NI 6509 User Guide and Specifications
10
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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 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 6509 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 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. You
cannot read the state of any lines that are configured for change detection
until the change detection interrupt occurs.
Watchdog Timer
The watchdog timer is a software configurable feature used to set critical
outputs to 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.
When the watchdog timer is enabled, if the NI 6509 does not receive
a
watchdog reset
software command within the time specified for the
watchdog timer, the outputs go to a user-defined safe state and remain in
that state until the watchdog timer is disarmed by the application and new
values are written, the NI 6509 is reset, or the computer is restarted. The
expiration signal that indicates an expired watchdog will continue to assert
until the watchdog is disarmed. After the watchdog timer expires, the
NI 6509 ignores any writes until the watchdog timer is disarmed.
Note
When the watchdog timer is enabled and the computer enters a fault condition, ports
that are set to tri-state remain tri-stated and do not go to user-defined safe states.
You can set the watchdog timer timeout period to specify the amount of
time that must elapse before the watchdog timer expires. The counter on the
watchdog timer is configurable up to (2
32
– 1) × 100 ns (approximately
seven minutes) before it expires.
