Measurement Computing CB-COM-7021 - Manual (Page 69)

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CB-7021, CB-7022, CB-7024 User’s Manual
3.3 Dual Watchdog Operation
Dual Watchdog = Module Watchdog + Host Watchdog
The Module Watchdog is a hardware reset circuit used to
monitor the module’s operating status. While working in harsh or
noisy environment, the module may go down caused by a noise
signal. The outputs go to the PowerOn Value. The module can
accept the host’s command to change the output value.
The Host Watchdog is a software function to monitor the
host’s operating status. Its purpose is to detect a network/
communication problem or a halted host. If a timeout occurs, the
module will place all outputs to their Safe Value to prevent any
problem in the controlled unit/process.
The CB-7000 module with Dual W atchdog makes the
control system more reliable and safer.
3.4 Reset Status
The Reset Status is set when the module is powered-on or
is reset by the Module Watchdog. It is cleared when the command
Read Reset Status ($AA5) sent. This is useful for user to determine
the module’s working status. When Reset Status is set (the module
is reset) the output is changed to the PowerOn V alue. When the
Reset Status is clear (the module is not reset) the output is not
changed.
3.5 Analog Output
The module’s output have three different condition:
<1> Safe Value. If the host watchdog times-out, the output