Measurement Computing Data Translation DT3034, Руководство пользователя

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Principles of Operation
85
Figure 26: Continuous About-Trigger Mode
Figure 27 illustrates the same example using software-retriggered triggered scan mode. The
multiscan count is 2 indicating that the channel list will be scanned twice per trigger or
retrigger. In this example, pre-trigger analog input data is acquired on each clock pulse of the
A/D sample clock for two scans; then, the board waits for the software retrigger event. When
the software retrigger occurs, the board begins acquiring pre-trigger data until the
post-trigger event occurs. Then, the board finishes scanning the channel list the specified
number of times, but acquires the data as post-trigger samples. On all subsequent software
retriggers, post-trigger data is acquired.
Figure 27: Continuous About-Trigger Mode with Triggered Scan
Data Format and Transfer
To represent unipolar signals, DT3034 boards use straight binary data encoding, such as 0000
to represent 0 V, and FFFFh to represent full-scale. To represent bipolar signals, DT3034
boards use offset binary data encoding, such 0000 to represent negative full-scale, and FFFFh
to represent positive full-scale. Use software to specify the data encoding as binary.
The ADC outputs FFFFh for above-range signals, and 0000 for below-range signals.
The board packs two input samples (an even and an odd sample) into each transfer to the host
computer. Samples corresponding to entries 0, 2, 4, and so on, in the channel list are
considered even samples; samples corresponding to entries 1, 3, 5, and so on, in the channel
list are considered odd samples.
Pre-trigger event occurs
Chan 0
Chan 1
Chan 0
. . .
A/D
Sample
Clock
Post-trigger event occurs
Pre-trigger data acquired Post-trigger data acquired
Chan 1
Chan 0
Chan 1
Chan 0
Chan 1
Chan 0
Chan 1
Chan 0
Chan 1
Pre-trigger event
occurs;
pre-trigger data is
acquired for 2 scans of
the channel list.
Chan 0
Chan 1
Chan 0
Chan 1
A/D
Sample
Clock
Re-trigger event
occurs; pre-trigger
data is acquired until
post-trigger occurs.
Chan 0
Chan 1
Chan 0
Chan 1
Chan 0
Chan 1
Post-trigger event
occurs; post-trigger
data is acquired until
the end of the number
of scans.
Re-trigger event
occurs;
post-trigger
data is acquired
for 2 scans of
the channel list.