Keithley KPCI-3101 Series, User Manual

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KPCI-3101 — KPCI-3104 Series User’s Manual Principles of Operation 2-13
About-Trigger acquisition
Use about-trigger acquisition mode (continuous about-trigger mode) when you want to acquire
data both before and after a specific external event occurs. This operation is equivalent to doing
both a pre-trigger and a post-trigger acquisition.
Using software, specify:
•
The dataflow as continuous about-trigger
• The pre-trigger source as the software trigger
•
The post-trigger source as the external digital (TTL) trigger
• The retrigger mode as the internal retrigger (if you are using triggered scan mode)
Refer to page 2-7 for more information on the supported conversion modes; refer to page 2-10
for information on the supported trigger sources.
NOTE When using about-trigger acquisition, you cannot use externally-
retriggered scan mode; refer to
page 2-8 for more information on
triggered scan mode.
The about-trigger acquisition starts when you start the operation. When it detects the selected
post-trigger event, the board stops acquiring pre-trigger data and starts acquiring post-trigger
data.
If you are using internally-retriggered scan mode and the post-trigger event has not occurred, the
board continues to acquire pre-trigger data using the internal retrigger clock to clock the opera-
tion. If, however, the post-trigger event has occurred, the board continues to acquire post-trigger
data using the internal retrigger clock to clock the operation.
The about-trigger operation stops when the specified number of post-trigger samples has been
acquired or when you stop the operation. Refer to
page 2-8 for more information on internally-
retriggered scan mode.
Figure 2-6 illustrates continuous about-trigger mode using a channel-gain list of two entries:
channel 0 and channel 1. In this example, pre-trigger analog input data is acquired continuously
on each clock pulse of the A/D sample clock until the post-trigger event occurs. When the post-
trigger event occurs, post-trigger analog input data is acquired continuously on each clock pulse
of the A/D sample clock.
Figure 2-6
Continuous about-trigger mode
Pre-trigger event occurs
Chan 0
Chan 1
Chan 0
. ..
A/D
Sample
Clock
Post-trigger event occurs
Pre-trigger data acquired Post-trigger data acquired
Chan 1
Chan 0
Chan 1
Chan 0
Chan 1
Chan 0
Chan 1
Chan 0
Chan 1