National Instruments Data Acquisition Device NI USB-621x - Manual (Page 138)

© National Instruments Corporation 12-1 NI USB-621x User Manual
12
Triggering
A trigger is a signal that causes an action, such as starting or stopping the
acquisition of data. When you configure a trigger, you must decide how
you want to produce the trigger and the action you want the trigger to cause.
All USB-621x devices support internal software triggering, as well as
external digital triggering. For information about the different actions
triggers can perform for each sub-system of the device, refer to the
following sections:
• The Analog Input Digital Triggering section of Chapter 4, Analog
Input
• The Analog Output Digital Triggering section of Chapter 5, Analog
Output
• The Counter Triggering section of Chapter 8, Counters
Triggering with a Digital Source
Your USB-621x device can generate a trigger on a digital signal. You must
specify a source and an edge. The digital source can be any input PFI
signal.
The edge can be either the rising edge or falling edge of the digital signal.
A rising edge is a transition from a low logic level to a high logic level.
A falling edge is a high to low transition. Figure 12-1 shows a falling-edge
trigger.
Figure 12-1. Falling-Edge Trigger
5 V
0 V
Falling Edge Initiates Acquisition
Digital Trigger