Data Translation DT3000 Series, Руководство пользователя, Страница: 74 / 156

Principles of Operation
73
Analog Input Conversion Modes
DT3000 Series boards support the following conversion modes:
• Single-value operations are the simplest to use but offer the least flexibility and efficiency.
Use software to specify the range, gain, and analog input channel (among other
parameters); acquire the data from that channel; and convert the result. The data is
returned immediately. For a single-value operation, you cannot specify a clock source,
trigger source, trigger acquisition mode, scan mode, or buffer.
Single-value operations stop automatically when finished; you cannot stop a single-value
operation.
• Scan mode takes full advantage of the capabilities of the DT3000 Series boards. In a scan,
you can specify a channel list, clock source, trigger source, scan mode, and buffer. Two
scan modes are supported: continuously paced scan mode and triggered scan mode.
These modes are described in the following subsections.
Using software, you can stop a scan mode operation by performing either an orderly stop
or an abrupt stop. In an orderly stop, the board finishes acquiring the specified number of
samples, stops all subsequent acquisition, and transfers the acquired data to host memory;
all subsequent triggers or retriggers are ignored. In an abrupt stop, the board stops
acquiring samples immediately; the acquired data is not transferred to host memory, but
all subsequent triggers or retriggers are ignored.
Continuously Paced Scan Mode
Use continuously paced scan mode if you want to accurately control the period between
conversions of individual channels in a scan.
When it detects an initial trigger, the board cycles through the channel list, acquiring and
converting the value for each entry in the channel list; this process is defined as the scan. The
board then wraps to the start of the channel list and repeats the process continuously until
either the allocated buffers are filled or until you stop the operation. Refer to
page 77 for more
information on buffers.
The conversion rate is determined by the frequency of the A/D sample clock; refer to page 71
for more information on the A/D sample clock. The sample rate, which is the rate at which a
single entry in the channel list is sampled, is determined by the frequency of the A/D sample
clock divided by the number of entries in the channel list.
To select continuously paced scan mode, use software to specify the dataflow as continuous
and to specify the trigger source as either internal or external. Refer to
page 77 for more
information on the supported trigger sources.
Figure 20 illustrates continuously paced scan mode using a channel list with three entries:
channel 0, channel 1, and channel 2. In this example, analog input data is acquired on each
clock pulse of the A/D sample clock. The board wraps to the beginning of the channel list and
repeats continuously.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com