Measurement Computing Data Translation DT3034, Руководство пользователя

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Principles of Operation
77
Note: An A/D Trigger Out signal is provided for your use. This signal is high when the
A/D subsystem is waiting for a trigger and low when a trigger occurs. In continuously-paced
scan mode, this signal goes low when the trigger occurs and stays low until you stop the
operation.
Triggered Scan Mode
DT3034 boards support two triggered scan modes: software-retriggered and
externally-retriggered. These modes are described in the following subsections.
Software-Retriggered Scan Mode
Use software-retriggered scan mode if you want to accurately control both the period between
conversions of individual channels in a scan and the period between each scan. This mode is
useful when synchronizing or controlling external equipment, or when acquiring a buffer of
data on each trigger or retrigger. Using this mode, you can acquire up to 262,144 samples per
trigger (256 times per trigger x 1024-location channel list).
When it detects an initial trigger, the board scans the channel list a specified number of times
(up to 256), then waits for an software retrigger to occur. When the board detects an software
retrigger, the board scans the channel list the specified number of times, then waits for
another software retrigger to occur. The process repeats continuously until either the allocated
buffers are filled or you stop the operation; refer to
page 85 for more information on buffers.
The sample rate is determined by the frequency of the A/D sample clock divided by the
number of entries in the channel list; refer to
page 74 for more information on the A/D sample
clock. The conversion rate of each scan is determined by the frequency of the Triggered Scan
Counter, a 24-bit counter with a 20 MHz clock located on the board.
Using software, specify the retrigger frequency. The minimum retrigger frequency is 1.2 Hz.
The maximum retrigger frequency is 250 kHz (250 kSamples/s).
Specify the retrigger frequency as follows:
Min. Retrigger = # of CGL entries x # of CGLs per t 2

s
Period A/D sample clock frequency
Max. Retrigger = 1
Frequency
Min. Retrigger Period
For example, if you are using 512 channels in the channel list, scanning the channel list 256
times every trigger or retrigger, and using an A/D sample clock with a frequency of 1 MHz,
set the maximum retrigger frequency to 7.62 Hz, since
7.62 Hz = 1
( 512 * 256) +2

s
1 MHz