Rockwell Automation 1794 FLEX I/O System with ControlLogix for SIL2 - Manual (Page 17)

Publication 1794-RM001G-EN-P - December 2011
SIL Policy 1-13
SIL Compliance
Distribution and Weight The programmable controller may conservatively be assumed to contribute
10% of the reliability burden. A SIL 2 system may need to incorporate multiple
inputs for critical sensors and input devices, as well as dual outputs connected
in series to dual actuators dependent on SIL assessments for the safety related
system.
Response Times The response time of the system is defined as the amount of time it takes for a
change in an input condition to be recognized and processed by the
controller’s ladder logic program, and then to initiate the appropriate output
signal to an actuator. The system response time is the sum of the following:
• input hardware delays
• input filtering
• I/O and communication module RPI settings
• controller program scan times
• output module propagation delays
See Table 1.1 for associated module information.
Each of the times listed above is variably dependent on factors such as the type
of I/O module and instructions used in the ladder program. For examples of
how to perform these calculations, see publication
1756-RM001 .