Measurement Computing Data Translation DT3034, User Manual

Page: 79 / 178

Chapter 6
78
To select software-retriggered scan mode, use software to specify the following parameters:
• The dataflow as Continuous, ContinuousPreTrigger, or ContinuousPrePostTrigger,
• Triggered scan mode usage as enabled.
• The retrigger source as Software.
• The number of times to scan per trigger or retrigger (also called the multiscan count).
• The frequency of the Triggered Scan Counter.
The initial trigger source depends on the trigger acquisition mode selected; refer to page 79 for
more information on the supported trigger acquisition modes and trigger sources.
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
software-retriggered scan mode, this signal stays low until the desired number of samples
have been acquired, then goes high until the software retrigger is generated.
Externally-Retriggered Scan Mode
Use externally-retriggered scan mode if you want to accurately control the period between
conversions of individual channels and retrigger the scan based on an external event. Like
software-retriggered scan mode, this mode allows you to acquire 262,144 samples per trigger
(256 times per trigger x 1024-location channel list).
Note: Use externally-retriggered scan mode with continuous post-trigger acquisitions only;
refer to page 81 for more information on post-trigger acquisitions.
When it detects an initial trigger (post-trigger source only), the board scans the channel list up
to 256 times, then waits for an external retrigger to occur. Specify any supported post-trigger
source as the initial trigger. For the retrigger, specify either an external digital (TTL) trigger.
When the retrigger occurs, the board scans the channel list the specified number of times, then
waits for another external 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 conversion rate of each channel is determined by the frequency of the A/D sample clock;
refer to
page 74 for more information on the A/D sample clock. The conversion rate of each
scan is determined by the period between external retriggers; therefore, it cannot be accurately
controlled. The board ignores external triggers that occur while it is acquiring data. Only
external retrigger events that occur when the board is waiting for a retrigger are detected and
acted on.