National Instruments NI 6014 - Manual (Page 55)

Chapter 4 Connecting Signals
© National Instruments Corporation 4-29 NI 6013/6014 User Manual
SISOURCE Signal
Any PFI pin can externally input the SISOURCE signal, which is not
available as an output on the I/O connector. The onboard scan interval (SI)
counter uses SISOURCE as a clock to time the generation of
the STARTSCAN signal. You must configure the PFI pin you select as
the source for SISOURCE in the level-detection mode. You can configure
the polarity selection for the PFI pin for either active high or active low.
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 generates SISOURCE
unless you select some external source. Figure 4-21 shows the timing
requirements for the SISOURCE signal.
Figure 4-21. SISOURCE Signal Timing
SCANCLK Signal
SCANCLK is an output-only signal that generates a pulse with the leading
edge occurring approximately 50 to 100 ns after an A/D conversion begins.
The polarity of this output is software-selectable but is typically configured
so that a low-to-high leading edge can clock external AI multiplexers
indicating when the input signal has been sampled and can be removed.
This signal has a 400 to 500 ns pulse width and is software enabled.
t
w
= 23 ns minimum
t
p
= 50 ns minimum
t
w
t
w
t
p