Function Frequency Counting - GE RSTi-EP User Manual

Chapter 5 Detailed Descriptions of I/O Modules

5.16.1 Function Frequency Counting

Defining the Measurement Cycle Period
The length of measurement cycle period has to be defined for each channel within the output
process data. As the 23 bit value has a resolution of 1 μs, values between 1 μs and 8.388.607 μs can
be defined. The measurement cycle period must be long enough to detect at least one rising edge.
Setting of Input Filter
In order to suppress any distrubances a digital input filter can be set for each channel via the
Parameter Input filter (Filter frequencies and respective pulse width see Overview of editable
parameter).
Start Measurement
Setting of Bit 8 in the control word of each channel starts the cyclic measurement.
Calcuation 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 an precision of 1 μs (valid value range between 0x00000008 and
0x7FFFFFF8).
Due to its resolution the value has to be devided by 8 within the control programm in order to get the
current period duration in microseconds. This value can be between 1 μs and 16.777.215 μs.
The frequency is being calculated programmwise as follows:
Frequency F = Rising edges in current period
Current measured period
In case there is no rising edge registred within a measurement cycle the current period duration will
be set to the maximum value of 16.777.215,875 μs.
If the current period duration as well as the rising edges are registred 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.
GFK-2958E RSTi-EP User Manual 205