Data Translation DT9857, Руководство пользователя, Страница: 79 / 192

Principles of Operation
79
Continuous Scan Mode
In continuous scan mode, you can acquire data from all the entries in the channel-gain list
continuously. Using software, you can specify a channel list, clock frequency, start trigger,
reference trigger, post-trigger scan count, and buffer using software.
On the DT9857-8 and DT9857E-8 modules, you can enter up to 13 entries in the channel list:
analog input channels 0-7, the tachometer input (channel 8), general-purpose counter 0
(channel 9), measure counter 1 (channels 10), measure counter 2 (channel 11), and the digital
input port (channel 12). On the DT9857-16 and DT9857E-16 modules, you can enter up to 21
entries in the channel list: analog input channels 0 to 15, the tachometer input (channel 16),
general-purpose counter 0 (channel 17), measure counter 1 (channel 18), measure counter 2
(channel 19), and the digital input port (channel 20). Using software, specify the channels that
you want to sample in sequential order.
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 80 for more information about
start and reference triggers.
The conversion rate is determined by the frequency of the input sample clock; refer to page 77
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 channels to acquire in the analog input channel list
• Specify the sampling frequency (refer to page 77 )
• Specify the start trigger source (refer to page 81 )
• Specify the reference trigger source (refer to page 82 )
• Specify the post-trigger scan count (the number of post-trigger samples to acquire after
the reference trigger occurs).