Allen-Bradley 1756-L6 Series, Reference Manual, Page: 22 / 134

22 Rockwell Automation Publication 1756-RM001I-EN-P - May 2012
Chapter 1 SIL Policy
Reaction 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
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
• Redundancy system switchover times (applicable in duplex systems)
Each of the times listed is variably dependent on factors such as the type of I/O
module and instructions used in the logic program. For examples of how to
perform these calculations, see
Appendix A , Reaction Times of the ControlLogix
System .
For more information on the available instructions and for a full description of
logic operation and execution, see the following publications:
• Logix5000 Controllers General Instruction Set Reference Manual,
publication
1756-RM003
• ControlLogix System User Manual, publication 1756-UM001
Reaction Times in
Redundancy Systems The worst-case reaction time of a duplex system is different than a simplex
system. The redundancy system has a longer reaction time because of the
following:
• There are a series of cross-loading operations that continuously occur
between the primary and secondary controllers. Cross-loading fresh data
at the end of each program scan increases scan time.
To minimize scan time by reducing cross-loading overhead, you can plan
your project more efficiently (for example, minimize the use of SINT,
INT, and single tags, and use arrays and user-defined data structures).
Generally, the primary controller in a duplex system has a 20% slower
response time than the controller in a simplex system.