000008 and 0x7FFFFFF8).
Due to its resolution, the value must be divided by 8 within the control program in order to
get the current period duration in microseconds. This value can be between 1μs and
16,777,215μs.
The frequency is being calculated program-wise as follows:
In case there is no rising edge registered within a measurement cycle the current period
duration will be set to the maximum value of 16,777,215.875μs.
If the current period duration as well as the rising edges are registered as zero, the current
measured period was too short to register a rising edge.
Modifying the Measurement Cycle Period
The measurement cycle period can be modified during a running measurement. In this case
the new value is not valid until the following measurement cycle, during the current
measurement cycle the old value will be kept.
Stop Measurement
Setting of Bit 9 in the control word of each channel stops the cyclic measurement. If the last
measurement cycle has not been run through completely, the current measured period will
be set to the maximum value of 0x7FFFFFFF, the rising edges will be set to zero.
Detailed Description of I/O Modules
Rising edges in current period
Frequency F =
Current measured period
Chapter 5
Oct 2019
211