System Reaction Time. Allen-Bradley Rockwell Automation 1734-AENT

System Reaction Time

Safety Integrity Level (SIL) Concept Chapter 1

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.

Sensor

Reaction

Time

+

Input

Reaction

Time

+

Safety Task

Reaction

Time

+

Output

Reaction

Time

+

Actuator

Reaction

Time

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.

Publication 1756-RM093F-EN-P - January 2010 21