USB-6525 User Guide and Specifications 8 ni.com
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 4 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 NI 6525 detects any change in the input data so you
can read the new data value.
• Bit 1 is connected to a limit sensor. The NI 6525 detects rising edges
on the sensor, which correspond to over-limit conditions.
• Bit 0 is connected to a switch. The software can react to any switch
closure, which is represented by a falling edge. If the switch closure is
noisy, enable digital filtering for this line.
In this example, the NI 6525 reports rising edges only on bit 1, falling edges
only on bit 0, and rising and falling edges on bits 7, 6, 5, and 4. The NI 6525
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. You
cannot read the state of any lines that are configured for change detection
until the change detection interrupt occurs.
Table 4. 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
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