Rockwell Automation 6008-SV2R VMEbus remote I/O Scanner - Manual (Page 19)

Chapter
Scanner Overview
1
1Ć11
The VME master processor must issue a valid command to the scanner at
least once in a user-specified time period (the default is 500 msec). If the
scanner fails to see a valid command from a VME master processor in this
time period (as counted by the watchdog timer), the scanner resets itself
and repeats its startup initialization sequence. This causes the I/O racks on
the link to fault within 100 msec and the I/O all turn off or remain in their
last state, depending on the switch setting on the I/O chassis.
You can disable the watchdog timer or change its timeout period with the
SETUP command.
To keep the watchdog from shutting down the scanner, periodically issue a
LINK STATUS command. This command provides the application
program with important diagnostic information about the status of the I/O
link and, at the same time, causes the least amount of overhead for the
scanner to complete the command.
To debug your application program you can select debug mode and disable
the watchdog timer using the SETUP command.
ATTENTION: Unwanted machine action can result from
disabling the VME master processor watchdog. When the VME
master processor watchdog is disabled, the scanner has no way
of knowing that communication has been lost with your VME
master processor and will continue to send data from the output
image table to the output modules.
VME Master Processor
Watchdog Timer