National Instruments NI USB-621x - Manual (Page 133)

© National Instruments Corporation 14-1 NI USB-621x User Manual
14
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
M Series 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 7, Analog
Output
• The Counter Triggering section of Chapter 9, Counters
Triggering with a Digital Source
Your DAQ 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 14-1 shows a falling-edge trigger.
Figure 14-1. Falling-Edge Trigger
5 V
0 V
Falling Edge Initiates Acquisition
Digital Trigger