© National Instruments | 1-33
cDAQ-9185/9189 User Manual
accuracy, use an NI Linux Real-Time controller as the host in a supported topology. However,
NI-DAQmx guarantees that two tasks configured to start at the same host time always start at
the same I/O Device Time in all scenarios, preserving precise synchronization between chassis
in this common use case. Refer to the Timestamps and Time Triggering topics in the NI-DAQmx
Help for more information on accessing time-based features in the NI-DAQmx API.
Watchdog Timer
The watchdog timer is a software-configurable feature used to set critical outputs to expiration
states in the event of a software failure, a system crash, or any other loss of communication
between the application and the cDAQ chassis.
When the watchdog timer is enabled, if the cDAQ chassis does not receive a watchdog reset
software command within the time specified for the watchdog timer, the outputs go to a
user-defined expiration state and remain in that state until the watchdog timer is disarmed by a
device reset. After the watchdog timer expires, the cDAQ chassis cannot perform any operation
until the cDAQ chassis is reset.
You can use the watchdog timer to do the following:
• Set a timeout period to specify the amount of time that must elapse before the
watchdog timer expires
• Set to expire upon loss of network connectivity
The counter on the watchdog timer is configurable up to (2
32
- 1) × 25 ns (approximately
107 seconds) before it expires. Analog output, digital output, and counter output channels can
be configured to transition to an expiration state when a watchdog timer expires.
Resetting the chassis after a watchdog expiration event results in all module outputs defaulting
to power up or startup states as defined in the module specifications.
Note Resetting the chassis after a watchdog expiration event may result in
undefined transient behavior on the outputs of the NI 9269 and NI 9474 modules.
Consult the module specifications for the expected I/O behavior.
Note Restarting or re-reserving the chassis may lead to undefined transient
behavior on the outputs of any modules, so these operations are not recommended as
means of a watchdog expiration event recovery.
Note No other operations may be running on the cDAQ chassis while the watchdog
timer task is being started; this includes all DAQmx tasks, calibration of modules,
and routing and configuration of signals on the chassis. After the watchdog timer task
starts, DAQmx tasks can be started and stopped and other operations can be
performed.
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