National Instruments Data Acquisition Device E Series - Manual (Page 49)

Chapter 2 Analog Input
E Series User Manual 2-16 ni.com
Outputting the AI Start Trigger Signal
You can configure the PFI 0/AI START TRIG pin to output the
ai/StartTrigger signal. The output pin reflects the ai/StartTrigger signal
regardless of what signal you specify as its source.
The output is an active high pulse. Figure 2-5 shows the timing behavior of
the PFI 0/AI START TRIG pin configured as an output.
Figure 2-5. PFI 0/AI START TRIG Timing Behavior
The PFI 0/AI START TRIG pin is configured as an input by default.
When acquisitions use a start trigger without a reference trigger, they are
posttrigger acquisitions because data is acquired only after the trigger. The
device also uses ai/StartTrigger to initiate pretrigger DAQ operations. In
most pretrigger applications, a software trigger generates ai/StartTrigger.
Refer to the AI Reference Trigger Signal section for a complete description
of the use of ai/StartTrigger and ai/ReferenceTrigger in a pretrigger DAQ
operation.
AI Reference Trigger Signal
You can use the AI Reference Trigger (ai/ReferenceTrigger) signal to stop
a measurement acquisition. In Traditional NI-DAQ (Legacy), a reference
trigger is referred to as a stop trigger. To use a reference trigger, specify a
buffer of finite size and a number of pretrigger samples (samples that occur
before the reference trigger). The desired number of posttrigger samples
(samples that occur after the reference trigger) is the buffer size minus the
number of pretrigger samples.
Once the acquisition begins, the DAQ device writes samples to the buffer.
After the DAQ device captures the specified number of pretrigger samples,
the DAQ device begins to look for the reference trigger condition. If the
reference trigger condition occurs before the DAQ device captures the
specified number of pretrigger samples, the DAQ device ignores the
condition.
t
w
t
w
= 50 to 100 ns