Chapter 2
Hardware Overview
2-12
ni.com
Note
For prototyping when equipment is unavailable, you can wire from V
iso
to
ISO Input 11 to simulate external equipment that indicates to the NI 8255R to operate
normally.
Watchdog Timer
The watchdog timer is a software configurable feature that can monitor
software on the NI 8255R 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.