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Chapter 4
Device Overview
6527 User Manual
If you anticipate noisy or rapidly changing input lines, use digital filtering
to reduce the changes to a manageable number; excessive notifications can
hurt system performance. For example, if you want to limit the rate of
notifications (and interrupts) to a maximum of one change per line every
10 ms, set a filter interval of 10 ms.
Table 4-2 shows configuring change notification for five 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 input
device. The 6527 detects any change in the input data so you can read
the new data value.
•
Bit 1 is connected to a limit sensor; the 6527 detects rising edges on
the sensor, which correspond to over-limit conditions.
•
Bit 0 is connected to a switch. Your software can react to any switch
closure, represented by a falling edge. If the switch closure is noisy,
you should also enable digital filtering for at least this line.
In this example, the 6527 reports rising and falling edges on bits 7, 6, 5,
and 4; rising edges only on bit 1; and falling edges only on bit 0. The 6527
reports no changes for bits 3 and 2. After receiving notification of a change,
you can read the port to determine the current values of all eight lines.
Table 4-2. Change Notification Example
Changes to
detect
Enable rising-
yes
edge detection
Enable falling-
yes
edge detection
7
6
5
4
yes
yes
yes
yes
yes
yes
4-6
Bit
3
2
1
—
—
no
no
yes
no
no
no
www.ni.com
0
no
yes
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