PCI/PXI/PCIe-6509 User Guide and Specifications
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© National Instruments
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19
Change Detection Example
Table 3 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
6509 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 6509 detects rising edges on the sensor, which
correspond to over-limit conditions.
•
Bit 0 is connected to a switch. The NI 6509 reacts 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 6509 reports rising edges on only bit 1, falling edges on only bit 0, and
rising and falling edges on bits 7, 6, 5, and 4. The NI 6509 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. However, you cannot read the lines that are configured for change detection until
a change detection interrupt occurs.
Watchdog Timer
The watchdog timer is a software-configurable feature that sets critical output lines to predefined
safe states in the event of a software failure, a system crash, or any other loss of communication
between the application and the NI 6509.
After you enable the watchdog timer, if the NI 6509 does not receive a
watchdog reset
software
command within the time specified for the watchdog timer, the output lines enter a user-defined
safe state and remain in that state until one of the following events occurs:
•
The application disarms the watchdog timer and writes new values to the output lines.
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The NI 6509 is reset.
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The computer is restarted.
Table 3.
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
