state. If capable, the controller will re-boot into the FAIL state. When a timeout of a
control-critical task occurs, the controller generates the appropriate alarms (Diagnostic
Alarm - Critical Task Watchdog Warning), but no other action is taken by the controller.
There is an exception to this behavior where a timeout occurs when the controller CPU is
heavily loaded and the CPUFREE parameter indicates less than 5%. The controller does
not take any action and does not re-boot into a FAIL state.
Tasks critical to view
Tasks executing in the controller, which are critical to view, such as communication with
I/O, display or peer devices, may not execute as required due to excessive loading of the
CPU. The CPU overloading due to tasks other than executing control and result in a
sustained level of CPU Free at 0% (CPU usage is 100%) may cause a loss of view of the
controller.
When a timeout of a view-critical task occurs, the controller generates the appropriate
alarms (Diagnostic Alarm - Critical Task Watchdog Warning), but no other action is
taken by the controller.
7.14
C300 Controller processing overload behavior
The C300 Controller is able to handle controller processing overload conditions when the
controller is required to perform more work than it has time for. The execution of control
strategies or control processing usually demands the most resources in terms of processor
time and is the most likely cause of an overload condition in a controller, (although
other non-control processing causes can produce controller overloads). When
the C300 controller detects an overload, it will 'borrow' time from control execution by
suspending control processing in a defined and predictable manner. This allows the
controller to continue operation to provide control and maintain view and communication
under these conditions.
Controller operation is structured in terms of a 'base cycle' in which key processing tasks
are executed. An overload condition may occur when the controller is scheduled to
perform more tasks than can be completed within a base cycle. For example, a controller
is scheduled to execute more control processing tasks than is recommended, which may
include control, I/O and communications processing tasks. If the scheduled tasks cannot
be completed by the controller within the base cycle, a control overrun occurs.
Control overruns are reported as a diagnostic event in the C300CEE block as 'cycle
overruns', (CRCYCLEOVRN and LSCYCLEOVRN parameters). The controller
manages cycle overruns when the scheduled processing for a cycle does not finish by
85% of the ‘base cycle’ time in the following ways depending on the range of scheduled
processing time.
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