Digital Change Detection in NI-DAQmx
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The National Instruments Getting Started with NI-DAQmx Series is aimed at helping you learn NI-DAQmx programming fundamentals. Through video and text tutorials, this series will take you from verifying your device's operation in Measurement & Automation Explorer (MAX) to programming data acquisition applications using LabVIEW. It is intended for both the beginner who wants to learn how to use the DAQ Assistant, as well as the experienced user who wishes to take advantage of advanced NI-DAQmx functionality.
Overview
Change detection is a digital operation in which a transition on one or more digital input lines causes the entire group or port to be captured in hardware. Change detection is a strobbed operation meaning that transfers are controlled by hardware (clock signal or handshaking lines). Once a change occurs, data is latched in hardware buffer, and then the pattern is transferred via Direct Memory Access (DMA) or Interrupt to the PC memory.
Change detection can quickly identify changes in digital port state and allows port monitoring without polling in a software loop. It also removes redundant data and registers for the entire port. Change detection is useful for monitoring lines that change at irregular intervals.
Digital Port Masking
Consider a digital operation with the following parameters:
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Port size: 16-bits
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Lines to monitor: xxxxxxxxxxxxxx11
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Mask: 000000000000011 (0 = don’t care)
Then the following occurs for the set of data transmitted:
- First data: 111111111111111 Latched
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Second data: 1011110001111011 No change
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Xth Data: 000100110100100 Change, entire port is latched
LabVIEW Example
Figure 1 shows a LabVIEW program that continuously monitors a change on the rising and falling edge of some the digital lines. When a change is detected, the values on the digital lines are read or latched into memory. Notice that you configure your digital input task to performe change detection in the DAQmx Timing VI.
Figure 1: Change Detection on Rising and Falling edge.
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