Chapter 3
Connecting the Signals
DAQPad-MIO-16XE-50 User Manual
3-32
ni.com
SISOURCE Signal
Any PFI signal can externally input SISOURCE, which is not available
as an output on the DAQPad-TB-52. The onboard scan interval counter
uses the SISOURCE signal as a clock to time the generation of the
STARTSCAN signal. You must configure the PFI signal you select as
the source for the SISOURCE signal in the level-detection mode. You can
configure the polarity selection for the PFI signal 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 the SISOURCE
signal unless you select some external source. Figure 3-20 shows the
timing requirements for the SISOURCE signal.
Figure 3-20.
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.
Figure 3-21 shows the timing for the SCANCLK signal. In this figure, the
falling edge of CONVERT* indicates the initiation of an A/D conversion.
Note
When using NI-DAQ, SCANCLK polarity is low-to-high and cannot be changed
programmatically.
t
w
= 23 ns minimum
t
p
= 50 ns minimum
t
w
t
w
t
p
SISOURCE
