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Description of the DP Functions, Data, and Error Codes
4.1.26 DP_watchdog
Purpose
This function starts or stops a watchdog on the CP 5613/CP 5614 that monitors the
activity of the DP master user program. After this function has been called, if the
DP master user program no longer accesses the watchdog element in the dual-port
RAM due to an error (for example an endless loop), this is detected by the
CP 5613/CP 5614 after the set time has elapsed. The CP reacts by changing from
the OPERATE DP mode to the CLEAR DP mode. This brings the slaves to a
defined state.
For more information on CLEAR see Section 2.6 "Modes of the DP Master"
Note 1
!
The watchdog can be used to prevent dangerous plant statuses that might occur
as a result of the failure of the DP master user program.
Without the watchdog, the DP master would remain the the OPERATE mode and
continue to send the last output data to the DP slaves, although the DP master
user program is no longer functioning correctly. If this is unacceptable, the
DP_watchdog function must be called with a monitoring time set to suit the
application.
Note 2
The watchdog of a DP master user program is off following DP_open or after
DP_close. If the value 0 is transferred in the "timeout" parameter, an active
watchdog is deactivated again.
Note 3
The value 1 set for the monitoring time (10 ms) is always rounded up to 2 (20 ms)
internally. The shortest selectable monitoring time is therefore 20 ms.
The accuracy of the monitoring time is approximately 10 ms.
122
DP Base Programming Interface for CP 5613/CP 5614
C79000-G8976-C108-02
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