You can start an acquisition based on the condition or state of an analog or digital signal using "triggering". In NI-DAQ, you can use software triggering to start acquisitions or use an external device to perform hardware triggering (This is our case).
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. In NI-DAQ you specify the triggering conditions that must be reached before acquisition begins. When the conditions are met, the acquisition begins. You also can analyze the data before the trigger.
There are two types of hardware triggers: digital and analog.
A digital trigger is usually a transistor logic TTL signal having two discrete levels. Usua lly, digital trigger signals are connected to STARTTRIG*, EXTTRIG*, DTRIG, EXT TRIG IN, or PFI pins on your DAQ device.
When using an analog trigger (several NI E-Series and SMIO boards have this type of trigger), you configure the DAQ device to wait for a certain condition of the analog input signal. Once the device identifies the trigger conditions, it starts an acquisition.
For learning more about Triggering and for seeing some LV examples, it would be a good idea to go to www.ni.com>>NI Developer Zone>>NI Development Library>>Measurement Hardware>>Analog Input/Output>>Analog Input>>Triggering.