Download LabVIEW Data Acquisition Basics Manual

Chapter 6
One-Stop Single-Point Acquisition
With hardware-timed control loops, your acquisition is not interrupted by
user interaction. Hardware-timed analog input automatically places the
data in your DAQ device FIFO buffer at an interval determined by the
analog input scan rate. You can synchronize your control loop diagram to
this precise analog input scan rate by repeatedly calling the AI Single
Scan VI to read the oldest data in the FIFO buffer.
The AI Single Scan VI returns as soon as the next scan has been acquired
by the DAQ device. If more than one scan is stored in the DAQ device FIFO
buffer when the AI Single Scan VI is called, then the LabVIEW diagram
was not able to keep up with the acquisition rate. You can detect this by
monitoring the data remaining output of the AI Single Scan VI. In other
words, you have missed at least one control loop interval. This indicates
that your software overhead is preventing you from keeping up with your
hardware-timed loop rate. In Figure 6-8, the loop too slow Boolean
indicator is set to TRUE whenever this occurs.
Figure 6-8. Analog IO Control Loop (HW-Timed) VI Block Diagram
In this diagram, the AI Config VI configures the device to acquire data on
channels 0 and 1. The application does not use a buffer created in CPU
memory, but instead uses the DAQ device FIFO buffer. input limits (also
known as limit settings) affects the expected range of the input signals. For
more information on input limits (limit settings), refer to Chapter 3, Basic
LabVIEW Data Acquisition Concepts. The AI Start VI begins the analog
LabVIEW Data Acquisition Basics Manual
6-8
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