Data Translation DT9837, Руководство пользователя

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Chapter 5
72
When it detects the start trigger, the module samples all the channels in the list
simultaneously.
If a reference trigger is not specified, data that is acquired after the start trigger is post-trigger
data. The sampled data is placed in the allocated buffer(s). The operation continues until you
stop it or until no more buffers are available.
If a reference trigger is specified, data that is acquired after the start trigger is pre-trigger data;
when the reference trigger occurs, pre-trigger data acquisition stops and post-trigger
acquisition starts at the next sample. The sampled data is placed in the allocated buffer(s). The
operation continues until the number of scans that you specify for the post-trigger scan count
have been acquired; at the point, the operation stops. Note that the sample at which the trigger
occurs is not counted as a post-trigger sample. Refer to
page 73 for more information about
triggers.
The conversion rate is determined by the frequency of the input sample clock; refer to page 69
for more information about the input sample clock.
Using software, you can stop a scan by performing either an orderly stop or an abrupt stop. In
an orderly stop, the module finishes acquiring the current buffer, stops all subsequent
acquisition, and transfers the acquired data to host memory; any subsequent triggers are
ignored. In an abrupt stop, the module stops acquiring samples immediately; the current
buffer is not completely filled, it is returned to the application only partially filled, and any
subsequent triggers are ignored.
To select continuous scan mode, use software to specify the following parameters:
• Specify the data flow as Continuous
• Specify the clock source as internal and specify the clock frequency (refer to page 69 )
• Specify the start trigger (refer to page 73 )
• Specify the reference trigger (refer to page 74 ).
• Specify the post-trigger scan count (the number of post-trigger samples to acquire after
the reference trigger occurs).
Figure 23 illustrates continuous scan mode (using a start and reference trigger) with a channel
list of four entries: channel 0 through channel 3. In this example, pre-trigger analog input data
is acquired when the start trigger is detected. When the reference trigger occurs, the specified
number of post-trigger samples (3, in this example) are acquired.