National Instruments NI 6238 - Manual (Page 105)

Chapter 7 Counters
NI 6238/6239 User Manual 7-40 ni.com
Figure 7-35. Other Internal Source Mode
External Source Mode
In external source mode, the device generates a delayed Source signal by
delaying the Source signal by several nanoseconds. The device
synchronizes signals on the rising edge of the delayed Source signal, and
counts on the following rising edge of the source, as shown in Figure 7-36.
Figure 7-36. External Source Mode
Source
Synchronize Count
Source
Delayed Source
Synchronize
Count