National Instruments Data Acquisition Device NI USB-621x, Руководство пользователя

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Chapter 5 Analog Output
NI USB-621x User Manual 5-6 ni.com
The so u rce also can be one of several internal signals on yo u r USB-621 x
device. Refer to Device Routing in MAX in the NI-DAQmx Help or the
LabVIEW Help in version 8.0 or later for more information.
Yo u also can specify whether the waveform generation begins on the rising
edge or falling edge of AO Start Trigger.
Routing AO Start Trigger to an Output Terminal
Yo u can ro u te AO Start Trigger o u t to any o u tp u t PFI terminal. The o u tp u t
is an active high p u lse.
AO Pause Trigger Signal
Use the AO Pa u se Trigger signal (ao/Pa u seTrigger) to pa u se the generation
of AO samples in a DAQ seq u ence. That is, when AO Pa u se Trigger is
active, no samples occ u r.
If the AO Sample Clock is derived from AO Sample Clock Timebase—for
example, when yo
u choose the onboard 20 MHz or 100 kHz
Timebase—the AO Sample Clock Timebase is divided down by a
programmable clock divider circ
u it and then drives AO Sample Clock,
as shown in Fig u re 5-3.
In this case, AO Pa u se Trigger masks off AO Sample Clock Timebase
p u lses from the programmable clock divider.
For example, an internal timebase is ro u ted to AO Sample Clock Timebase
and the Timebase divisor is 5, as shown in Fig u re 5-4. AO Sample Clock
normally asserts once for every five periods of AO Sample Clock
Timebase; the programmable clock divider co
u nts down from 4 to 0. When
AO Pa u se Trigger is asserted, the programmable clock divider ignores
p u lses of AO Sample Clock Timebase.
Figure 5-4. AO Pause Trigger Example
AO P aus e Trigger
AO Sa mple Clock Time bas e
AO Sa mple Clock
0 4 3 2 1 0 4 3
Progr a mm ab le Clock Divider Co u nt
2 1 0
The Progr a mm ab le Clock Divider
ignore s AO Sa mple Clock Time bas e
when AO P aus e Trigger i s tr u e.