Programming manual M238 | 3 MB - BERGER - POSITEC

Tasks
System and Task Watchdogs
Introduction
2 types of watchdog functionality are implemented for the Modicon M238 Logic
Controller:
System Watchdogs: These watchdogs are defined in and managed by the
controller OS (firmware). These are not configurable by the user.
Task Watchdogs: Optional watchdogs that can be defined for each task. These
are managed by your application program and are configurable in SoMachine.
System Watchdogs
2 system watchdogs are defined for the Modicon M238 Logic Controller. They are
managed by the controller OS (firmware) and are therefore sometimes referred to
as hardware watchdogs in the SoMachine online help. When one of the system
watchdogs exceeds its threshold conditions, an error is detected.
Task Watchdogs
The threshold conditions for the 2 system watchdogs are defined as follows:
If all of the tasks require more than 80% of the processor resources for more than
3 seconds, a system error is detected. The controller enters the EMPTY state.
If the lowest priority task of the system is not executed during an interval of 20
seconds, a system error is detected. The controller responds with an automatic
reboot into the EMPTY state.
NOTE: System watchdogs are not configurable by the user.
SoMachine allows you to configure an optional task watchdog for every task defined
in your application program. (Task watchdogs are sometimes also referred to as
software watchdogs or control timers in the SoMachine online help). When one of
your defined task watchdogs reaches its threshold condition, an application error is
detected and the controller enters the HALT state.
When defining a task watchdog, the following options are available:
Time: This defines the allowable maximum execution time for a task. When a
task takes longer than this the controller will report a task watchdog exception.
Sensitivity: The sensitivity field defines the number of task watchdog exceptions
that must occur before the controller detects an application error.
A task watchdog is configured on the Configuration sub-tab of the Task
Configuration tab for the individual task. To access this tab, double-click the task in
the device tree.
NOTE: For more information on watchdogs, refer to the CoDeSys part of the
SoMachine online help.
38 EIO0000000384 04/2012