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Chapter 2
NI 651x Features
In this example, the DIO device 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 DIO device 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.
Note
NI 6512/6513/6516/6517 devices do not support change detection.
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 651
x
device.
Note
The NI-DAQmx Watchdog feature is meant to protect a system from software
errors and hangs. In the case of a PXI system with remote control through MXI, a lost
MXI connection could result in unexpected Watchdog behavior and therefore
improperly implemented Watchdog states.
When the watchdog timer is enabled, if the NI 651
x
device 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 651
x
device 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 651
x
device ignores any writes
until the watchdog timer is disarmed.
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.
Note
NI 6510/6511 devices do not support the watchdog timer.
