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National Instruments Corporation
9
NI 6520/6521 User Guide
Change Detection
You can program the NI 6520/6521 device to send an interrupt when a
change occurs on any input line.
The DIO device can monitor changes on selected input lines or on all input
lines. It can monitor for rising edges (0 to 1), falling edges (1 to 0), or both.
When an input change occurs, the DIO device generates an interrupt, and
the NI-DAQ driver then notifies the software.
Note
Excessive change detections can affect system performance. Use digital filtering to
minimize the effects of noisy input lines.
The DIO device sends a change detection when any one of the changes
occurs, but it does not report which line changed or if the line was rising or
falling. After a change, you can read the input lines to determine the current
line states. The maximum rate of change detection is determined by the
software response time, which varies from system to system.
An overflow bit indicates that an additional rising or falling edge has been
detected before the software could process the previous change.
Refer to the software documentation for information about how to set up
and implement the change detection.
Change Detection Example
Table 2 shows a change detection example for six bits of one port.
This example assumes the following line connections:
•
Bits 7, 6, 5, and 4 are connected to data lines from a four-bit TTL
output device. The DIO device detects any change in the input data
so you can read the new data value.
•
Bit 1 is connected to a limit sensor. The DIO device detects rising
edges on the sensor, which correspond to over-limit conditions.
Table 2.
Change Detection Example
Bit
7
6
5
4
3
2
1
0
Changes to detect
—
—
Enable rising-edge detection
yes
yes
yes
yes
no
no
yes
no
Enable falling-edge detection
yes
yes
yes
yes
no
no
no
yes