National Instruments E Series - Manual (Page 72)

Chapter 4 Signal Connections
DAQCard E Series User Manual 4-36  National Instruments Corporation
UISOURCE Signal
Any PFI pin can externally input the UISOURCE signal, which is not
available as an output on the I/O connector. The UI counter uses the
UISOURCE signal as a clock to time the generation of the UPDATE*
signal. You must configure the PFI pin you select as the source for the
UISOURCE signal in the level-detection mode. You can configure the
polarity selection for the PFI pin for either active high or active low.
Figure 4-23 shows the timing requirements for the UISOURCE signal.
The maximum allowed frequency is 20 MHz, with a minimum pulse
width of 23 ns high or low. There is no minimum frequency limitation.
Either the 20 MHz or 100 kHz internal timebase normally generates
the UISOURCE signal unless you select some external source.
Figure 4-23. UISOURCE Signal Timing
t
p
t
w
t
w
t
p
t
w
= 50 ns minimum
= 23 ns minimum