Rockwell Automation 1756-IH16ISOE ControlLogix Sequence of Events Module - Manual (Page 25)

Publication 1756-UM528A-EN-P - April 2004
How Does the Sequence of Events Module Operate in a ControlLogix System? 2-9
The “reserved” spot on the network and the module’s RPI are
asynchronous to each other. This means there are Best and Worst Case
scenarios as to when the owner-controller will receive updated
channel data from the module in a remote chassis.
Best Case RPI Multicast Scenario
In the Best Case scenario, the module performs an RPI multicast with
updated channel data just before the “reserved” network slot is made
available. In this case, the owner-controller receives the data almost
immediately.
Worst Case RPI Multicast Scenario
In the Worst Case scenario, the module performs an RPI multicast just
after the “reserved” network slot has passed. In this case, the
owner-controller does not receive data until the next available
network slot.
Table 2.4 summarizes the Best Case and Worst Case scenarios, from
the time an input changes state to the time the owner-controller will
receive the data:
IMPORTANT
Enabling the Enable CST Capture feature on an
input module in a remote chassis allows the
module to multicast data at both the RPI rate and
when the input changes state. This helps to
reduce the Worst Case time.
Table 2.4 Best and Worst Case Scenarios For Remote Input Data Transfer
Best case scenario Worst case scenario
Enable CST Capture
disabled Backplane/Network
transfer times (<1mS) Twice the RPI
Enable CST Capture
enabled Backplane/Network
transfer times (<1mS) RPI