National-instruments Data Acquisition Device NI USB-621x Manual de usuario descargar pdf (Pagina 46)

Chapter 4 Analog Input

Pretriggered data acquisition allows you to view data that is acquired before

the trigger of interest, in addition to data acquired after the trigger.

Figure 4-7 shows a typical pretriggered DAQ sequence. AI Start Trigger

(ai/StartTrigger) can be either a hardware or software signal. If AI Start

Trigger is set up to be a software start trigger, an output pulse appears on

the AI Start Trigger line when the acquisition begins. When the AI Start

Trigger pulse occurs, the sample counter is loaded with the number of

pretriggered samples, in this example, four. The value decrements with

each pulse on AI Sample Clock, until the value reaches zero. The sample

counter is then loaded with the number of posttriggered samples, in this

example, three.

Figure 4-7. Pretriggered Data Acquisition Example

If an AI Reference Trigger (ai/ReferenceTrigger) pulse occurs before the

specified number of pretrigger samples are acquired, the trigger pulse is

ignored. Otherwise, when the AI Reference Trigger pulse occurs, the

sample counter value decrements until the specified number of posttrigger

samples have been acquired.

USB-621x devices feature the following analog input timing signals:

• AI Sample Clock Signal

• AI Sample Clock Timebase Signal

• AI Convert Clock Signal

• AI Convert Clock Timebase Signal

• AI Hold Complete Event Signal

• AI Start Trigger Signal

• AI Reference Trigger Signal

• AI Pause Trigger Signal

n/a

0 1 2

1 0 2 2 2

AI Start Trigger

AI Reference Trigger

AI Sample Clock

AI Convert Clock

Scan Counter

1 2 ... 41 42 43 44 45 46 47 48 49 50 51 ... 184 185

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National-instruments Data Acquisition Device NI USB-621x Manual de usuario Pagina 46