Rockwell Automation 1756-DMF30 ControlLogix Drive Module - Manual (Page 36)

Publication 1756-UM522B-EN-P - February 2003
3-10 Drive Module Features
Requested Packet Interval
The Requested Packet Interval (RPI) is a configurable parameter that
defines when the module multicasts its data onto the local chassis
backplane.
In addition to the RPI, Change of State (COS) functionality also causes
the module to produce its data to the consuming controller whenever
the values of the data changes. The RPI timer is asynchronous to the
COS functionality. Both cause the module to produce data when
triggered.
Buffered, Direct and Diagnostic Data - RPI Effect on Output Data (from the
controller)
As a producing controller writes data to the Drive module, the output
data is placed in a local buffer until the next RPI reset occurs. When
the RPI timer expires, the output data is moved from the controller’s
local buffer to the Drive module.
The RPI timer is asynchronous to the program execution. Therefore, a
worst case update to the Drive module can be calculated by adding
the program execution time to the RPI timer setting, as configured by
the user.
Chapter Summary and
What’s Next In this chapter, you learned about the ControlLogix Drive module
features. Move to Chapter 4, Installing the Drive Module, to learn how
to install the module.