Chapter 2 Digital I/O
© National Instruments Corporation 2-11 NI PCI-8254R User Manual
Shutdown
Shutdown mode is a software-enabled feature that, when activated, allows
an external device to halt the NI 8254R device processing operations.
Additionally, Shutdown mode allows you to specify user-defined
shutdown states for all fault conditions.
When Shutdown mode is enabled and the shutdown input signal,
ISO In put 11, turns off, the NI 8254R registers an external shutdown
condition. When a fault occurs, outputs operate according to user-defined
shutdown states. Each TTL output is configurable to drive high, drive low,
or tri-state, and each isolated output is on/off configurable.
Note For prototyping when equipment is unavailable, you can wire from V to
ISO Input11 to simulate external equipment that indicates to the NI 8254R to operate
normally.
Watchdog Timer
The watchdog timer is a software configurable feature that can monitor
software on the NI 8254R and take action if the software is unresponsive.
The millisecond counter on the watchdog timer is configurable up to
65,534 ms, in 1 ms increments, before it expires.
Configure the watchdog timer to take one of the following actions when it
expires.
Caution Use the Indicator Only option only to test the watchdog timer. If software
becomes unresponsive, it cannot be relied upon to send notification to the host.
•Indicator Only—This option sends the expiration signal back to the
development machine through software. True indicates an expired
watchdog timer. False indicates an unexpired watchdog timer. The
expiration signal that indicates an expired watchdog timer continues to
assert until the watchdog timer is disarmed. Disarming the watchdog
timer resets the software indicator.
•TTL Output 0—This option outputs a signal on TTL Output 0. High
indicates that the watchdog timer has expired. Low indicates that the
watchdog timer has not expired. If the watchdog timer has expired, the
expiration signal continues to assert until the watchdog timer is
disarmed.
•Shutdown—If Shutdown mode is enabled, the outputs go to the
user-defined shutdown states.