Siemens SIMATIC ET 200S Operating Instructions Manual page 167

If the "1 Pulse per dynamic measuring time" estimated measured value is less than the last
measured value during the frequency and speed measurement, this estimated measured
value is output as the new measured value. With the period measurement, the dynamic
measuring time is output as the estimated period if the dynamic measuring time is greater
than the last measured period.
Figure 3-22
The 1Count5V measures continuously. When assigning parameters, you specify an update
time.
During the time until the end of the first elapsed update time, a value of "-1" is returned.
The continuous measurement begins after the gate is opened with the first pulse of the pulse
train to be measured. The first measured value can be calculated after the second pulse, at
the earliest.
A measured value (frequency, period, or speed) is output in the feedback interface each time
the update time elapses. The end of a measurement is indicated with the STS_CMP1 status
bits. This bit is reset with the RES_STS and RES_STS_A bits according to the complete
acknowledgement principle.
If the direction of rotation is reversed during an update time, the measured value for this
measurement period is undefined. By evaluating the STS_C_DN and STS_C_UP feedback
bits (direction evaluation), you can respond to any process irregularities.
Technological Functions
Operating Instructions, 06/2010, A5E00124867-07
Measuring Principle
1Count5V
3.7 Measurement Modes
167