User manual

Publication 1756-RM093F-EN-P - January 2010 21
Safety Integrity Level (SIL) Concept Chapter 1
System Reaction Time
The system reaction time is the amount of time from a safety-related
event as an input to the system until the system sets corresponding
outputs to their safe state. Faults within the system can also have an
effect upon the reaction time of the system. The system reaction time
is the sum of the following reaction times.
Each of the times listed above is variably dependent on factors such as
the type of I/O module and instructions used in the program.
Safety Task Reaction Time
The Safety Task Reaction Time is the worst-case delay from any input
change presented to the controller until the processed output is set by
the output producer. It is less than or equal to the sum of the safety
task period and the safety task watchdog.
Safety Task Period and Safety Task Watchdog
The safety task period is the interval at which the safety task executes.
The safety task watchdog time is the maximum permissible time for
safety task processing. If safety task processing time exceeds the
safety task watchdog time, a non-recoverable safety fault occurs in the
controller and outputs transition to the safe state (off) automatically.
You define the safety task watchdog time, which must be less than or
equal to the safety task period.
The safety task watchdog time is set in the task properties window of
RSLogix 5000 software. This value can be modified online, regardless
of controller mode, but it cannot be changed when the controller is
safety-locked or once a safety task signature is created.
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Sensor
Reaction
Time
Input
Reaction
Time
Safety Task
Reaction
Time
Output
Reaction
Time
Actuator
Reaction
Time