Chapter 2
Analog Input Timing/Control
DAQ-STC Technical Reference Manual
2-14
©
National Instruments Corporation
2.4.3 Acquisition-Level Timing and Control
The SC counter is available to control the acquisition-level timing. Two trigger signals are
available for acquisition-level timing—the START1 trigger and the START2 trigger. The
DAQ-STC has three acquisition-level timing modes—posttrigger mode, pretrigger mode, and
continuous acquisition mode. This section discusses the operation of the START1 and
START2 triggers in the context of these three timing modes.
2.4.3.1 Posttrigger Acquisition Mode
In the posttrigger acquisition mode, only one parameter is required—the number of scans to
complete. The START1 pulse initiates the scan sequence. The SC counter counts the number
of scans and terminates the acquisition upon completion of the programmed number of scans.
Posttrigger acquisitions can be retriggerable or nonretriggerable. In the retriggerable mode,
additional START1 pulses that occur after SC_TC initiate additional acquisition sequences.
In the nonretriggerable mode, only one acquisition sequence is allowed. Externally generated
START1 and START2 triggers enter the DAQ-STC through one of PFI<0..9> or
RTSI_TRIGGER<0..6>.
Figure 2-11 shows a single-posttrigger acquisition sequence consisting of three scans.
Figure 2-11.
Posttrigger Acquisition Mode
2.4.3.2 Pretrigger Acquisition Mode
In the pretrigger acquisition mode, two parameters are required. The first parameter gives the
pretrigger count requirement; that is, the number of scans that must occur before START2 can
be recognized. The second parameter gives the posttrigger count requirement; that is, the
number of scans that will occur after START2 is recognized.
START
STOP
CONVERT
SC Counter
SC_TC
START1
2
1
0
