National Instruments NI 6238 - Manual (Page 101)

Chapter 7 Counters
NI 6238/6239 User Manual 7-36 ni.com
Figure 7-31. Duplicate Count Prevention Example
On the first rising edge of the Gate, the current count of 7 is stored. On the
next rising edge of the Gate, the counter stores a 2 since two Source pulses
occurred after the previous rising edge of Gate.
The counter synchronizes or samples the Gate signal with the Source
signal, so the counter does not detect a rising edge in the Gate until the next
Source pulse. In this example, the counter stores the values in the buffer on
the first rising Source edge after the rising edge of Gate. The details of
when exactly the counter synchronizes the Gate signal vary depending on
the synchronization mode. Synchronization modes are described in the
Synchronization Modes section.
Example Application That Works Incorrectly
(Duplicate Counting)
In Figure 7-32, after the first rising edge of Gate, no Source pulses occur.
So the counter does not write the correct data to the buffer.
Gate
Source
Counter Value
Buffer 2
7 7
6 7 1 2 1
Rising Edge
of Gate
Counter detects rising edge
of Gate on the next rising
edge of Source.