Programming instructions

© National Instruments Corporation 8-1 LabVIEW Data Acquisition Basics Manual
8
Controlling Your Acquisition
with Triggers
The single-point and waveform acquisitions described in the previous
sections start at random times relative to the data. But, there are times that
you may need to be able to set your analog acquisition to start at a certain
time. An example of this would be if you wanted to measure the
temperature of an object after applying heat to it. An electrical thermometer
sends a step voltage to your data acquisition (DAQ) device after the heating
process completes. If you have no way to begin measuring data
immediately after your device receives the step voltage, then you must
acquire more points, some before the step voltage and some after it in order
to capture the data you need. As you can see, this solution is an inefficient
use of computer memory and disk space, because you must allocate and use
more than is necessary. Sometimes the data you need may be closer to the
front of the buffer and other times it may be closer to the end of the buffer.
However, there is a way to start an acquisition based on the condition or
state of an analog or digital signal. This technique is commonly called
triggering
. Generally, a
trigger
is any event that causes or starts some form
of data capture. There are two basic types of triggering—hardware and
software triggering. In LabVIEW, you can use software triggering to start
acquisitions or use it with an external device to perform hardware
triggering.
Hardware Triggering
Hardware triggering lets you set the start time of an acquisition and gather
data at a known position in time relative to a trigger signal. External devices
produce hardware trigger signals. In LabVIEW, you specify the triggering
conditions that must be reached before acquisition begins. Once the
conditions are met, the acquisition begins immediately. You can also
analyze the data before trigger.
There are two specific types of hardware triggers: digital and analog. In the
following two sections, you will learn about the necessary conditions to
start an acquisition with a digital or an analog signal.