Watchdog Timers - GE PACSystems RXi User Manual

5.8.3

Watchdog Timers

Software Watchdog Timer
A software watchdog timer in the CPU is designed to detect failure to complete sweep
conditions. The timer value for the software watchdog timer is set using the programming
software. The allowable range for this timer is 10 to 2550 ms; the default value is 200 ms.
The software watchdog timer always starts from zero at the beginning of each sweep.
The software watchdog timer is useful in detecting abnormal operation of the application
program that prevents the CPU sweep from completing within the user-specified time.
Examples of such abnormal application program conditions are as follows:
■
Excessive recursive calling of a block
■
Excessive looping (large loop count or large amounts of execution time for each iteration)
■
Infinite execution loop
When selecting a software watchdog value, always set the value higher than the longest
expected sweep time to prevent accidental expiration. For Constant Sweep mode, allowance
for oversweep conditions should be considered when selecting the software watchdog timer
value.
The watchdog timer continues during interrupt execution. Queuing of interrupts within a single
sweep may cause watchdog timer expiration.
To extend the current sweep beyond the software watchdog timer value, the application
program can restart the software watchdog timer using Service Request function #8.
However, the software watchdog timer value may only be changed from the configuration
software.
CPU Response to a Software Watchdog Timeout:
If the software watchdog timeout value is exceeded, the CPU goes to Stop-Halt Mode (refer
to page 87). A fault is placed in the Controller Fault table and outputs go to their default state.
Hardware Watchdog Timer
A backup circuit provides additional protection for the CPU. If this backup circuit activates, the
CPU is immediately placed in Reset mode. Outputs go to their default state and no
communications of any form are possible, and the CPU will halt.
CPU Response to a Hardware Watchdog Timeout:
■
The CPU restarts and goes into Stop-Halt Mode (refer to page 87).
■
The CPU does not retain Controller and I/O Fault tables following recovery from the
Stop-Halt state.

During startup following hardware watchdog reset, the CPU logs an informational fault
with Error Code 446, which indicates a watchdog auto-reset occurred.
Note:
Stop-Halt Mode
The controller has the following capabilities in Stop-Halt mode:
■
The CPU can communicate with the Proficy Machine Edition and PACs Analyzer
software through the GbE port, and with the IDM. No other communications or operations
are possible.
■
You can view the fault tables, including all faults logged before the timeout.
GFK-2816F
PACSystems does not support Fatal Fault Retries.
August 2015
Chapter 5. CPU Operation
87