Rockwell Automation 2711P Software Development Kit User Manual - Manual (Page 35)

35 Publication 2711P-UM005A-EN-P - March 2007
Chapter
4
PanelView Plus CE-Specific Extensions to the
WinCE API
Overview This chapter covers these topics:
• Watchdog control
• Hardware watchdog
• Software watchdog
• System parameters
• System timers
• Hardware monitor
• Keypad
• System event log
• Recommended PanelView Plus CE mechanisms
Watchdog Control The watchdog is a monitor mechanism that automatically resets the
system when there is a loss of control. The hardware watchdog is
enabled by default and must be periodically tagged to keep the entire
system alive. The action of tagging or kicking a watchdog is a widely
used method to insure that system control is intact.
The watchdog service is a DLL and is responsible for tagging the
hardware watchdog. An application can check into the watchdog
service and register itself such that the watchdog service must be
periodically tagged by the application. This latter behavior is referred
to as the software watchdog. Once enabled by an application, the
software watchdog service must be periodically tagged by the
application; otherwise, a timeout occurs and it is assumed that the
application or some underlying software has lost control.
Consequently, an action is initiated that terminates the application or
resets the entire system.
If an application chooses to use the watchdog service, then the
application is also responsible for constructing itself such that some
protection is afforded by the Watchdog. The Watchdog knows when
an application has failed to tag it at the prescribed rate, nothing more.
That is the definition of loss of control within this context, and there
are cases such as tagging the Watchdog too early which are
undetected.